759-788 CAP 12.2 INGLESE - Treccani · a-olefins and cyclo-olefins; d) monometallic catalysts. PE...

12.2.1 Introduction

The first studies on polymers took place in the secondhalf of the Nineteenth century, essentially through theefforts of researchers working on certain naturalsubstances, particularly the chemical structure and thechemico-physical behaviour of natural rubber, celluloseand proteins, as well as scientists of organic chemistrywho often encountered polymeric substances during theirwork of synthesis and characterization. In 1861, ThomasGraham coined the term ‘colloid’ (from the Greek kolla,glue) to refer to natural polymers because of the highviscosity of their solutions (Graham, 1861). Chemists didnot immediately consider the possibility that polymersmight be constituted of large molecules. In 1904, CarlHarries, while studying natural rubber, concluded that itwas constituted of a set of cyclic dimers of isoprenecombined together to form bigger aggregations throughsecondary chemical bonds (Harries, 1904). It wasHermann Staudinger in 1920, with regard to the structureof styrene, formaldehyde and natural rubber polymers,who hypothesized the open chain formulae that are nowcommonly accepted. He attributed the colloidal propertiesof the high polymers exclusively to their high molecularweight and was the first to propose calling them‘macromolecules’ (Staudinger, 1920). The problemsconnected with the structure of macromolecules togetherwith those regarding the various possible physical statesof polimeric substances and the correlation betweenproperty and structure, began to be tackled from 1934,principally by Herman Mark (Mark and Whitby, 1940).As a result of this intensive work, at the beginning of the1950s macromolecular chemistry was based on a solidscientific foundation but it had also attained fundamentalgoals regarding industrial applications. In fact, thepolymer industry was in a phase of major developmentand was producing fibres such as nylon and polyester,rubbers such as buthyl rubber and styrene-butadiene

rubber, various thermoplastic resins such as polyethylene(PE) and polyvinylchloride (PVC), and thermosetpolymers such as aminoplastic resins and epoxy resins.

Nevertheless, starting from the most commonmonomers such as vinyls and diolefins, the methods ofpreparation available to macromolecular chemistry werenot able to provide ordered structure macromolecules. Infact, amorphous or very low crystalline polymers werebeing obtained. These were significantly different fromthe structural order of natural polymers, many of whichare highly crystalline. A major step forward in this fieldwas made in 1953-54, with the achievement of anotherfundamental goal in the history of macromolecularchemistry. In 1953, Karl Ziegler at the Max-Planck-Institut für Kohlenforschung where he had been workingfor some time on the chemistry of alkyl aluminiumcompounds, discovered that the product of the reactionbetween Al(C2H5)3 and TiCl4 was capable of catalysingthe polymerization of ethylene into a high molecularweight (MW) linear polymer (Ziegler, 1964). In 1954,Giulio Natta, at the Politecnico di Milano, using similarcatalysts obtained a class of highly crystalline polymersfrom the principal higher a-olefins, which were called‘isotactic’ because they were characterized by longsequences of monomeric units with the same spatialconfiguration (Natta, 1964). Fig. 1 shows the page inNatta’s 1954 diary where he noted the first synthesis ofpolypropylene (PP). Ziegler and Natta were honoured forthese discoveries with the 1963 Nobel Prize forChemistry.

The Montecatini company’s plant in Ferrara was thefirst to begin production of PP in 1957. This marked thereal beginning of the development of the polyolefin (PO)industry. Highly regular polymers were also obtainedfrom butadiene and from other diolefins.

The advent of the Ziegler-Natta (Z/N) catalysisopened up a new period in macromolecular chemistryfrom both the scientific and, above all, the applications

759VOLUME II / REFINING AND PETROCHEMICALS

12.2

Polyolefins

point of view, influencing the plastic materials, elastomersand fibres industry in a highly significant way.

It is unanimously held that the ‘plastics era’ reachedits zenith in 1979, when the volume of production ofplastic materials exceeded that of steel (Martuscelli,1995). POs, which today constitute the most widespreadfamily of plastic materials on a global scale, made amajor contribution to reaching this milestone. Moreover,it should be remembered that, if the worldwideproduction of polymeric materials is still expanding, thisgrowth is accentuated even more with respect to POs. Infact, analysing the data from the last decades, the averageannual increase for POs is seen to be of the order of 7%compared with 4-5% for other plastic materials (by wayof comparison, the equivalent increase of somecommodities such as wood, aluminium and copper, wasless than 2% and that of steel of 0.7%).

This disproportionate growth in favour of POs is alsoanticipated for additional future growth. Table 1 shows thedata, in both quantities and percentages, with respect toproduction of the principal high volume plastic materials.From this data it can be seen that, even though production

of all thermoplastic polymers are continuously increasing,the PO family (and PP in particular) has the highest rateof growth, rising from 60.9% in 1992 to an anticipated66.1% in 2005. This highly favourable trend is due to: a) low cost and wide availability of monomers; b) thenon-toxicity of polymers; c) the wide range ofhomo/copolymers obtainable; d ) recyclability ofpolymers; e) identification of increasingly efficient andsophisticated catalytic systems; f ) economical andenvironmentally friendly production processes.

As far as the usage is concerned, the extensive spreadof POs is due to the fact that the main feature of thesepolymeric materials is their great versatility in terms ofstructural characteristics and physico-mechanicalproperties. In fact, both the homopolymers of ethylene(C2), propylene (C3), 1-butene (C4) and 4-methyl-1-pentene, and the copolymers C2/C3, C2/C3/diene, C2/C4,C2/C3/C4, C2/1-hexene, C2/1-octene and C2/cyclo-olefins,obtainable in wide ranges of composition, give rise tomaterials with properties that range, for example, fromthe high rigidity typical of technopolymers to materialswith the characteristics typical of elastomers. Fig. 2 showsthe principal producers of PE and PP with their respectivemarket share up-dated as at 2002.

12.2.2 Catalytic systems for the polymerization of olefins

Catalysts for the polymerization of olefins can begrouped into four main families: a) catalytic systemssupported on oxides, used essentially for polymerizationof ethylene; b) Ziegler-Natta catalytic systems, used in thehomo/copolymerization of a-olefins and cyclo-olefins;c) homogeneous catalytic systems based on metallocenes, used in the homo/copolymerization of

760 ENCYCLOPAEDIA OF HYDROCARBONS

POLYMERIC MATERIALS

Fig. 1. Page of the diary of Giulio Natta containing the noteof the first synthesis of PP (Pasquon, 2004).

Table 1. Evolution of the world market in thermoplastic polymers (CMAI, 2002)

Year 1992 1997 2005 (forecast)

Polymer t (106) % t (106) % t (106) %

Polypropylene (PP) 15.1 19.0 23.1 21.6 39.0 24.7

High density polyethylene(HDPE) 13.3 16.7 18.8 17.6 29.0 18.4

Linear low densitypolyethylene (LLDPE) 6.1 7.6 9.7 9.0 17.4 11.0

Low density polyethylene(LDPE) 14.0 17.6 15.7 14.7 19.0 12.0

ABS resins 3.0 3.8 3.9 3.6 5.6 3.5

Polystyrene (PS) 9.3 11.7 12.0 11.3 17.0 10.8

Polyvinylchloride (PVC) 18.8 23.6 23.8 22.2 31.0 19.6

Total 79.6 100 107.0 100 158.0 100

a-olefins and cyclo-olefins; d ) monometallic catalysts.PE is also obtained in high pressure, high temperature

processes in the presence of radical initiators. This isexactly how PE was synthesised by researchers for thefirst time in 1933 at Imperial Chemical Industries (ICI),who noticed the presence of traces of a waxy polymerwhen ethylene and benzaldehyde were put to react in anautoclave at 170°C and 190 MPa of pressure. The relativepatent was filed in 1936, and in 1939 a small productionplant began operation.

Catalysts supported on oxides

These types of catalysts are obtained supporting metaloxides or organic-metallic compounds of transitionmetals on Al2O3 and SiO2.

Catalysts of the Standard Oil Co.In 1951, the Standard Oil Co. filed a patent claiming

the first catalytic system capable of producing HDPE(High Density PolyEthylene) using a low pressureprocess. It used Mo2O3 supported on Al2O3 and activatedat 500°C in the presence of H2 (Roebuck and Zletz,1954). The yield of the polymerization at a temperaturebetween 200 and 280°C and a partial pressure of theethylene between 4 and 7 MPa was extremely low (from0.5 to 2.5 g of PE per g of catalyst per hour). Thiscatalytic system was subsequently modified with theobjective of obtaining polymers with a higher MW(Tadokoro et al., 1967).

However, polymerization processes which used thiscatalytic system, operated in solution and did not undergoany significant industrial development.

Catalysts of the Phillips Petroleum CompanyThese catalysts were described for the first time in

a patent filed by the Phillips Petroleum Co. (Hoganand Banks, 1958) and have enjoyed notablecommercial success, to the extent that even today morethan 50% of HDPE produced worldwide is synthesisedusing this technology. The methodology of thesynthesis provides for the impregnation ofsilica-alumina with an aqueous solution of CrO3 andsubsequent dehydration and activation with air in afluid bed at high temperature (500-800°C) for severalhours.

A sizeable volume of scientific publications describesthe activation mechanism of this catalytic system anddeals, above all, with the state of oxidation of chromium,the number of active centres and the kinetics ofpolymerization (Guyot, 1965).

Spectroscopic studies (Karakchiev et al., 1967) andanalytical research (Hogan, 1970) have demonstrated thesurface formation of support of chromates anddichromates resulting from the reaction between CrO3and the silanols present on the support as shown in thefollowing diagram:


POLYOLEFINS

othersothersSumitomo Mitsui Polyolefins

Sumitomo Mitsui Polyolefins

Formosa PlasticsAtofina Petrochemicals

Atofina Petrochemicals

Nova Chemicals

polyethylene2002 world production: 67.1 millions of t

polypropylene2002 world production: 38.7 millions of t

Polimeri EuropaFormosa Plastics

Basell Polyolefins

Basell Polyolefins

Equistar ChemicalsChevron Phillips Chemicals

BP SolvaySabic

Sabic

ExxonMobil Chemical

ExxonMobil Chemical

BP Solvay

Dow Chemical

Sunoco Chemicals

Dow Chemical

Reliance Industries Borealis

Borealis

0 5 10 15 40(%) (%)

45 0 5 10 15 40 45

Fig. 2. Principal producers of polyolefins with relative market shares (CMAI, 2002).

Si OH

Si OHO � 2CrO3

Si O

Si OO

Cr

CrO

O

O

O

O

Si OH

Si OHO � CrO3

Si O

Si OO Cr

O

O

The formation of silyl chromates on the surface ofthe support allows the stabilization of Cr(VI) which isotherwise unstable in air at temperatures above 500°C.

This catalyst, which is very sensitive to moisture,polymerises ethylene with a significant induction time. Itcan be assumed that the Cr2�, produced in the initialcontact phase between the monomer and the catalystduring which the catalyst changes colour, is in fact theactive species. The reduction and subsequent activationof the catalyst can be carried out through processingwith CO; in this case, the polymerization reaction beginswith no induction time. The regulation of the MW of thepolymer chains presents a serious problem, since it is notpossible to use the traditional chain transfer such as, forexample, hydrogen, which in this case, because of thereaction with the chromates, would oxidize to watercontaminating the catalytic system. The MW control is,therefore, obtained by choosing the appropriate supportto use both directly and after having subjected it tospecial treatments. The most commonly used silica must,however, have a large surface area, high porosity and apore diameter of between 20 and 50 nm. There isliterature available that describes pretreatments usingtitanium compounds or fluorine, which convert thesilica’s hydroxylic groups into fluorides, reducing thereactivity of the support; the same effect can be obtainedby significantly increasing the temperature of thecalcination. Also used are special polymerizationconditions that favour the termination of the growingpolymer chain, which takes place through the b-hydrogen shift reaction:

The high temperature favours this reaction whichleads to the formation of a macromolecule with aterminal vinyl group and an ethyl group bound to themetal. The most recent developments with regard tocatalytic systems belonging to this family are first, thework of researchers of Union Carbide Corporation(UCC: Carrick et al., 1972), describing the use ofbis(triphenylsilyl) chromate, strikingly similar to theactive site of polymerization as previously described;and the second, based on catalysts consisted of acompound containing an atom of chromium co-ordinated with a p system. The one especially used isdicyclopentadienyl chromate. One of the mostinteresting aspects of this catalytic system is itssensitivity to hydrogen, which makes it possible to

obtain polymers with a wider range of molecularweights. The molecular weight distribution (MWD) iswider in comparison with that obtained with the UCCtype of catalysts.

Organometallic compounds of supported transitionmetals

The most commonly used supports are SiO2 andAl2O3 activated with different thermal treatments. Thepolymerization activity is generally very low, so much sothat none of these catalytic systems have been used inindustrial production plants.

The performance of these catalysts, expressed as PE(kg)/M (mmol) · atm · h (where M represents the metal),polymerising at 80°C is as follows: Cr(allyl)3 onAl2O3�58; Zr(allyl)4 on Al2O3�280; Hf(allyl)4 onAl2O3�1; Hf(allyl)4 on SiO2�25; Zr(benzyl)4 onAl2O3�390; Ti(benzyl)4 on Al2O3�170.

Ziegler-Natta catalytic systems

Catalytic systems for polyethyleneIn 1953, Ziegler discovered that PE could be

obtained at low pressure in a wide range of temperaturesusing a bi-component catalytic system based oncompounds of a transition metal and organometalliccompounds. Generally halides of Ti, Zr, V and Nb wereused, being made to react with organometalliccompounds of Al, Li, Na, K, Zn, Cd, Ga and B. Thecombinations which found industrial applications wereTiCl3 and TiCl4 which were made to react withAl(C2H5)3 and Al(C2H5)2Cl, today called ‘Low Yield’(LY) catalytic systems, which were very quicklyreplaced by catalytic systems based on supportedcatalytic solid components defined as ‘High Yield’ (HY).Belonging to this class are catalytic componentsobtained through the reaction of a titanium compoundwith magnesium compounds, so as to generate an activeform of MgCl2. The first HY catalyst was synthesized inFerrara by researchers at Montedison in 1968. Fig. 3shows the powder X-rays diffraction spectra of MgCl2before and after activation. This shows that afteractivation, MgCl2 displays an increase in the degree ofdisorder, which can be seen from the disappearance ofthe reflection at 15° which has been replaced by a halo.Moreover, an increase in the surface area and a reductionin the dimensions of the crystallites can be observed. Alarge number of Montedison’s patents describe theprocess for obtaining these catalysts through the simplegrinding together of TiCl3 or TiCl4 with MgCl2 (Mayr etal., 1984; Mayr et al. 1985). The original structure ofMgCl2 consists of a compact cubic packing of chlorineatoms; alternate planes of the octahedral cavities of thispacking are filled by Mg2� ions. The resulting structureis of triple alternating layers of Cl�Mg�Cl, where the


POLYMERIC MATERIALS

X

X

X

X

R

M X

X

X

X

R CH2

CH2MX

X

X

X

R

M

R

X

X

X

X

CH2

CH2M

Mg is hexacoordinate. During the dry milling, becauseof the presence of titanium salts, the layers of chlorineatoms are subjected to translation and rotation whichdestroys the crystal order in the staking direction(Giannini, 1981). Many other ways have been perfectedto obtain catalytic compounds of this type, for examplethrough the reaction of titanium halides with magnesiumalcoholates (patented by Hoechst) or through thereaction of TiCl4 with Grignard reagent or dialkylmagnesium derivatives. In this case, the titanium presenton the support is in large measure reduced totitanium(II). Of particular interest is a family ofmagnesium chlorotitanates obtained through the reactionbetween TiCl4 and MgCl2 in a large excess of a Lewisbase (LB), such as alcohols, acids, esters and amines(Giannini et al., 1977). The compounds thus obtainedhave a MgTiCl6·xLB stechiometry and are extremelyactive in the polymerization of ethylene in combinationwith AlR3. The structure, which represents a structuralhypothesis regarding the conformation of the active site,was worked out from these compounds (Bart et al.,

1981). Table 2 shows the polymerization performance at80°C of some supported catalysts on MgCl2 (with atitanium content in the catalyst of 3.6% by weight).

In these catalytic systems, the regulation of the MWof the growing chain of PE is carried out usinghydrogen. The most modern catalytic systems aresynthesized so that the solid catalytic component has acontrolled morphology (granular or spherical). Theprocess perfected by Montedison, for example, involvesthe spherulization of adducts of magnesium chloridewith ethyl alcohol in a melt state and subsequentreaction of the microspheres thus obtained with a largeexcess of boiling TiCl4, so as to remove the excess ofalcohol and so obtain TiCl4 supported on active MgCl2in a spherical form (Giannini et al., 1979). Thesecatalytic components are particularly suited toprocessing in gas phase, inasmuch as the nascentpolymer is capable of reproducing the morphology ofthe catalyst with a replication factor dictated by theactivity of the catalyst.

The action mechanism of the Z/N catalysts wasdescribed for the first time by Peter Cossee and EvertArlman (Arlman and Cossee, 1964) and is depicted inthe following diagram:

Analysing the reaction between TiCl4 andAl(CH2R)3, where R is an alkylic radical, the transitionmetal bears four atoms of chlorine, a group�CH2R


POLYOLEFINS

14,000

inte

nsit

y

angle (°)

12,000

10,000

8,000

6,000

4,000

2,000

0 10 20 30 40 50 60

inte

nsit

y

angle (°)

500

1,000

1,500

2,000

2,500

3,000

0 10 20 30 40 50 60

B

A

Fig. 3. X-ray diffraction spectra of MgCl2 before (A) andafter (B) activation.

X

X

Mt � Al(CH2R)3

alkylation and activation

transition state

insertion

co-ordination

X

CH2R

Mt

X

CH2RMt

Al(CH2R)3CH2R

Mt

CH2RMt �

CH2RMt

R'

R'

CH2RMt

R'

CH2R

Mt R'

CH2R

Mt R'CH2R

Mt

R'

derived from the reaction between the metal itself andthe cocatalyst linked by a s bond (alkylation andactivation) and a vacant site where the olefin can be co-ordinated. The mechanism carries out a first step ofco-ordinating the olefin and, after a subsequent transitionstate, a second step of insertion into the metal/carbon s bond. The site where the olefin was co-ordinated in thesecond step of the reaction bears the growing polymericchain, while the site which contained the alkylic group isnow available for the co-ordination of a new monomericunit. Repeating the steps of co-ordination andsubsequent insertion gives rise to the polymerizationreaction.

Catalysts for polypropylene: general considerationsBefore dealing at length with catalysts for

polypropylene, it would be appropriate to make someobservations about the stereochemistry ofmacromolecules.

Propylene and monomers with the general formulaCH2�CHR are prochiral molecules in that they containa stereogenic carbon atom. Therefore, unlike ethylene,the two C atoms of the double bond are not equivalentand, as a result, the polymer segments containingdifferent types of concatenation head-to-tail, tail-to-tail,tail-to-head or head-to-head (addition 1.2 or 2.1) presentphenomena of position isomerism or regioisomerism.

There are three levels of understanding regarding thetypology of a polymeric chain: its constitution,configuration and conformation.

The constitution defines the sequence of the atoms orof the bonds in a macromolecule (regioisomerism). Theconfiguration defines the layout relative to thesubstituents around an element of stereoisomerism. Theconformation takes account of the position in space ofthe atoms obtained through rotation around the simplebonds.

The study of the configuration of a polymer consistsof determining the tacticity, that is the type of orderingpresent. The head-to-tail vinyl polymers are made up ofa succession of alternating methylene groups and atomsof pseudo-assymetrical carbon atoms, bearing an alkylicgroup, which are real centres of achirotopic

stereoisomerism. Isotactic is defined as those polymersin which all the tertiary carbon atoms have the samespatial configuration; syndiotactic refers to those inwhich each tertiary carbon atom has an oppositeconfiguration to that of the adjacent one (Farina andPuppi, 1993). If there is no identifiable stereoregularityin the sequence of the tertiary carbon atoms, the polymeris defined as atactic. If instead the polymer possessesregular features (iso or sindio types) interspersed withirregular features, it is referred to as a stereoblockpolymer. The term pseudo-assymetrical is used when, inconsidering a non-terminal tertiary carbon atom in achain of PP, it bears a methyl, an atom of hydrogen andtwo fragments of a polymer chain, formally equivalent.

Macromolecules of isotactic polymers have ahelicoidal structure, as shown in Fig. 4, which illustratesa chain of isotactic PP and a chain of syndiotactic PP ina crystalline state, with an orthogonal view of the axis ofthe helix and a view projected along the axis of the helix.

The performance of a catalytic system for PP isexpressed taking account of certain essential parameters:the polymerization yield generally expressed in kg ofpolymer/g of catalyst or kg of polymer/g of titanium; thestereospecificity expressed as the isotactic polymer as a


POLYMERIC MATERIALS

Table 2. Polymerization performance at 80°C, expressed as PE (g)/Ti (mmol)�bar�h,

of certain catalysts supported on MgCl2

Catalyst Cocatalyst Performance

MgCl2-TiCl4 Al(C2H5)3 1,650

MgCl2-TiCl4 Al(C2H5)2Cl 330

MgCl2-TiCl3 Al(C2H5)3 430

MgCl2-TiCl3 Al(C2H5)2Cl 20

A BA B

Fig. 4. Chains of isotactic (A) and syndiotactic (B) PP with orthogonal view of the axes of the helix (above) and seen projected along the axis of the helix (below)(Natta, 1965).

percentage of the total polymer produced (II, index ofisotacticity, obtainable empirically as the percentage ofpolymer insoluble in boiling n-heptane); the MW andthe distribution of the molecular weights of the polymersobtained.

Heterogeneous catalysts for first generationpolypropylene

The first catalytic system used by Natta in 1954 forthe polymerization of propylene consisted of acombination of TiCl4 and AlR3 (where R represents ethylor isobutyl), which had previously been employed byZiegler for the polymerization of ethylene. This catalystgave a modest yield and a stereospecificity of about 30-40%. TiCl3 violet (II�85%) was of much moreinterest, substantiating the hypothesis, subsequentlyrevealed to be untrue, that stereospecificity wasexclusively correlated to a solid, crystalline catalyticcomponent. TiCl3 has been used for many years in theindustrial production of isotactic PP in combination withAlR3 or AlR2Cl.

TiCl3 displays four different types of crystallinemodification – a, b, g and d – and can contain AlCl3 insolid solution depending on the method of preparation(Natta et al., 1958; Natta et al., 1959; Natta, 1960; Nattaet al., 1961). The a, g and d-forms are violet in colourand have a layered structure formed by a Cl�Ti�Clstack, that is made up of a titanium ion between twolayers of chlorine. The a-form TiCl3shows a hexagonalclose packing of chlorine atoms, and the g-form a cubicclose packing structure. The structure of TiCl3 is verysimilar to that of MgCl2 from which it can be obtainedoccupying with atoms of titanium 2/3 of the octahedralcavity of the compact lattice of the chlorine ions, whichin MgCl2 are occupied by atoms of magnesium. Becauseof this 2/3 occupation factor, the titanium atoms findthemselves in a chiral environment responsible forcontrolling the stereochemistry of the polymerizationreaction. The same type of chirality is produced in a solidMgCl2/TiCl4 type catalytic component, but on a singleface of MgCl2 (Busico et al., 1986). In the d-form TiCl3,the package is irregular, typical of a disordered structure.The bTiCl3 is brown in colour and displays a linearstructure. The most commonly employed methods ofsynthesis of the various TiCl3 types in industrial usageare: reduction of TiCl4 with H2 at 800°C forming aTiCl3;reduction of TiCl4 with metallic aluminium at 150°C inthe presence of traces of AlCl3, producinggTiCl3·0.33AlCl3 which through subsequent milling istransformed into the d-form which is particularly activein polymerization; reduction of TiCl4 with AlR2Cl, whichleads to the formation of bTiCl3·xAlCl3 which, throughthermal activation, is transformed into the g-form.

The polymerization performance of TiCl3-basecatalytic systems, derive from its crystalline form and

from the type of cocatalyst used. The polymerizationyield of TiCl3 and TiCl3 in particular, displays thefollowing progression as far as the cocatalyst isconcerned: AlEt3�AlEt2Cl �AlEt2Br �AlEt2I, whilethe stereospecificity displays a completely oppositeprogression.

Syndiotactic PP was isolated for the first time byNatta (Natta et al., 1960) from the n-heptane solublefraction of an essentially isotactic product obtained witha catalytic system made up of TiCl3/AlR2Cl. On theother hand, it was synthesized for the first time byAdolfo Zambelli who polymerized propylene at �78°Cusing a homogeneous catalytic system formed fromVCl4/AlEt2Cl (Natta et al., 1962).

Heterogeneous catalysts of second generationpolypropylene

At the beginning of the Seventies, Solvay patented acatalytic system based on TiCl3 having specialcharacteristics in terms of yield, stereospecificity andability to control the morphology of the polymer(Hermans and Henrioulle, 1973). The synthestic methodis based on the reduction of TiCl4 in a hydrocarbonsolvent at low temperature with AlEtCl2; throughsubsequent heating to 65°C, bTiCl3·xAlCl3·yAlEtCl2was obtained. This solid displayed a small surface areaand low catalytic activities. Through subsequentprocessing with isoamyl ether at 35°C, a large part of thealuminium compounds were removed and subsequentprocessing with an excess of TiCl4 at 70°C, and thenwashing with hydrocarbons led to the formation of adTiCl3 having a large surface area (��200 m2/g), highporosity (�0.2 cm3/g) and high catalytic activity. Theperformance data of this catalytic system are reproducedin Table 3.

Heterogeneous catalysts for polypropylenesupported on MgCl2(third, fourth and fifth generations)

As previously stated concerning PE catalysts,numerous ways can be followed to obtain a catalyticsystem based on activated MgCl2, definable as a supportwith a disordered crystalline structure; in particular, as aconsequence of this disorder, due to the rotation andtranslation of the Cl�Mg�Cl layers, many magnesiumatoms become coordinatively unsaturated and positionedon the two most probable fracture surfaces of the MgCl2.Experimental data show that the preferential lateral cutare those corresponding to (110) and (100) planes (Fig. 5). These two types of lateral cut lead to twodifferent local situations: pentacoordinated magnesiumatoms on surface (100) and tetracoordinated magnesiumatoms on surface (110) (Fig. 6).

During synthesis of the catalyst through reactionwith TiCl4, single molecules of the same or Ti2Cl8


POLYOLEFINS

dimers can be positioned, in epitactic growth, on thesurfaces of the MgCl2 (Albizzati, 1993). PaoloCorradini, on the basis of evaluation of non-bondedinteractions for the proposed active site, demonstratedthat the epitactic growth of Ti2Cl6 dimers (derived fromthe reduction of Ti2Cl8 with AlR3) on surface (100) ofMgCl2 leads to the formation of a stereospecificcatalytic site. On the other hand, a non-stereospecific siteresults from the co-ordination of TiCl3 or Ti2Cl6 onsurface (110) of MgCl2 (Corradini et al., 1983). Fig. 7compares the stereospecific site model on MgCl2 andthat on TiCl3: as can be seen, they are extremely similar.

The MgCl2·TiCl4/AlR3 catalytic system displays highcatalytic activity in the polymerization of ethylene, while


POLYMERIC MATERIALS

Table 3. Comparison of performances of catalysts containing different diethers as internal electron donors (iED)*

* Conditions of polymerization: liquid monomer, 70°C, 2 h, cocatalyst Al(C2H5)3

iED Distance O-O (nm) Productivity (kg of PP/g of catalyst) II (%)

1,3-dimethoxypropane (1,3 DMP) 0.45 4.0 64.9

2-dimethyl-1,3-DMP 0.39 30.0 74.9

2,2-dimethyl-1,3-DMP 0.48 35.0 89.8

2-isopropyl-2-methyl-1,3-DMP 0.29 40.0 94.5

2,2-diisopropyl-1,3-DMP 0.30 42.0 96.4

2-isobutyl-2-isopropyl-1,3-DMP 0.28 60.0 96.9

110cut

100

cut

Cl

Mg

Fig. 5. Preferential breakage plains of crystals by MgCl2.

Cl

Mg

Fig. 6. Co-ordination vacancies for atoms of magnesium on surface (100) (above) and on surface (110) (below).

A

B

Cl Ti Mg

Fig. 7. Models of stereospecific sites: A, Ti2Cl6 positionedon surface (110) of the TiCl3; B, Ti2Cl6 grown hepitacticallyon surface (100) of the MgCl2.

A

B

Cl

Mg

for propylene, besides a high yield, it displays lowstereospecificity (II�40-50%), a value which is entirelyconsistent with the model of the active sites justdescribed and based on the contemporaneous presenceof isospecific and aspecific sites on MgCl2. Researchersat Montedison at the beginning of the Seventiesdiscovered that it was possible to increase thestereospecificity of the MgCl2 based catalysts usingLewis bases as electron donors (ED) (Giannini et al.,1978). In particular, the best results were obtained bycombining two EDs: the first positioned on the solidcatalytic component (iED, internal Electron Donor) andthe other combined with AlR3 (oED, outside ElectronDonor). Experimental tests demonstrated that duringpolymerisation, an exchange takes place between theiED and the oED; the mechanism of the process can beexplained as follows (Giannini et al., 1987): selectivepoisoning or modification through the formation of acomplex with ED on the surface of MgCl2 which wouldgenerate aspecific centres, and a competition of ED withTiCl4 for a selective co-ordination with the surface of theMgCl2 bearing Mg atoms coordinatively unsaturated.Bearing in mind that surface (100) of MgCl2 displays ahigher basicity than face (110) in terms of the possibilityof co-ordinating TiCl4, it can be positively stated thatstereospecific sites of Ti are positioned on surface (100),while the ED saturates the co-ordination vacancies of theMg atoms present on surface (110) impeding thepositioning there of aspecific Ti species, as shown in Fig. 8. The most effective iEDs aretetramethylethylenediamine (TMEDA), ethyl benzoate(EB) and diisobutyl phthalate (DIBP), while as regardsoEDs the best are EB, methyl p-toluate (MPT), 2,2,6,6-tetramethylpiperidine (TMP) and variousalkylalkoxysilanes (Albizzati, 1993).

The high polymerization activity manifested by theMgCl2 based catalytic systems is attributable to ahigher number of active centres and a higherpropagation constant in the polymerization reaction(both of a higher order of magnitude than TiCl3).These catalysts have been the subjects of continuousresearch activity on the part of numerous industrialgroups with the aim of the continual improvement oftheir performance in terms of yield, stereospecificityand ability to control the morphology of the polymerproduced. The most recent developments (fifthgeneration) concern a family of solid catalyticcomponents based on MgCl2 and 1,3-diethers (Albizzatiet al., 1990), to be precise 2,2-dialkyl-dimethoxypropaneshaving the general formula (R1,R2)C(CH2OR)2,capable of providing PP in high yields andstereospecificity in the absence of oEDs.

In line with the theory expounded above, withregard to the role of Lewis bases in thestereoregulation of polymerizations, the hypothesispresents itself that one of the most important aspectsof a bi-functional ED should be the ability to kelatetetracoordinated magnesium atoms situated on surface(110) of the MgCl2. This hypothesis is confirmed bythe behaviour of numerous bi-functional EDs, such asfor example phthalates and alkylalkoxysilanes. Even inthe diether family it is only those that have a moreprobable conformation which guarantee a specificdistance between the oxygen atoms, that areparticularly effective (Albizzati, 1997). In particular,through studies carried out using the ConformationStatistical Distribution Methodology (CSDM), onlywhen the distance between the two electron donoratoms was close to 0.3 nm is it possible to obtainexcellent polymerization performance (see again Table3). This distance is the best for its co-ordination on amagnesium divacant atom situated on surface (110) ofMgCl2. Recently Basell Polyolefins presented anotherfamily of catalysts containing as iED succinateshaving the general formula (R2C(CO2R))2 (Morini etal., 2000). In Table 4 there is a summary of theperformances of catalysts belonging to the differentgenerations.

As previously indicated, it is possible, using specialways of synthesizing, to obtain solid catalyticcomponents in micro-spherical form and hence in thepolymerization, according to a replication ratiodictated by the catalytic activity, to produce a sphericalpolymer, with subsequent ever simpler productionprocesses. In fact, the high yield makes it possible toavoid removal of the catalytic residue, the highstereospecificity to avoid the separation of theamorphous polymer and the spherical shape to avoid the pelletization of the final polymer (Giannini et al., 1979).


POLYOLEFINS

LBLB

LB

LB

LB

LB

LB

LB

LB LBLB

LB

LB

LB

LB

LB

LB

LB

LB

LB Ti2Cl6

Ti2Cl6

Ti2Cl6

Ti2Cl6

Ti2Cl6LB

LB LB LB LB LBLB

110cut

110cut

100

cut

100

cut

110 cut

before titanation after titanation

110 cut

Fig. 8. Schematic representation of the positioning of theLewis base (LB) and of Ti2Cl6 on surfaces (110) and (100) of MgCl2 before and after reaction with an excess of TiCl4.

Metallocene based catalytic systems

The most important innovation, which has appearedin recent years in the field of Z/N co-ordinated anioniccatalysis, is certainly the discovery of metallocene basedcatalytic systems (MBC, Metallocene Based Catalyst).These catalytic components are d0 pseudotetrahedral,organometallic compounds, in which twocyclopentadienyl ligands are co-ordinated h�5 to atransition metal, typically of group IV (Ti, Zr, Hf).These complexes were previously used by Natta(Natta et al., 1957) and Breslow (Breslow andNewburg, 1957), in combination with aluminiumalkyls for the polymerization of ethylene, achievingan extremely low catalytic activity. At the end of theSeventies, Hansjörg Sinn and Walter Kaminsky (Sinn andKaminsky, 1980), combining metallocenes with a newcocatalyst, polymethylaluminoxane (MAO), obtainedvery high yields of PE and atactic PP with low MW.

MAO is the product of a reaction, carried out in verycontrolled conditions, between trimethylaluminium(AlMe3) and water. To achieve these conditions, the waterwas made available from some hydrated salts, such as forexample Al2(SO4)3·16H2O, CuSO4·5H2O, FeSO4·7H2O(Giannetti et al., 1985), or is used in the form of ice(Sinn, 1988). From cryoscopic measurements, GPC (GelPermeation Chromatography) and NMR (NuclearMagnetic Resonance; Resconi et al., 1990a), it washypothesized that MAO consisted of a mixture of linearand/or cyclic oligomers, containing significant quantitiesof co-ordinated AlMe3 (Sinn, 1995). The formationmechanism of the catalytically active species, which turnsout to be a dicyclopentadienyl alkyl cation, is:

It is important to emphasise how the p ligands of themetallocene remains co-ordinate with the transitionmetal even after the reaction with the cocatalyst andduring the course of the polymerization, thus influencingthe catalytic activity, the speed of propagation/transfer ofthe chain, the stereospecificity of the reaction and thereactivity of the co-monomers, in the case ofcopolymerization. In 1984, John Ewen obtained amixture of atactic and isotactic PP using a combinationof meso/racemic isomers of a stereorigid metallocene:ethylene-bis-indenyl-titanium-dichloride (r/m-EBITiCl2)(Ewen, 1984). This was the first direct proof that the useof a chiral catalyst is necessary for obtaining astereoregular poly-a-olefin. At the same time,Kaminsky, using the equivalent hydrogenatedzirconocene in a pure racemic form (r-EBTHIZrCl2),prepared by Hans-Herbert Brintzinger (Schnutenhausand Brintzinger, 1979; Brintzinger, 1988), obtained highyield isotactic PP and polybutene (Kaminsky, 1986).These catalytic systems are also called Single SiteCatalysts (SSC) or more correctly, Single CentreCatalysts (SCC): in fact on transition metals there aretwo catalytic sites which, however, statistically producethe same type of macromolecule. The most efficientcocatalysts used in conjunction with MBCs are, otherthan MAOs, other aluminium compounds such as somediisoalkylalumoxanes (isobutyl-, isooctyl- and 2-phenyl-propyl-; Resconi et al., 1990b). Moreover,boron compounds such as tris-pentafluorophenyl boraneand tetrapentafluorophenyl borate can also be used.

The outstanding performances in the polymerizationof MBCs gave a glimpse of industrial applications ofrelevant interest, thus generating a major stimulus toresearch in the numerous laboratories both of industryand of the universities. In fact, in the ten years followingSinn and Kaminsky’s discovery, over 800 patents werefiled claiming the synthesis of metallocenes and ofcocatalysts and their use in the homo/copolymerizationof olefins and cyclo-olefins. All the major PO producingcompanies active at that time, such as Exxon, Hoechst,Fina, BASF, Dow, Idemitsu, Mitsui Toatsu, Mitsui


POLYMERIC MATERIALS

* Conditions of polymerization: liquid monomer, 70°C, 2 h, R� �C2H5

Table 4. Comparison of performances of catalysts of different generations*

Generation Catalytic system Productivity (kg of PP/g of catalyst) II (%) Morphology

1a TiCl30.33AlCl3�AlR2Cl 2-4 90-94 powder

2a TiCl3�AlR2Cl (Solvay) 10-15 94-97 granular

3a TiCl4/iED/MgCl2�AlR3/oED 15-30 90-95 spherical

4a TiCl4/diester/MgCl2�AlR3 silane 30-60 95-99 spherical

5a TiCl4/diether/MgCl2�AlR3 100-130 96-99 spherical

OX

X

Cp2Mt Al

[Aln(CH3)n�1OnX]

� �

X

CH3

Cp2Mt

CH3

�[Aln(CH3)n�1OnX]

�

�

�

CH3

Cp2Mt�

Petrochemical and Montell, were active participants inthis research effort.

The peculiar characteristics of MBC, which incertain cases encouraged their industrial development,can be summarized thus: it is possible to use them tosynthesise all the traditional POs (understood to bepolymers obtainable with the different generations ofHY catalysts), often endowed with some superiorproperties; they are able to synthesize new polymers,not obtainable with traditional catalytic systems, suchas for example highly stereoregular syndiotactic poly-a-olefins, such as PP (Ewen et al., 1988), poly-1-butene, poly-1-pentene and poly-4-methyl-1-pentene(Asanuma et al., 1991) and polyallyltrimethylsilane(Ziegler et al., 1994). Moreover, they can yield atacticPP with high molecular weights (Resconi et al., 1994).Even in the field of EPR elastomers (see below) theyprovide crystalline and amorphous copolymers andterpolymers with innovative molecular structures andphysico-mechanical properties. For example, non-vulcanized ethylene/propylene copolymersendowed with elastomeric characteristics (Galimbertiet al., 1994a; Guerra et al., 2002) or characterized byan almost totally alternated succession of the twocomonomers (Galimberti et al., 1995, 1994b) or by thecontemporaneous presence of sequences of both of thecomonomers (Galimberti et al., 1998).

In some cases, these characteristics can also give riseto an improved utilization of and a simplification ofsome existing production processes.

In 2003, about 15% of polyethylenes (HDPE andLLDPE), 10% of PP and 20% of EPR producedworldwide was synthesized with MBC.

Making a comparison between the traditionalcatalytic systems supported on MgCl2 and MBC forthe synthesis of PO, some fundamental similarities canbe seen, as well as some major differences. Among thesimilarities it is important to remember that both thecatalyses generate a polymerization for inserting amonomer onto the bond between the metal and thegrowing chain, definable as a coordinated ionicreaction, in which the transition metal carries apartially positive charge and the double bond of theolefin is coordinated on the catalytic centre. The mostsignificant differences can be summarized as:• The metallocenes are compounds, which are

chemically and structurally easily distinguishableand soluble in aromatic and aliphaticpolymerization solvents. Unlike a heterogeneouscatalyst, it is usually defined through analysis of itschemical composition (for the solid component),and from the formula (for the cocatalyst), whichboth describe only the precursor of the actualcatalytic system.

• Almost all the metal atoms are active in

polymerization (Chien and Wang, 1990), unlike whathappens with heterogeneous catalysts, in which onlya small part of the titanium is active inpolymerization (Albizzati, 1993).

• The mechanism of the polymer chain’s growth isdifferent, with two co-ordination sites for MBCs,while only one is available for titanium located onthe surface of the HY catalysts, and for this reason itis also more accurate to define MBCs as singlecentre rather than single site (Resconi et al., 1995).

• In the MBCs, the steric and electronic environmentaround the active centre can be modified much moreeasily.

• In MBCs, the atom of metal has a pseudo-octahedralconfiguration while the surface titanium in the HYcatalysts has an octahedral conformation.

• The homogeneity of the active centres of MBCsensures that they produce polymers with a narrowMWD and narrow chemical composition distribution(CCD).The following observations can be made about this

last-mentioned property with regard to a-olefinhomo/copolymerization: in the case ofhomopolymerization to a predominantly stereoregularpolymer, the same catalytic centre promotes both regularand irregular insertions of the monomer; hence any errordue to regio- or stereoirregularity is distributedstatistically along the macromolecule. A similar resultcomes about from the distribution of a comonomer incopolymerization, so much so that the irregular unit canbe perceived to be a monomer. This is substantiallydifferent from heterogeneous titanium based catalyticsystems, in which some centres give rise to an essentiallystereospecific propagation and others to aspecificpropagation.

Another interesting aspect relating to these catalyticsystems is the correlation that exists, in the case ofpolymerization of propylene, between the symmetry ofthe transition metal compound and the stereochemistryof the polymer obtained. This correlation, which canlead to isotactic, syndiotactic, hemiisotactic and atacticpolypropylene, is described in Table 5 and Fig. 9.

In 1991, in industrial (Stevens et al., 1991) and scientific literature, there appeared the CGCs (Constrained Geometry Catalysts), whose parent is the complex {h1:h5�[(tert-butylamide)dimethylsilyl] (2,3,4,5-tetramethyl-1-cyclopentadienyl )}TiCl2:


POLYOLEFINS

CH3

CH3CH3

ClCl �115°

Ti

Si

N

H3C

H3CH3C

H3C

H3C H3C

These compounds are often erroneously associatedwith MBCs and are characterized in the instancereferred to above by a constrained geometry, as regardsthe angle N�Ti�Cp. The cocatalysts, typical of MBCs,homo/copolymerize ethylene with high yields andpropylene in an aspecific way (they produce anamorphous PP with high MW). They have foundindustrial applications in the production of thecopolymers C2/C8, C2/styrene and EPDM containingENB (5-ethylidene-2-norbornene) as a third monomer(termonomer).

Monometallic catalysts

Relatively stable organometallic compounds of Ti,Zr, V and Cr have been used in the polymerization ofethylene with very low yields. Of definite scientificinterest was the appearance of benzyl derivatives of Tiand Zr active in the polymerization of ethylene andabove all of prochiral monomers such as propylene and4-methyl-1-pentene (Giannini et al., 1970). As regardspolymerization of ethylene compounds, having a generalformula (halogen)xMeBzy (where Me�Ti, Zr and

Bz�CH2-C6H5), they display a growing polymerizationactivity with the following sequence:TiBz3Cl�TiBz3Br�TiBz3I�TiBz4. In the case ofpolymerization of propylene and 4-methyl-1-pentenepartially isotactic polymers are obtained, this result is ofimportant scientific interest in that it represents the firstexample of stereospecific polymerization in ahomogeneous phase.

12.2.3 Properties, structure and applications of polyolefins

As previously stated, in the field of polymeric materialsPOs constitute a very versatile group as regards theirproperties and consequently also their applications(Ciardelli et. al. 1983; Ciardelli et al. 1986). In fact,starting from a limited number of monomers (propylene,ethylene, 1-butene or higher a-olefins such as 1-esene,1-octene) it is possible to construct an enormous numberof molecular structures, which give rise to materials withvery different properties. Through control of thepolymerization process, the polyolefin chain can beconstructed so as to contain a suitable quantity ofcomonomers which give rise to ramifications of differentlength and consequently to a number and typology ofsuitable defects. The possibility of having segments,which can or cannot be crystallized, will depend on thenature, the quantity and the distribution of these defectsalong the macromolecular chain. The structure of the POmacromolecules can be described through: MW; MWD;the presence of defects dictated by regioregularity orstereoregularity; CCD due to the presence, in quality andquantity, of different comonomers along the chain.

It is also noted that semicrystalline polymers, such asPOs, are arranged in the solid state according to a well-defined hierarchical structure. The macromolecularsegments, if their structure permits it (for example in thecase of polyethylenic sequences or isotactic orsyndiotactic segments), are arranged in a crystallinelattice. A ‘superstructure’ is thus obtained. The regularityof the chain is a necessary condition, although notsufficient in itself, to have crystallinity. The type and


POLYMERIC MATERIALS

Table 5. Correlation between the symmetry of the metallocene and the type of polymer obtainable

Symmetry of the metallocene Example of metallocene Polymer obtained

C2v Cp2MtCl2, Cp2*MtCl2 atactic

C2 Rac-EBI MtCl2 isotactic

Cs Me2C(Cp) (Flu)MtCl2 syndiotactic

Ci Me2C(MeCp) (Flu)MtCl2 hemiisotactic

Cp�cyclopentadienyl; Cp*�pentamethylcyclopentadienyl; rac-EBI�racemo-ethylenebisindenyl; Flu�fluorenile; MeCp�3-methyl-cyclopentadienyl; Mt�Ti, Zr, Hf

(CH3)C2

(CH3)2Si

ZrCl2

ZrCl2

ZrCl2isotactic atactic

syndiotactic

Fig. 9. Correlation between the structure of the metalloceneand the type of polymer obtainable.

degree of crystallinity does not depend on themacromolecular structure alone, but also on thecondition of the transformation process and, inparticular, on the thermodynamic history of the material,from granule to pellet to final product. These processesinvolve phases of fusion and solidification, whichgenerally take place in conditions, which are neitherfavourable nor thermodynamically balanced. Hence theproperties of POs are determined by their structure, theirsuperstructure (morphology) and finally by theconditions of transformation. The MWD is veryimportant, especially for processibility. In Z/N typeanionic co-ordinated catalysis, the termination of thegrowing chain of the various macromolecules can comeabout through different mechanisms, such as a transferreaction with the MW regulator (for example H2), or elsea transfer reaction with the monomer or the deactivationof the catalytic site. As a consequence, each polymer isgenerally composed of macromolecules with differingdegrees of polymerization, and therefore of lengths,varying according to the mechanism, the polymerizationprocess and the heterogeneity of the catalytic sites. Theresultant MWD can be determined through GPC andnumerically, as the ratio between the average ponderalmolecular weight and the average numeral molecularweight (Mw/Mn). For an ideal monodispersed polymer,Mw/Mn is equal to 1. The technological objectiveconsists of manipulating all the above-mentionedparameters in such a way as to obtain the desiredproperties. This objective can be achieved by working onthe type of polymerization process and on the operatingconditions as well as on the catalytic system in terms, forexample, of the molar ratios between the variouscomponents (Al/Ti/donor).

Polyethylene

PE is a thermoplastic polymer produced in greaterquantities all over the world. There are various types ofPE, with different structures, determined by theproduction process used or the presence of comonomersin the polymerization in order to obtain co/terpolymers.The chemical structure and the distribution of thelengths of the chains affect the properties both of themolten polymer (rheology) and of the polymer in a solidstate as well as those of the end product.

Thanks to a higher packaging and greater order ofthe macromolecules, the crystalline areas of PE have ahigher density than the amorphous areas. In PE, as thecrystallinity increases, and hence the density, so does therigidity. It is common practice to use density as aparameter for specifying the different types of PE: a) HDPE (High Density Polyethylene), characterized bylinear macromolecules with very few short branches; b) LDPE (Low Density PolyEthylene), consisting of

macromolecules with a high number of irregularlydistributed branches of varying lengths which hinder acomplete crystallization; c) LLDPE (Linear LowDensity PolyEthylene), consisting of homogeneouslymodified linear macromolecules with short lateralchains (obtained by copolymerization with a-olefins C4-C12) which lower, albeit in a controllable manner, thecrystalline packing; d ) VLDPE (Very Low DensityPolyEthylene), consisting of macromolecules containinga high number of lateral chains. UHMWPE (Ultra HighMolecular Weight PolyEthylene), a polymer with amolecular weight between 3 and 10 million, has alsoseen significant commercial development.

Along with density, the Melt Flow Index (MFI) isalso commonly used to characterize the various types ofPE. Originally, the extent of this measurement was usedto establish the processibility of the polymer assessing itsbehaviour in the melt state. Measurement of the MFI, iscarried out by applying a standard load on a piston andmeasuring the amount of the polymer extruded, relativeto time (g/10 mins) through a given nozzle.

In addition to the traditional test based on weight(MFR, Mass Flow Rate), there is also a test basedon volume (MVR, Melt Volume Rate). The mostmodern equipment can provide additionalinformation such as the density of the polymer inthe melt state, its thermal stability and its shearsensitivity (Guaita et al., 1998). The melt density isdetermined by measuring the ratio of the MFR tothe MVR and represents an important piece of datasince, unlike the density of the solid, it is notaffected by crystallinity. Its reciprocal, the specificvolume, is the physical measurement used to predictthe shrinkage of the polymer during processing, forexample during injection moulding. Thermalstability, defined as the variation of MFI (in weightor volume) over a unit of time, is a parameter whichreveals the variations which take place in thepolymer in the melt state. Shear sensitivity is theratio between the MFI values obtained under twodifferent loads, which usually differ by a factor of10. This value can be easily correlated to the MWD.

Areas of application of polyethyleneDepending on the properties detailed in Table 6, the

various types of PE are used in the different applicationslisted in Table 7. Fig. 10 shows a diagram of theapplications of the different polyethylenes as a functionof their density.

Film. PE film is the most widely used material forpackaging due to its transparency and flexibility allied toits strength and the ease with which it assumes the shapeof the object to be protected. The films are produced bya process called film blowing and through extrusion andcollection on cooled cylinders (cast film). An LDPE with


POLYOLEFINS

a density of 0.92 g/cm3 and an MFI of 2 g/10 min isgenerally used for blown film, and the diameter of theblown bubble can reach 2 m, while for cast film apolymer with a density of 0.93 g/cm3 is used. In addition

to packaging, these films are also used forwaterproofing and the production of strong bags. Inorder to optimize the properties of the film, anLDPE/LLDPE blend is often used. With HDPE, the film


POLYMERIC MATERIALS

Table 6. Properties of some representative polyethylenes

Property LDPE HDPE LLDPE Method

MFI (g/10 min) 1.1 1.1 0.85 190°C/2.16 kg

MFI (g/1 min) 57.9 50.3 24.8 190°C/21.6 kg

Density (g/cm3) 0.924 0.961 0.922 ASTM D1505

Crystallinity (%) 40 67 40 DSC

Melting point (°C) 110 131 122 DSC

Vicat softening point (°C)

93 127 101 ASTM D1525

Number of CH3/1,000 C atoms 23 1.2 26 IR

Comonomer 1-butene 1-butene

Mw 200,000 136,300 158,100 GPC

Mn 44,200 18,400 35,800 GPC

Tensile modulus (MPa) 12.4 26.5 10.3 ASTM D638

Elongation at break (%) 653 906 811

Table 7. Percentage of use of polyethylene based on its world-wide applications

Market HDPE (%) LDPE and LLDPE (%)

Film 28 73

Blow moulded containers 35 1

Injection moulded containers and other items 22 5

Extrusion coating containers 10

Pipes 10 1

Cable insulation 1 5

Other 4 5

1,000

100

10

0.1

0.01 0.89 0.9 0.91 0.92 0.93

density (g/ml)

MF

R (

dg/m

in)

0.94 0.95 0.96

1 VLDPE LLDPE

films andsheets

MDPEfilms and

sheets

HDPEinjectionmoulding

LLDPEinjectionmoulding

rotomoulding

HDPEblow

mouldingHDPE

pipes andfilms

Fig. 10. Applications of the variouspolyethylenes according to theirdensities.

obtained has poorer optical qualities, making itsfavoured uses those where the primary requirement ismechanical strength.

Blow moulded containers. HDPE is the preferredmaterial for producing containers for liquids through theprocess of blow moulding, since it has good anti-ageingcharacteristics and rigidity. The industrial applicationsare aimed at the production of containers for bleach,detergents, milk and petroleum products. Generally, anHDPE with an MFI of 0.2 g/10 min, a density of 0.95g/cm3 and if possible, a wide MWD obtainable througha two stage process of polymerization is used.

Containers and other objects from injectionmoulding. This is a technology used for the productionof caps and other closures in the field of packaging, ortoys and industrial containers. All types of polyethylenesmay be used, depending on the flexibility required.LLDPE with a narrow MWD, which is capable ofproviding the right balance of the requiredcharacteristics, is particularly suitable.

Extrusion coating containers. LDPE is widely usedin conjunction with cardboard, paper or aluminium toproduce containers for liquid food products; in this case,the requirement is for a polymer with a very lowimpurity content food grade polymers. The extrusionprocess is carried out at about 300°C and polymers witha strong tendency to swell are particularly suitable (highdie swell); a polymer typically used for theseapplications is an LDPE with an MFI of 4 g/10 min anda density of 0.92 g/cm3.

Pipes. This is the main use of PE as an engineeringmaterial. It is used to produce pipes for water and naturalgas and must guarantee resistance to ageing for at least50 years. Generally a PE of medium density or an HDPEwith a wide MWD is used.

Cable insulation. Because of its dielectric properties,PE has had an important use for many years in thecovering of cables for the distribution of electricalcurrent and for telecommunications. An LDPE is used,whose performance is enhanced by a process ofreticulation using two different technologies: treatmentwith peroxides (the most widely used is dicumylperoxide) during the extrusion or grafting phase, in thepresence of peroxides, of alkenyl alkoxysilanes and asubsequent condensation reaction in the presence ofcatalysts such as dibutyltin dilaurate, so as to formsiloxane type bonds between the polymer chains.

UHMWPE based items. The processibility of thismaterial is particularly complex because of the very highviscosity of the melt; in fact, traditional technologieswould result in a degradation of the material. The mostcommonly used method is compression moulding. Thismaterial finds a use where there is a need for itsexceptional anti-scratch properties, its low frictioncoefficient and its chemical inertness. It has a crystalline

melt point of 130°C and a temperature of continuoususage not exceeding 90°C.

Polypropylene

The distinctive characteristics of PP are its lowdensity, high chemical resistance, its toughness and itsability to be orientated (Moore, 1996). The applicationsto which PP has been found to be particularly suited arein the field of films and fibres and to this end differenttechnologies have been developed which make itpossible ‘to stretch’ the polymer, significantly enhancingits properties.

FilmThe orientation of PP is obtained by heating the item

to a temperature at which the crystals are partiallymelted, 120-160°C, stretching it to the desired shape andthen cooling it during the stretching to reform thecrystals in such a way as to retain their orientation. Themacromolecular chains are forced to align themselvesduring this processing and can very easily form crystals:in fact, the most macroscopic effect is a notable increasein crystallinity. Alongside this phenomenon caused byorientation, there is also an increase in toughness and inthe flexural modulus, which grow in proportion to theamount of stretching. The increase in toughness andrigidity is determined by the high number of chainsaligned in the direction of the stretching, so that asmaller section is able to bear a heavier load. Theorientation also has an enhancing effect on the lowtemperature impact resistance, while it causes areduction of the elongation at break. If it is reheated to atemperature close to its melt point, the item shrinks andhas the tendency to assume a form similar to that of anon-orientated piece. Very important from theapplication point of view are PP bi-orientated films(BOPP). The properties of a BOPP are shown in Table 8,in comparison with those of other films obtained fromother polymers.

In addition to having excellent qualities of toughnessand rigidity, BOPP has other distinctive characteristicssuch as, for example, its effect as a barrier to humidity,excellent transparency (clarity), high dielectric rigidity.The barrier effect against humidity derives from theincrease in crystallinity: in order to penetrate, the watermolecules are faced with a more tortuous path, aphenomenon which comes about only through theamorphous phase. Nevertheless, BOPP film ispermeable to oxygen; to overcome this inconvenienceand make it suitable for application in the field of foodpackaging it is combined with a film of PVC or with ametalized film. The morphology of PP during biaxialorientation varies from spherulitic, with numerouscrystalline/amorphous interfaces positioned in all


POLYOLEFINS

directions, to planar, characterized by a reduced numberof interfaces in the plane of the film with dimensions ofless than a micron, which significantly diminishes thepotential for light to be refracted as it passes through thefilm. Thus the transparency is assisted by orientation.The normal surface roughness of a non-orientated filmassociated with its spherulitic morphology causes lowgloss due to the diffusion of the reflected light; in thecase of BOPP on the other hand, it is reduced by theorientation to a vertical dimension of virtually zero andso the film exhibits a high gloss. Its barrier effect,transparency and gloss make BOPP particularly suited tofood packaging. In a similar way to the barrier effectagainst humidity, the biaxial orientation increasesresistance to the passage of electrical current. Thedielectric rigidity of BOPP film is about three timeshigher than that of a non-orientated film. This propertypermits the use of thinner insulating layers andestablished the use of BOPP in condensers and cablingapplications. The latter technology, which takes the namePPL (Paper Polypropylene Laminated), brings about theinsulation of the high voltage conductor by combiningPP film and special types of paper. Industrial productionof BOPP film began at the start of the Sixties and was aparticipant in the significant growth resulting from theability to replace cellophane in many applications. Asevidence of its spread, in 2002 about 800,000 t of BOPPwere produced in Europe.

FibresAnother important application of the PP

homopolymer, consists of its use in obtaining orientatedfibres which are used, for example, in the production ofthe common raffia. The processing method consists ofobtaining a sheet of homopolymer through an extrusionprocess and then passing it through a series of blades inorder to extract the ribbons, which are then stretched.Another method of obtaining oriented fibre is to extrudemolten strands and simultaneously cool them with high-speed air. The strip of fibre thus obtained iscollected on a mobile support and hot-welded into annonwoven fabric (spunbonded). Homopolymers with

medium-high fluidity and a narrow MWD are used forthese applications. In fact, in conditions of highdeformation, which arise during the transformationprocess, a narrow MWD makes it possible to easilymaintain the induced orientation. In another type ofprocess, the fibres are ‘melt-blown’, orientated throughthe effect of the air which comes out of the nozzle alongwith the polymer. The melt-blown process enables verythin fibres to be obtained, which are not heat-welded.Some of the properties of orientated PP fibre are shownin Table 9 and compared with nylon and polyester fibres.

The fibres thus obtained are used in the productionof raffia, which in turn is used to produce ropes. Thedevelopment of this technology, which took place at thebeginning of 1960s, was facilitated by the fact that it waspossible to adapt the equipment typically used in themanufacture of ropes without having to carry out anyspecial modifications. The products obtained can beused to produce fabric for bags, waxed fabrics, andgeo-membranes for the construction of embankmentsand for civil engineering applications. PP also hasnumerous applications in the carpet sector, both for theso-called primary or base part and for the secondary part.Its low melting point and the impossibility of dyeing it,are the principal factors which exclude PP from thetraditional applications in the field of the top band oftextiles, suitable for use in clothing. The commercial


POLYMERIC MATERIALS

Table 8. Comparison of the properties of film obtained from PP and BOPP and other polymeric materials

Property LDPE HDPE BOPP PP

Traction resistance (MPa) 17-24 34-69 40-60 140-240

Flexural modulus (MPa) 140-210 550-1,250 690-960 1,720-3,100

Elongation at break (%) 300-600 400-800 50-130

Tear resistance (N/mm) 80-160 16-160 1.5-2

Opacity (%) 5-8 high 1-4 1-4

Transport velocity O2ASTM D1434 450 150 240 160

Table 9. Comparison of the properties of orientated PPfibres and fibres obtained from other polymeric materials

Property PP Nylon Polyester

Melt temperature (°C) 165 260 240

Density (g/cm3) 0.9 1.14 1.4

Toughness (MPa) 567 648 526

Elongation at break (%) 21-28 18-28 9-11

Modulus (MPa) 2,187 3,078 6,804

Shrinkage at 100°C (%) 5.5 9.7 8.4

Absorption of humidity (%) 0.03 4.5 0.4

success of PP fibres, begun around the mid-1960s, hasmade necessary the development of various ever moresophisticated production technologies. Currently themarket for films and orientated fibres accounts for abouthalf the volume of PP produced worldwide since, due totheir wide range of properties and their inexpensiveness,they are used almost exclusively in many applications.

Specialized applicationsNonwoven-fabrics As already pointed out, another

important application for PP is in the field of nonwovenfabrics, made possible by the availability of a polymerwith high fluidity and a narrow MWD. There are threetypes of nonwoven fabrics, characterized by differentproperties and different appearances, obtained throughthe following production methods: spunbonded, cardedand thermobonded webs from staple fibers and melt-blown. The first are very tough, while the secondare voluminous and soft. Melt-blown technology uses PPwith very high fluidity and produces extremely finefibres (2-4 microns diameter), suitable for fabrics withhigh absorbing power and selective filtering capability.As a consequence, the sector with the highestconsumption of nonwoven fabrics today is that ofdisposable diapers, of which they form the outercovering. The non-hygroscopic nature of PP makes itparticularly suited to such applications, preventingprolonged contact of the fluid with the skin. The highcost of laundering and sterilizing hospital garments hasopened-up an important sector for the use of nonwovenPP in the field of single-use garments: the fabric islight, non-allergenic and resistant to chemical agentsand moulds; moreover, it can be disposed of throughvery simple procedures such as incineration. Fabricsobtained through melt-blown have the capability ofselectively absorbing oil and mixtures of oil/water, aproperty widely used for the purification of reservoirsand water courses.

Automotive components. Another application inwhich PP has found an important use since the Eightiesis in the automotive sector. The profile of its physico-mechanical properties, such as for example the balanceof rigidity/impact resistance, combined with itsaesthetic properties, such as its receptiveness topainting, has allowed PP to compete with other polymermaterials, such as technopolymers or other engineeringmaterials. Today on a world level, PP-based materialsaccount for 90% of the market for car bumpers. Thedesign of bumpers has undergone rapid evolution inrecent years, to the extent that, over and above theirmechanical function, they now have an aesthetic value,often being an integral part of the bodywork, beingpainted and housing the lighting equipment. Aninteresting aspect is linked to the fact that the continuedimprovement in the mechanical properties of the

materials for bumpers has led to a progressive reductionin the thickness of the items and therefore a saving ofmaterial, giving a significant reduction in weight and aconsequent reduction in fuel consumption. Even theoptimization of the rheologic characteristics has alloweda simplification of the moulding process through areduction of the cycle time. There have been similardevelopments in various external and internal carcomponents, such as wiring and hot melt adhesives forthe back of carpets and imitation leathers. It should beemphasized that this massive use of PP in cars makes itpossible to propose a ‘single material’ solution thatwould make recycling much easier.

To give a quantitative idea of the trend in the use ofPP based materials in cars, it is sufficient to rememberthat it rose from an average of 22 kg per car in 1990 toover 40 kg in 1995, with the figure reaching about 50kg in 2004.

Rigid packaging product. Rigid packaging is a termused to describe articles such as containers for margarineand yoghurt, jars, bottles and boxes of various types. Thecontainers come in various shapes and sizes and areproduced through three principal processes: injectionmoulding, blow moulding and thermoforming. Asregards the latter process, orientation of the PP makesthe item more transparent, rigid and impact-resistant.The fields of application are sealed containers for ovenor microwave ready convenience foods and for freezers;in addition to which the favourable cost/performancebalance has led to the adoption of PP for blown bottlesas well. PP is also the preferred resin for closures such ascaps and lids. The high rigidity and fluidity of thematerial that can be obtained with the new productionprocesses, make it possible to mould containers withvery thin walls, in compliance with the generalguidelines from environmental protection agencies toreduce the volume of packaging.

Propylene-based heterophasic copolymers

The major weak point of the PP homopolymer is itsfragility, especially at low temperature. To get aroundthis problem, already by the Seventies thought wasbeing given to the possibility of improving its impactresistance through the addition of an elastomeric phase(Moore, 1996). These materials, which go under thename of heterophasic copolymers, in earlier times wereproduced through the mechanical mixing of PP andEPR; more recently though, they have been obtainedthrough direct synthesis in the polymerization reactorvessel, by following the homopolymer production stepwith a copolymerization step, generally in the presenceof ethylene. The principal effect obtained through theintroduction of an elastomeric component, characterizedby a very low flexural modulus, consists in an increase


POLYOLEFINS

in impact resistance and a reduction of the rigidity andhardness of the homopolymer. Heterophasic copolymersdisplay a degree of structural complexity due to thepresence of the elastomeric phase, which plays afundamental role in determining the product’s finalproperties. The rubber phase can vary in MW andMWD, volume, dimension and composition andmoreover, it can present a certain percentage ofstereoregularity (Jang et al., 1984, 1985). Depending onthe temperature and the speed of loading, the rubberyparticles dispersed in the semi-crystalline matrix assistthe process of cavitation (crazing) or of shear (shearyielding) which determine the level of deformationbefore a fracture occurs. Low temperatures and highloading speeds favour the formation of crazing, whilehigh temperatures and low speed favour shear yielding.The combination of the two mechanisms makes itpossible the absorption of the applied energy, withoutreaching a local level of force such as to cause abreakage. These mechanisms operate above the glasstransition temperature (Tg) of the dispersed phase and,as previously noted, their efficiency is linked to thenature of the dispersed rubber phase, to its proportion tothe volume of the system, and to the dimensions and thedistribution of the particles in the matrix. The nature ofthe rubber determines the behaviour on deformation ofthe dispersed particles and their ability to absorb andredistribute the energy, while the concentration and thedimensions of the particles (therefore their number)define the probability of intersecting the line of fractureand of activating the mechanism which counteracts itspropagation (Riccò, 1988). The MW of the rubberphase is crucial for controlling the dimensions of theparticles in the matrix. A suitable ratio between theviscosities of the phases is essential for transferring thecrazing force across the continuous phase anddispersing the rubber phase, during the mixingprocesses. To summarize, in order to achieve effectiveimpact resistance, the specific characteristics of therubber phase are Tg, its capacity to adhere to the PPmatrix and its ability to crystallize, which governs itsshrinkage during cooling. As regards the composition ofthe elastomeric phase, a high propylene content willincrease its Tg limiting the improvement of its impactresistance, while as the ethylene content increases the Tg

diminishes, favouring an increase in its impactresistance, with the risk, however, that the polyethylenicsequences might crystallize. In this case, the rubberparticles display a greater shrinkage during coolingcompared with what would happen if it were completelyamorphous. When a heterophasic copolymer is cooled,the crystallization of the polypropylene matrix causes areduction of volume notably greater to that of anamorphous rubber phase, creating a compression forceon the particles of the dispersed phase, together with

tensions in the matrix. Hence, even relatively smalldeformations can easily cause a force in the systemsufficient to form crazing in the matrix. If rubber isused which is rich in ethylene to the extent that is hasthe crystallinity of PE, the contraction in volume of therubber particles during cooling increases and tends tomatch that of the matrix. This reduces the level of thetensions present, retarding the formation of crazing. PEhas a Tg sufficiently low that it does not compromise itsimpact resistance. The presence of a crystallizablefraction in PE has, however, the positive effect ofeliminating the whitening of the heterophasiccopolymers and can be obtained by blending withHDPE or, in this case also, directly in the productionreactor vessel through a post-polymerization withethylene. The adhesion and impact resistance reach amaximum with percentages of ethylene in the rubberphase of between 50 and 60%. Hybrid processes areparticularly suited to the obtaining of high-qualityheterophase copolymers (Spheripol process, see below).Multiphase systems with the PP matrix and a dispersedrubber phase are generally less transparent than ahomopolymer and this is due to the diffusion of light atthe PP/EPR interface, which is added to the diffusioncaused by the spherulitic structure. An increase inopacity (haziness) is linked to the increase in the rubbercontent, to the dimensions of the same and to thedifference in refraction indices of the system’scomponent phases. To reduce the opacity in the PP/EPRmixture, the dimensions of the rubber particles must bereduced through an appropriate choice of the intrinsicviscosities of the phases and through the minimizationof the differences in the phases’ refraction indices; inthe case of POs, that means reducing the difference inthe densities of the phases themselves. In fact, there is alinear correlation between the densities and therefraction indices. Nevertheless, the dimensions of therubber particles must not diminish below a minimumvalue so that the impact resistance, typical of thesematerials, is not compromised. Hence, in summary, agood balance between transparency and impactresistance is the result of a compromise which alsotakes account of the rheologic properties and thereforeof the processibility of the material, of itsphysico-mechanical properties and of the aestheticproperties of the finished item. Again, with regard totransparency, the development of innovativepolymerization processes, capable of producingmultiphase systems in increasingly complex reactorvessels and characterized by an ever- increasing range ofproperties, has enabled the production of materialswhich are super-rigid, super-soft or else rigid andresistant to impacts while still possessing excellentoptical properties. The Catalloy process (see below) isparticularly suited to obtaining these materials.


POLYMERIC MATERIALS

Ethylene/propylene copolymers

In industrial practice, the properties of PE and PP areusually modified through copolymerization with smallquantities of comonomers, giving rise in the case of PE,for example, to LLDPE or in the case of PP to randomcopolymers. Both cases can be defined as modified PEor PP. On the other hand, essentially amorphouscopolymers C2/C3 represent an important class ofsynthetic elastomers obtainable in a wide range ofcompositions, possibly in the presence of a thirdmonomer, generally a non-conjugated diene, that iscapable of introducing a free double bond in aramification of the polymer, and is usable forvulcanization with sulphur, without introducingunsaturation in the principal chain of themacromolecule. These elastomers are called EPM(Ethylene Propylene Methylene) and EPDM (EthylenePropylene Diene Methylene), and more generically, bothbelong to the EPR (Ethylene Propylene Rubber) family.The termonomers most often used in the development ofthese polymers are 1,4-hexadiene (HD), 5-ethylidene-2-norbornene (ENB) anddicyclopentadiene (DCPD); currently ENB is usedalmost exclusively in industrial practice. The EPRs weresynthesized for the first time by Natta (Natta et al.,1955) using active catalytic systems in thepolymerization of propylene. Currently, the catalyticsystems commonly used in industrial practice are ofthree types: those based on vanadium compounds,titanium compounds supported on MgCl2 andmetallocene type catalysts. As far as the catalyticsystems based on vanadium are concerned, thecompounds most often used are VOCl3 and vanadiumtriacetylacetonate that are used in combination with ahalogenated aluminium alkyl and in the presence of anactivator, generally a chlorinated organic ester, such as,for example, ethyl trichloroacetate and n-butyl-perchloro-crotonate or benzotrichloride. Theactive species in the polymerization is V(III) whichdecomposes rapidly into V(II), inactive in thepolymerization. The role of the chlorinated ester is toreactivate the catalyst by reoxidising the V(II) to V(III).The molar ratio Al/V is between 10 and 30, while that ofactivator/V is between 5 and 10. The catalytic systemsused in the titanium-based synthesis of EPDM are thosedescribed for PE. They give rise to a polymer with ahigher degree of crystallinity from PE and are notwidely used. The regulation of the MW is carried outusing Zn(C2H5)2. Examples of catalysts active in thepolymerizatioon of EPDMs belonging to the MBCfamily are bis-indenyl-zirconium dichloride andrac-ethylenebis-tetrahydroindenyl-zirconium dichlorideused in combination with MAO. The polymerizationprocesses currently used in industrial practice are of

three types: in solution, in slurry (in a suspension ofliquid propylene) and in gas phase. In particular: a) a process in solution for EPDM containing HD astermonomer using vanadium based Z/N catalystsmarketed by DDE (DuPont-Dow Elastomers); b) a process in solution for EPDM containing ENB astermonomer using MBC (DDE); c) a process in solutionfor EPDM containing ENB as termonomer usingvanadium based Z/N catalysts (Bayer); d ) a process ingas phase for EPDM containing ENB as termonomerusing heterogeneous Z/N catalysts prepolymerised foruse in a fluid bed reactor in gas phase (UCC); e) aprocess in suspension of a liquid monomer for EPDMcontaining ENB as termonomer using vanadium basedZ/N catalysts (Bayer). The installed capacity of EPDMworldwide is of about 1.3 million t/yr with anEPM/EPDM ratio of about 15/85.

EPDMs find application in waterproofing, in theautomobile industry and in the manufacture of cables forthe transmission of electrical energy. For this last use, thecopolymers produced in plants working in solution areused out of preference as these processes provide an easierstage of removal of the catalytic residues, which are verydamaging to the final dielectric properties of the polymer.One of the most important characteristics is the resistanceto ozone and oxygen, due to the lack of insaturation in theprincipal chain. In industrial practice, EPDMs are mixedwith numerous fillers such as kaolin, silica carbon, blacksteam and talcum. They can be vulcanized with sulphur orwith peroxides; in general vulcanisation with peroxidesproduces compounds with a superior compression set.Resistance to aliphatic, aromatic and chlorinated solventsis low, while resistance to the polar solvents such asketons and alcohols is excellent.

Poly(1-butene)

Poly(1-butene) (PB) is a semi-crystalline polymerwith characteristics similar to PE and PP. In particular itshows interesting properties of flexibility, rigidity,Environmental Stress Cracking (ESC) and resistance toabrasion. It is produced using stereospecific catalysts forPP that make it possible to obtain a polymer that isessentially isotactic with a crystallinity of about 50%.The isotactic fraction is determined as a percentage ofthe polymer insoluble in xylene at 25°C. One of thepeculiarities of PB is its polymorphism; in fact, likemany isotactic polymers, it crystalliszes in variousforms, more specifically orthorhombic, hexagonal(twinned and untwinned) and tetragonal. Theinterconnection between the various forms comes aboutaccording to the schedule found in Fig. 11.

Shapes I´ and III are obtained by crystallization fromsolution and do not have any industrial relevance, whileshapes II and I, that are obtained by crystallization from


POLYOLEFINS

melt, have found practical applications. The PBhomopolymer has a density of about 0.91 g/cm3, a Vicatsoftening point of 110°C and a melt point of between125 and 140°C.

The most important producer of PB in the world,both as a homopolymer and a copolymer with ethylene(maximum quantity of comonomer 8% by weight), isBasell Polyolefins, that established a facility of 45,000t/yr in May 2004. It is a process in solution at hightemperature that uses Ziegler-Natta stereospecificcatalysts in order to produce a polymer with highisotacticity (up to 99% of the polymer fractioninsoluble in xylene). Applications of PB areprincipally concerned with pipework for hot water andhot fluids generally, film for special applications suchas packaging for foods when high cooking temperatureis required, and polymeric alloys, especially togetherwith PP with which it is particularly compatible. Othercharacteristics, such as chemical resistance,permeability and mechanical properties in general, arevery similar to those of PE and PP.

Poly-4-methyl-1-pentene

Poly-4-methyl-1-pentene (PMP) is produced withZ/N catalysts preferably HY and highly stereospecific toavoid the phase of removal of the catalytic residues andof the atactic polymer from the final polymer. The firstproducer, in 1965, was ICI, who marketed the polymerunder the name of TPX. It is now produced with aprocess in suspension; the rate of polymerization of 4-methyl-1-pentene is much lower than that of propylenebecause of the effects of steric hinderance owing to thepresence of a voluminous group in position 3 relative tothe double bond. The most important producer is MitsuiPetrochemical Industries, which however has a capacitybelow 10,000 t/yr. The principal advantages that PMP hasover the other POs consist of its optical properties and itsresistance to high temperatures for short times. Eventhough it is moderately crystalline, the transparency ofthe material is good because the amorphous phase andthe crystalline phase have the same refraction index.Other significant properties of the material are itsresistance to polar solvents and aqueous solutions ofmineral acids. Because its behaviour is similar to that ofother POs while its cost is certainly higher, PMP is usedonly when the qualities with which it is endowed have to

be maintained during variations in temperature. It findsapplications in moulded containers for foodstuffs thathave to be subjected to successive processes of freezingand heating as well as in stoppers for bottles, syringes,laboratory equipment and lamps for the lighting industry.It also shows excellent properties of electrical insulation,even similar to those of fluorinated polymers.

12.2.4 Production processes

Before examining the principal production processes it isworthwhile to return to Fig. 2, where the principalworldwide producers of PE and PP are listed with theirrespective market shares. As far as PP is concerned, theundoubted leader is Basell Polyolefins, which also playan important role in the production of PE. The processesdeveloped by this firm in particular, or by the firms thatgenerated it, will therefore be examined: Montedisonand Hercules that gave rise to Himony in 1982; Shelland Himont that created Montell in 1993; Montell,BASF and Hoechst that finally generated Basell in 1999.

Raw materials

As far as raw materials are concerned, the modernplants for production of ethylene and propylene supplypolymerization grade monomers. In the Z/N catalysts themain poison of the catalytic system is carbon monoxide(CO), so much so that some scientists use it to determinethe number of its active centres (Corradini et al., 1992).In fact, CO gives rise to a preferential insertion withrespect to a monomer on the metal carbon bond of thegrowing polymer chain, blocking the polymerization.Through acid hydrolysis of the intermediateTi�CO�CH2-polymer the formation takes place,through the breaking of the Ti�CO bond, of a polymericchain with a terminal aldehyde group that can be studiedwith common spectroscopic techniques to arrive at thenumber of active centres present in the catalyst.

The cocatalyst used, aluminium alkyl, as well asalkylating the transition metal and giving rise to the startof the polymerization, being employed in large excesscompared with the metal (molar ratio Al/Ti �20), alsoacts as a scavenger of the polymerisation diluent, since itdisplays high reactivity in comparison with anyimpurities present (essentially polar compounds).


POLYMERIC MATERIALS

orthorhombic(form III)

hexagonal untwinned(form I')

tetragonal(form II)

hexagonal twinned(form I)

93°C, 4 h 104°C, 5 h 25°C, 72 h

Fig. 11. Schedule of interconnections between the various forms of poly(1-butene).

H2 is used as a regulator of the MW in the Z/Ncatalyst; however it becomes a poison in the Phillips typeof catalysts. An interesting phenomenon that is observedin the polymerization of propylene and not of ethylene isa notable activating effect, relative to the speed ofpolymerization, exercised by the H2 (Guastalla andGiannini, 1983). The explanation for this phenomenonlies in the capacity of H2 to re-activate some ‘dormant’catalytic sites generated by a 2,1 insertion of propylenethrough the reaction:

Ti�CH(CH3)�CH2-polymer (dormant site)�H2��

�� Ti�H (active site) � polymer-CH2�CH2�CH3

The catalytic site Ti�H is, in fact, very active as faras the polymerization reaction is concerned.

Production processes for polyethylene

Modern production processes are capable ofproviding a wide range of products. Thanks to thisversatility, high pressure processes for example, can besuitably modified to produce besides LDPE, LLDPEalso, just as low pressure HDPE processes can produceboth LLDPE and VLDPE.

High pressure processesThe polymerisation reactor used can be of two types:

autoclave and tubular. The working pressure is between150 and 200 MPa for the autoclave processes andbetween 200 and 350 MPa for a tubular reactor vessel.An example of these two reactors is found in Fig. 12.

Autoclave type reactor vessels. The autoclavefunctions like an adiabatic reactor vessel of theContinuous Stirred-Tank Reactor (CSTR) type, in whichthe removal of the reaction heat comes about by meansof the fresh monomer being fed in. The conversion of the

monomer is governed by the difference between thetemperature of the monomer being fed in, and the finaltemperature of the reaction. The most up-to-date reactorvessels have two zones kept at different temperatures,the first at 180°C and the other at 290°C, and the controlof the reaction temperature is affected by regulating thequantity of the radical initiator being fed in.

Tubular reactor vessels. A tubular reactor vesselconsists of some hundreds of metres of jacketed tubeswith an internal diameter of between 25 and 75 mm anda thickness capable of guaranteeing operation at highpressure, and calculated so as to maintain a ratiobetween the external diameter and the internal of about2.5. The temperature at which the polymerization startsdepends on the type of radical initiator used and variesfrom 190°C, when oxygen is used, to 140°C, whenperoxydicarbonate is used. The polymerizationtemperature is kept constant both by regulating thefeeding of the initiator, and by removing excess heat bymeans of the cooling liquid running through the jacket.Using this production process higher conversions areobtained compared with the autoclave process, which isoffset, though, by the higher energy cost due to the stageof compression of the monomer.

A conversion of 35% can be estimated in the case ofa tubular reactor vessel and 20% for the autoclaveprocess. Both the high pressure processes described canbe used for the copolymerization of ethylene with polarmonomers such as vinyl acetate and the esters of acrylicacid. They are also suitable, after appropriatemodifications, for using Z/N type catalysts for theproduction of HDPE and LLDPE.

Low pressure processesProcesses in slurry. Polymerization takes place in a

hydrocarbon diluent at a temperature at which the PE is


POLYOLEFINS

polymerpolymer

monomersmonomers

tubular reactor autoclave reactorFig. 12. Reactor vessels usedfor the polymerization of ethylene at high pressure.

insoluble. Until 1960, the polymerization yield was notsufficient to avoid the step of removing the catalyticresidues from the polymer, thus rendering the processextremely complex and economically unfavourable; withthe HY catalysts, this inconvenience has been overcome.Normally, a diluent of polymerization with a not veryhigh boiling point is used so as to make the removalphase as easy as possible; the diluent most often used isn-hexane. With this process, HDPE and LLDPE caneasily be obtained, with the limitation of having tointroduce reduced quantities of comonomer because ahighly modified PE with long a-olefins becomespartially soluble in the diluent of polymerization, causingnot a few complications to the production process. Thelimit of density obtainable is not less than 0.93 g/cm3. Forexample, the process in slurry described by Hoechstworks at a pressure of between 0.5 and 1 MPa and at atemperature of 80-90°C in a reactor vessel of about 100m3. The optimal concentration of the slurry is acompromise between the productivity per unit of volumeof the reactor vessel and the removal of the reaction heat.In practice, the concentration used is between 30 and45%. The slurry is passed through a centrifuge to removethe greater part of the polymerization diluent, which isrecycled directly into the reactor vessel. To obtain apolymer with a wide and bimodal MWD, two or morereactor vessels in series are used, in each of which apolymer of different MW is synthesized, varying theconditions of polymerization and, above all, theconcentration of H2 used as the MW regulator.

Process in gas phase. The first process in gasphase was perfected at the end of the Sixties by UCC.This technology, compared with the processespreviously described, has the advantage of a lowerinvestment cost and offers the possibility of producingin a fluid bed reactor vessel a wide range of polymerswith densities that vary from 0.89 to 0.97 g/cm3. Thecomonomers that can be used are 1-butene, 1-hexeneand 4-methyl-1-pentene. In Fig. 13 there is the flowdiagram of a gas phase production plant of BasellPolyolefins called Spherilene; worldwide there are 12of these plants in operation with a capacity of 1.5 million t/yr of PE.

The availability of a catalyst with a granular orpossibly spherical controlled morphology is offundamental importance to the operation of a fluid bedreactor vessel. High yield microspheroidal catalysts baseon MgCl2 are particularly suitable. The fluid bed reactorvessel is nothing more than a CSTR operating at atemperature of between 80 and 100°C and a pressure ofbetween 0.7 and 2 MPa, in the presence of acomonomer, if necessary. Originally, operation at lowpressure was used to avoid condensation of the butene orthe hexene in the cooling cycle, whereas today, somepatents claim that the vaporization heat of the liquid

olefin absorbed by the particles of the growing polymeris used to control the temperature of the system, therebyincreasing its productivity. Because the reactor vesselworks at a temperature not far from the melt temperatureof the LLDPE, it is essential that this be controlled verycarefully; when uncontrolled thermal peaks occur, CO2is used to poison the catalyst and reduce the speed of thepolymerization in a reversible way.

Processes in solution. This technology has beendeveloped by many companies, such as DuPont, Dow,DSM and Mitsui Petrochemical, for the production ofHDPE and LLDPE. Many of the disadvantages of thischoice are the same as those mentioned for theprocesses in slurry, specifically the difficulty ofobtaining highly modified PE and the need to removethe catalytic residues from the polymer. For example,in the DuPont process the ethylene is dissolved incyclohexane and fed into the polymerization reactorvessel at about 10 MPa. The polymerization phase isadiabatic and the temperature is between 200 and300°C. The feed is about 25% ethylene, 95% of whichis converted into polymer, and the time of residence isabout 2 minutes. The catalytic system used is generallybased on halides of transition metals and aluminiumalkyls. The solution containing the PE leaving thereactor vessel is passed over a bed of alumina todeactivate the catalytic system; after drawing off thesolvent and removing the monomer that has notreacted, a further stage of final drying is needed.

Polypropylene production processes

The evolution over time of the PP productionprocesses is closely connected to the discovery andperfecting of new and ever more efficient catalyticsystems (Moore, 1998). Whenever a new catalyticsystem capable of improved performance in terms ofyield, isotacticity index and spectrum of productsobtainable became available, the producers of PP hadtwo possibilities before them: modify the existingplant (retrofit) to make it suitable for the newcatalysts; or design new production plants that couldtake full advantage of the new discoveries. The biggestcompanies often chose the second option.

The production processes of PP that have hadmajor industrial development occurred in plantsoperating in solution, in a slurry of hydrocarbondiluent, in a slurry of a liquid monomer and in gasphase. The first production plant was opened inFerrara by Montecatini in 1957 and was a reactorvessel of 1 m3 that, using TiCl3/Al(C2H5)2Cl in aheptane slurry and polymerising at 60°C, produced PPwith a low yield. The descriptions of processes thatfollow, refer to production plants that use secondgeneration high yield catalysts and require a smaller


POLYMERIC MATERIALS

number of operation units compared with theprocesses using low yield catalysts (Fig. 14).

Processes in solutionOnly Eastman has used this process, which is well

known for its serious technical difficulties that haveseriously limited its adoption. In fact, the principalcharacteristics are its extremely limited flexibility andits need for catalysts that are active at hightemperatures.

Processes in slurry (hydrocarbon diluent)This production process was used by the principal

producers of PP until 1976, using first generationcatalysts, and is characterized by a low yield inpolymerization and insufficient isotacticity, so much soas to make it necessary to remove both the catalyticresidues and the amorphous polymer produced. Thehydrocarbon diluent most widely used was n-hexane orn-heptane; the monomer that did not react after thepolymerization, was recompressed and recycled. The


POLYOLEFINS

polymerization section monomers recovery and recycling

catalyst

monomers

steam

steam

drier

drying

product

CW

CW

CWCW

Fig. 13. Spherilene process for production of HDPE, LLDPE and VLDPE. CW, cooling water.

hydrogenpropylene

monomerrecovery

polymerization

catalyst

solvent

flash monomerhydrogenpropylene

additives

polymerization

catalyst

flash monomer

solventcleansing

freshsolvent

solventrecovery

atacticrecovery

centrifuge

drying

additives

extrusion

polypropylene

DIAGRAM 1 DIAGRAM 2

polypropylene

atactic

Fig. 14. Flow diagram of plants for production of PP.

SCHEME 1 SCHEME 2

catalytic residues were removed by means of treatmentwith water and alcohol. The polymer was isolated bycentrifugal action and the atactic polymer was collectedby evaporation of the hydrocarbon diluent. As reportedfor the LLDPE, in this case also the introduction of othermonomers for copolymerization proved problematic,because they had the effect of increasing the fraction ofpolymer soluble in the polymerization diluent.

Processes in slurry (monomer liquid)The main advantage of this technology consists of its

high productivity per unit of volume of the reactorvessel, owing to the concentration of the monomer andthe absence of diluents. Even considering thetechnological problem of controlling the higherpressures, the simplification of the process due to theabsence of after-treatments of the polymer are evident.In fact, the catalytic residues, with the right approach,are found to be soluble in small quantities of solvent andeasily removable together with the atactic polymer. Themost important limitation is linked to the difficulty ofintroducing significant quantities of ethylene and H2 intothe reactor vessel; this greatly reduces the spectrum ofsynthesizable products. The principal companies that

have developed this technology since the Eighties havebeen Rexene, Phillips, Exxon and Sumitomo. Flowdiagram of plants for production in slurry are found inFig. 15 (Hercules process) and Fig. 16 (Montedisonprocess).

Process in gas phaseThe first process was perfected by BASF at the

beginning of the Sixties; the first industrial plant wasopened in 1969 at Wesseling in Germany and gave rise,after the conclusion of a joint venture with Shell, to theproduction process called Novolen. The process tookplace in an agitated vertical reactor vessel where theatactic polymer was left in the polymer; thiscounteracted some properties of the final product suchas, for example, its flexional rigidity, its colour and itsresistance to oxidation. Although it appeared to be asimple process, the design of the reactor vessel proved tobe very sophisticated having to guarantee an effectivemixing of the polymer with the liquids and the gases fedin, by means of agitation of the polymerization bed. Thereaction heat was removed by evaporation of themonomer, with subsequent condensation outside thereactor vessel itself and recycling. In this case also, the


POLYMERIC MATERIALS

PP to additives

and extrusion

neutralizeflocculatesludgethickendewaterbiological treatment

waste waterand catalystresidues totreatment

to diluent and aqueous recovery

atactic PP

diluentdiluentpropylenediluent recovery

dilu

ent r

ecov

ery

thin-filmevaporator

alcohol

alcohol

wor

kup

steam

drie

rcentrifuge

diluent wash

aqueous recovery

catalystpropylene

diluentH2

NaOHwater

N2

CWCW CW

CW

CW

CW

sepa

rato

r

propylene and comonomers

poly

mer

izat

ion

monomerreact down

50-80°C5-15 bar

Fig. 15. Process in slurry perfected by Hercules.

possibility of obtaining a wide range of copolymers wasvery complex, inasmuch as a particle of a polymerhaving a high comonomer content turned out to beparticularly ‘sticky’ and, therefore, difficult to process.The two main processes operating in gas phase are theone perfected by UCC/Shell (Unipol process in a fluidbed) and that by BASF/Amoco (a mechanically stirredbed). UCC in particular, thanks to the experience gainedin perfecting the Unipol process for PE and using acatalytic system developed by Shell, launched aproduction plant in Seadrift (USA) in 1985 that has hadsignificant success since it was capable of supplying awide range of products. The BASF process has a verysimple flow diagram, with fourth generation catalyticsystems that make it possible to obtain a wide range ofproducts, thanks also to the use of a second reactorvessel that broadens the versatility of the process.

The advent of controlled morphology HY catalystshas enabled many of the inconveniences previouslynoted in the description of the processes to be resolved.In fact, after the agreement in 1975 between Montedisonand Mitsui Petrochemical, a new family of catalyticsystems became available. The specific object of theagreement was to develop a series of catalysts supportedon activated MgCl2 used in combination with differentelectron donors capable of supplying PP with a yield andindex of isotacticity that were very high, so much as to

make it possible to omit the stage of removal of thecatalytic residues and of the atactic polymer.

Spheripol processOne of the most important steps in the history of PP

production, closely connected to the continuousevolution of catalytic systems, has been the definition ofa hybrid process (polymerization in liquid monomer andsubsequent copolymerization in gas phase) that haspermitted a truly new range of polymeric materials to beobtained. The most innovative aspect relative to solidcatalytic components has been the perfecting of asynthesis technology capable of controlling itsmorphology and porosity and at the same time ofgenerating a uniform distribution of the active sites inthe particles of the catalyst. In this way, control of thepolymerization temperature even in the innermost partsof the polymer particles can be obtained, thus avoidingoverheating which is damaging to the quality of thepolymer itself (Albizzati, 1997). In other words, everysingle particle of growing polymer behaves like a microreactor (reactor granule technology), where it ispossible to control not only its morphology but also itsthree-dimensional structure and at the same time iscapable of homo/copolymerising one or more monomersso as to accommodate significant quantities ofheterophases (Fig. 17). This process has taken the name


POLYOLEFINS

PP to additives

and extrusion

to diluent recovery

atactic PP

diluentdiluentpropylene

dilu

ent r

ecov

ery

thin-filmevaporator

steam

drie

r

centrifuge

diluent wash

catalystpropylene

diluentH2

N2

CWCW CW

CW

CW

CW

propylene and comonomers

poly

mer

izat

ion

monomerreact down

5-80°C5-15 bar

Fig. 16. Simplified process in slurry perfected by Montedison.

Spheripol; the first plant was built in Brindisi in 1982 byMontedison and constitutes the basis for the creation of anew company called Himont, a joint venture betweenMontedison and Hercules. Today there are about 100Spheripol plants operating worldwide capable ofproducing nearly 15 million t/yr of polymer, whichrepresents about 40% of the overall world production.Fig. 18 shows a flow diagram of the Spheripol plant, andFig. 19 a photograph of the plant in Brindisi.

The peculiarities that characterize the Spheripolprocess are: a) polymerization in liquid monomer withshort times of residence and low reaction volumes; b)sections for separation and recovery of the monomer thathas not reacted; c) polymerization in gas phase for theproduction of heterophase copolymers; d) polymer

finishing section (stripping monomer and drying). This configuration combines the efficiency of

polymerization in liquid monomer with the versatility ofreactions in gas phase; all of which is made possible byextremely powerful catalytic systems. The potential ofthe Spheripol process derives from the fact that it iscapable of producing homopolymers, randomcopolymers and high impact heterophase copolymerswith an excellent impact/ rigidity balance; this is due tothe capability of generating a polypropylenic matrix withhigh crystallinity (II�99%) in the first reaction step andof distributing the elastomeric phase in the gas phasereactor vessels. This characteristic has delivered newproducts to the marketplace with low production costs,allowing PP and its copolymers to take the places inapplications that were typically the domain of otherpolymeric materials.

Catalloy processA further evolution of Spheripol is the Catalloy

process developed by Montell at the beginning of theNineties (Moore, 1998). This is a modular technology,made up of three mutually independent poymerizationreactors operating in gas phase, that allows thepolymerization of different monomers separately or inseries on the same growing spherical particle,transferring them from one reactor vessel to another. Thecomposition of the gas phase of the three polymerizationreactors is independent and consequently there is


POLYMERIC MATERIALS

Fig. 18. Spheripol process perfected by Himont.

catalyst

pre-polymer

polymer

dryi

ng

PP to additives

and extrusion

first polymerization stageliquid monomer (homopolymer,random copolymer)

second polymerization stagegas phase (impact copolymer)

propyleneH2

TEAdonor

catalyst precontact

prepolymerizationpropylene

steam

steam

disactivation

comonomersN2

60-80°C25-35 bar

CW

CW

to monomer recovery

Fig. 17. Reactor granule technology: microscope photographs showing the growth of the polymer particleduring the various phases of the polymerization process.

significant flexibility in the type of monomer and thequantity of polymer obtained in each polymerisationreactor, making it possible to design and obtain a widerange of products having innovative properties. Theprincipal characteristics of the process are low energyconsumption, high qualitative consistency and lowproduction costs compared to similar productsobtainable with compounding technologies. Fig. 20shows the Catalloy process and Table 10 the spectrum ofproperties of the polymers that such a process is able toproduce. In particular, the following are obtained from


POLYOLEFINS

Fig. 19. Picture of the Spheripol plant in Brindisi.

PP to steaming, drying and stabilization

first polymerization stage

(homopolymer, random copolymer)

propyleneH2

TEA donor

catalyst

precontact

prepolymerization

propylene

comonomerscomonomers

CW CW CW

70-90°C20-30 bar

third polymerizationstage

(special copolymers)

second polymerizationstage

(special copolymers)

rotary compressors

Fig. 20. Catalloy process perfected by Montell.

Table 10. Spectrum of properties of polymers obtainable from the Catalloy process

Property Minimum Maximum

Melt flow rate (dg/min) 0.2 1,000

Flexural modulus (MPa) 70 2,300

Shore D hardness 30 90

Elongation at break (%) 200 900

IZOD-50°C (J/m) 30 1,000

Rubber content (%) 0 65

Melting point (°C) 125 165

the process: a) super-rigid homopolymers with a wideMWD; b) transparent high impact copolymers; c) lowweld copolymers; d) low module copolymers; e)copolymers with exceptional impact resistant properties.

Hypol processThis is a process perfected by Mitsui Petrochemical

in liquid monomer using HY catalysts in granular formand it is made up of two agitated reactor vessels used inseries, cooled by evaporation of part of the liquidpropylene. A reactor vessel of reduced dimensions in gasphase is used downstream of the two principal reactorvessels to complete the conversion of the monomer andto produce copolymers with modest quantities ofcomonomer.

Spherizone processThe most up-to-date process for the production of

heterophasic copolymers was perfected in the secondhalf of the Nineties by Basell Polyolefins and is calledSpherizone (Covezzi, 2004). Using fifth generationcatalysts containing diethers or succinates such as iED, amodel of a gas phase circulating bed polymerizationreactor was perfected. This method is an advance overthe idea of producing material in two differentpolymerization reactors and is based on the fact that thegrowing polymer particle follows an oscillating processbetween environments with different compositions ofmonomer and H2. The two interconnected zones are theriser and the downcomer; each zone has a differentfluodynamic regime and by means of a fluid barrier theycan have different compositions of gas generatingdifferent materials. The rising frequency of oscillation ofthe particles across the two environments facilitates theproduction of polymeric particles, which are moreuniform in terms of composition and therefore haveimproved properties.

Finishing of the polymerThe production plants described above can produce

polymer in powder or with a controlled morphology.Nearly all the production processes provide for a finalstage of extrusion and pelletization that is carried out inan extruder with a suitable die plate pelletizer, in order toproduce pellets between 2 and 5 mm long. The mostmodern plants use a corotating twin-screw extruder.Only the Spheripol process supplies a spherical polymercapable of being marketed directly. The extrusion andpelletization stage can be used to add some additives andstabilizers to the polymer. Obviously, in the case ofspherical polymers this operation calls for moresophisticated technology. The need to add stabilizers isdue to a structural weakness in PP, in fact the presence ofone tertiary carbon atom can lead to a possible break inthe chain through a radical attack. This reaction

generates hydroxylic and carbonylic groups that give riseto a weakening of the material and the appearance of ayellow coloration. The stabilizers most often used arephosphites, capable of decomposing some forms ofperoxide that are present, so as to improve the stability ofthe polymer in the melt state and of the hindered phenolsthat protect the polymer from the effects of ageing. Thequantities of these additives added are generally less than0.2% by weight and only in some particular cases mightreach 0.5%, for example when using esters ofthiodipropionic acid which is particularly active in thereaction of decomposition of the peroxides. In the caseof applications that provide for a prolonged exposure toatmospheric agents, it is necessary to add additivescapable of minimizing the effects of UV radiation. Themost widely used for this purpose arealcoxybenzophenones and hydroxybenzotriazoles orhindered amines (HALS, Hindered Amine LightStabilizers) in the case of PP film. Anti-static agents arealso used in quantities between 0.2 and 1%, andgenerally consist of amides, amines or glycerylmonostearates. Calcium stearate and hydrotalcites areused as anti-acid additives, to neutralize any traces ofcatalytic residues present. Finally, derivatives of sorbitoland terbutylbenzoate salts, sodium aryl phosphate andfinely ground talcum can be used as nucleants (Moore,1996).

References

Albizzati E. (1993) MgCl2 high yield supported catalysts,«La Chimica e l’Industria», 75, 107-111.

Albizzati E. (1997) Catalysts for olefins polymerization, «LaChimica e l’Industria», 79, 1053-1060.

Albizzati E. et al. (1990) US Patent 4971937 to Himont.Arlman E.J., Cossee P. (1964) Ziegler-Natta catalysis. III:

Stereospecific polymerization of propene with the catalystsystem TiCl3-AlEt3, «Journal of Catalysis», 3, 99-104.

Asanuma T. et al. (1991) Preparation of syndiotactic polyolefinsby using metallocene catalysts, «Polymer Bulletin», 25,567-570.

Bart J.C.J. et al. (1981) Synthesis and X-ray crystal structureof the TiMgCl6 (CH3COOC2H5 )4 adduct, «Zeitschrift fürAnorganische und Allgemeine Chemie», 482, 121-132.

Breslow D.S., Newburg R.N. (1957) Bis-(cyclopendadienyl)-titalium dichloride-alkylaluminum complexes as catalystsfor the polymerization of ethylene, «Journal of the AmericanChemical Society», 79, 5073.

Brintzinger H.H. (1988) Synthesis and characterization ofchiral metallocene cocatalysts for stereospecific a-olefinpolymerizations, in: Sinn H., Kaminsky W. (editors)Transition metals and organometallics as catalysts for olefinpolymerization, Berlin, Springer, 249-256.

Busico V. et al. (1986) Polymerization of propene in thepresence of MgCl2-supported Ziegler-Natta catalysts. II:Effects of the co-catalyst composition, «MakromolekulareChemie», 187, 1115-1124.


POLYMERIC MATERIALS

Carrick W.L. et al. (1972) Ethylene polymerization with supportedbis (triphenylsilyl) chromate catalysts, «Journal of PolymerScience. Part A: Polymer Chemistry», 10, 2609-2620.

Chien J.C.W., Wang B.P. (1990) Metallocene–methylaluminoxanecatalysts for olefin polymerization. V: Comparison of Cp2ZrCl2and CpZrCl3, «Journal of Polymer Science. Part A: PolymerChemistry», 28, 15-38.

Ciardelli F. et al. (a cura di) (1983) Macromolecole. Scienzae tecnologia, Pisa, Pacini, v.I.

Ciardelli F. et al. (a cura di) (1986) Macromolecole. Scienzae tecnologia, Pisa, Pacini, v.II.

CMAI (Chemical Market Associates Inc.) (2002).Corradini P. et al. (1983) A possible model of catalytic sites

for the stereospecific polymerization of a-olefins on first-generation and supported Ziegler-Natta, «Gazzetta ChimicaItaliana», 113, 601-607.

Corradini P. et al. (1992) Hydrooligomerization of propene. A«fingerprint» of a Ziegler-Natta catalyst. II: A reinterpretationof results for isospecific homogeneous systems, «Die Makro-molekulare Chemie. Rapid Communications», 13, 21-24.

Covezzi M. (2004) Sviluppi di ricerca e nuove tecnologie nelsettore dei materiali polimerici, «La Chimica e l’Industria»,86, 58.

Ewen J.A. (1984) Mechanisms of stereochemical control inpropylene polymerizations with soluble group 4Bmetallocene/methylalumoxane catalysts, «Journal of theAmerican Chemical Society», 106, 6355-6364.

Ewen J.A. et al. (1988) Syndiospecific propylenepolymerizations with group 4B metallocenes, «Journal ofthe American Chemical Society», 110, 6255-6256.

Farina M., Puppi C. (1993) Electrophilicity of titanium atomssupported on magnesium chloride, «Journal of MolecularCatalysis», 82, 3-9.

Galimberti M. et al. (1994a) European Patent 0586658 toSpherilene.

Galimberti M. et al. (1994b) European Patent 0624604 toSpherilene.

Galimberti M. et al. (1995) European Patent 0632066 toSpherilene.

Galimberti M. et al. (1998) Ethene/propene copolymerizationwith high product of reactivity ratios from a single center,metallocene-based catalytic system, «Macromolecules»,31, 3409-3416.

Giannetti E. et al. (1985) Homogeneous Ziegler-Nattacatalysis. II: Ethylene polymerization by IVB transitionmetal complexes/methyl aluminoxane catalyst systems,«Journal of Polymer Science. Part A: Polymer Chemistry»,23, 2117-2134.

Giannini U. (1981) Polymerization of olefins with high activitycatalysts, «Macromolecular Chemistry and Physics.Supplement», 5, 216-229.

Giannini U. et al. (1970) Polymerization of olefins with benzylderivatives of titanium and of zirconium, «Journal ofPolymer Science», 8, 405-410.

Giannini U. et al. (1977) European Patent DE 2632591 toMontedison.

Giannini U. et al. (1978) US Patent 4107414 to Montedison.Giannini U. et al. (1979) US Patent 4149990 to Montedison-

Mitsui Petrochemical Industries.Giannini U. et al. (1987) Some aspects of the polymerization

of olefins with high-activity heterogeneous catalysts, in:

Fontanille M., Guyot A. (editors) Recent advances inmechanistic and synthetic aspects of polymerization,Dordrecht, Reidel, NATO ASI Series, 215.

Graham T. (1861) Anwendung der Diffusion der Flüssigkeitenzur Analyse, «Philosophical Transactions», 151, 183-224.

Guaita M. et al. (1998) Fondamenti di scienza dei polimeri,Pisa, Pacini.

Guastalla G., Giannini U. (1983) The influence of hydrogenon the polymerization of propylene and ethylene with amagnesium chloride supported catalyst, «MakromolekulareChemie. Rapid Communications», 4, 519-527.

Guerra G. et al. (2002) Influence of regio- and stereoregularityof propene insertion on crystallization behavior andelasticity of ethene-propene copolymers, «Journal of theAmerican Chemical Society», 124, 1566-1567.

Guyot A. (1965) Catalysts for ethylene polimerization, «RevueGenerale des Caoutchoucs & Plastiques», 2, 37.

Harries C. (1904) Über den Abbau des Parakautschuksvermittelst Ozon, «Berichte der deutschen chemischenGesellschaft», 37, 2708-2711.

Hermans, J.P., Henrioulle P. (1973) US Patent 3769233 toSolvay Cie.

Hogan J.P. (1970) Ethylene polymerization catalysis overchromium oxide, «Journal of Polymer Science. Part A:Polymer Chemistry», 8, 2637-2652.

Hogan J.P., Banks R.L. (1958) US Patent 2825721 to PhillipsPetroleum Company.

Jang B.Z. et al. (1984) Ductile-brittle transition in polymers,«Journal of Applied Polymer Science», 29, 3409-3420.

Jang B.Z. et al. (1985) Rubber-toughening in polypropylene,«Journal of Applied Polymer Science», 30, 2485-2504.

Kaminsky W. (1986) Preparation of special polyolefins fromsoluble zirconium compounds with aluminoxane ascocatalyst, in: Studies in surface science and catalysis, 25,Amsterdam, Elsevier, 293-304.

Karakchiev L.G. et al. (1967) Supported chromium oxidecatalysts for ethylene polymerization. The reason forstabilization of chromium (VI) ions, «Kinetika i Kataliz»,8, 188-192.

Lebedew S.W., Filonenko E.P. (1925) Polymerization. X:Action of some silicates on unsaturated compounds,«Berichte der deutschen chemischen Gesellschaft», 58,163-168.

Mark H., Whitby G.S. (edited by) (1940) Collected papersof Wallace Hume Carothers on high polymeric substances,New York, Interscience.

Martuscelli E. (1995) Polymer perspective, «La Chimica el’Industria», 77, 86-98.

Mayr A. et al. (1984) US Patent 4495338 to Montedison. Mayr A. et al. (1985) US Patent 4476289 to Montedison.Moore E.P. (1996) Polypropylene handbook. Polymerization,

characterization, properties, applications, München-Wien-New York, Hanser.

Moore E.P. (1998) The rebirth of polypropylene. Supportedcatalysts, how the people of the Montedison laboratoriesrevolutionized the PP industries, München-Wien-New York,Hanser.

Morini G. et al. (2000) Patent WO0063261 to Montell.Natta G. (1960) Aspetti catalitici delle polimerizzazioni

stereospecifiche, «La Chimica e l’Industria», 42, 1207-1225.


POLYOLEFINS

Natta G. (1964) Von der stereospezifischen Polymerisationzur asymmetrischen autokatalytischen Synthese vonMakromolekülen, «Angewandte Chemie», 76, 553-566.

Natta G. (1965) Macromolecular chemistry, «Science», 147,261-272.

Natta G. et al. (1955) Italian Patent 554803 to Montecatini. Natta G. et al. (1957) Polymerization of ethylene catalyzed

by titanium compounds, «La Chimica e l’Industria», 39,1032-1033.

Natta G. et al. (1958) Polymorphism of crystalline titaniumtrichloride, in: Atti dell’Accademia Nazionale dei Lincei.Rendiconti. Classe scienze fisiche, matematiche e naturali,24, 121-129.

Natta G. et al. (1959) Crystal structure of the g form of titaniumtrichloride, in: Atti dell’Accademia Nazionale dei Lincei.Rendiconti. Classe scienze fisiche, matematiche e naturali,26, 155-163.

Natta G. et al. (1960) Propylene linear high polymers withsyndiotactic structure, in: Atti dell’Accademia Nazionaledei Lincei. Rendiconti. Classe scienze fisiche, matematichee naturali, 28, 539-544.

Natta G. et al. (1961) The different crystalline modificationsof TiCl3, a catalyst component for the polymerization of a-olefins. I. a-, b-, g-TiCl3. II. d-TiCl3, «Journal of PolymerScience», 51, 399-410.

Natta G. et al. (1962) Stereospecific catalysts for the head-to-tail polymerization of propylene to a crystallinesyndiotactic polymer, «Journal of the American ChemicalSociety», 84, 1488-1490.

Pasquon I. (2004) Giulio Natta. Education, science, industry,and society, «La Chimica e l’Industria», 86, 30-32.

RESCONI L. et al. (1990a) European Patent 0384171 to Ausimont-Himont-Montedison.

Resconi L. et al. (1990b) Study on the role of methylalumoxanein homogeneous olefin polymerization, «Macromolecules»,23, 4489-4491.

Resconi L. et al. (1994) High molecular weight amorphuspolypropylene from metallocene/MAO catalysts, «PolymerPreprints», 35, 664.

Resconi L. et al. (1995) Effect of monomer concentration onpropene polymerization with the rac-[ethylenebis (1-

indenyl)] zirconium dichloride/methylaluminoxane catalyst,«Macromolecules», 28, 6667-6676.

Riccò T. (1988) Sviluppo della tenacità in materiali compositipolimero-polimero. Poliammide-6 modificata con gomma,in: Atti del 10° Convegno Scuola AIM, Gargnano, 5-10Giugno, 73-86.

Roebuck A.K., Zletz A. (1954) US Patent 2692258 toStandard Oil.

Schnutenhaus H., Brintzinger H.H. (1979) 1,1’-trimethylenebis h5 – 3-tert-butylcyclopentadienyl- titanium(IV) Dicloride, a Chiral ansa – Titanocene Derivative,«Angewandte Chemie. International Edition in English»,18, 777-778.

Sinn H. (1988) Some new results on methyl-aluminoxane, in:Sinn H., Kaminsky W. (editors) Transition metals andorganometallics as catalysts for olefin polymerization,Berlin, Springer, 257-268.

Sinn H. (1995) Proposal for structure and effect ofmethylalumoxane based on mass balances and phaseseparation experiments, «Macromoecular Symposia», 97,27-52.

Sinn H., Kaminsky W. (1980) Ziegler-Natta catalysis,«Advances in Organometallic Chemistry», 18, 99-143.

Staudinger H. (1920) Über Polymerisation, «Berichte derdeutschen chemischen Gesellschaft», 53, 1073-1085.

Stevens J.C. et al. (1991) European Patent 0416815 to DowChemical.

Tadokoro Y.T. et al. (1967) Polymerization studies usingnickel-carbon catalysts. I: Polymerization of ethylene witha modified nickel-carbon catalyst, «Kogyo Kagaku Zasshi»,70, 144-147.

Zeigler R. et al. (1994) Allyltrimethylsilane polymers frommetallocene catalysts: tacticity and structural characterization,«Polymer», 35, 4648-4655.

Ziegler K. (1964) Folgen und Werdegang einer Erfindung,«Angewandte Chemie», 76, 545-553.

Enrico Albizzati

Pirelli Labs - Materials InnovationMilano, Italy


POLYMERIC MATERIALS

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