CH2402 - Polymerisation Catalysis

8/3/2019 CH2402 - Polymerisation Catalysis

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Polymerisation Catalysis

Calculating Molecular Weights and Related Terms

There are several ways of measuring the average molecular weight of a polymer, it is possible to

calculate the number average molecular weight and weight average molecular weight.

Equation 1 - Calculating the number average molecular weight

Equation 2 - Calculating the weight average molecular weight

The polydispersity index (PDI) is related to both of these terms, ideally this value would be equal to

1 as this would represent entirely uniform chain length, i.e. each catalyst displays perfect specificityand forms chains of the exact same length.

Equation 3 - Calculating the PDI

Mn is always less than Mw and so PDI values are larger than 1, ca. >5 for heterogeneous catalysts

(multiple active sites leads to different rates of reaction and termination) and


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Olefin Polymerisation

Figure 1 - A general description of olefin polymerisation

The properties of a polymer depend on;

molecular weight and the distribution or range of molecular weights (the polydispersityindex, PDI)

the distribution and nature ofchain branches(R/R H) either through polymer re-incorporation, chain isomerisation or additional co-monomer

in -olefins the control of relative stereochemistry at the chiral carbon, i.e. the tacticity

modification post-polymerisation, e.g. end-group functionalization

The need to control and select these characteristics in order to achieve polymers with different uses

and properties has led to the development of several types of catalyst, including;

Phillips heterogeneous, solid supported chromium ions or complexes, there is oftenconfusion or lack of clarity over the number of chromium oxidation states involved which

can cause irregular behaviour

Ziegler-Natta solid supported titanium compounds used in conjunction withorganoaluminium co-catalysts

Single-site catalysts consisting of a single molecule or metal complex, these behavehomogenously despite not strictly being so. They are high specificity, and often provide a

low range of molecular weights (cf. to Phillips and Ziegler-Natta catalysts this is definitely

true) due to the uniform rate of polymerisation. They are also more likely to distribute

monomers, i.e. build regular units in systems with competing monomer units, e.g. ethylene

and hexane

The last of these three is the most commonly used in industry for many reasons, some of which are

described above. On an industrial scale these catalysts are solid supported, most often in pores of

silica or MgCl2, and are bound only by adsorption they retain their identity as a complex hence the

ability to act as a homogeneously catalyst.

As the polymer forms and the chain gets larger the solid support begins to fragment, this reveals

more pores and in turn more active catalyst and so this process continues until the reaction is

complete, the catalyst has been incorporated into the polymer and the solid support has degraded

into much smaller beads. If the catalysts were to be used in solution they would cling to the reactor

walls, increasing downtime, and so this solid supported method which maintains homogeneous

behaviour of the catalyst also has economic advantages. The most frequently used single-site

catalysts are the metallocenes.

Metallocene Catalysts

These are essentially homogeneous versions of the Ziegler-Natta catalyst, they consist of an early

transition metal in a high oxidation state (i.e. Ti(IV), Zr(IV), Hf(IV), though catalysts based on


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scandium, yttrium and lanthanides are known) made net neutral with 2 cyclopentadiene ligands and

2 chloro- or alkyl ligands in the cis isomer.

Early transition metals are used in a high oxidation state as this reduces the interaction with the

methyl ligands, they are small d0 atoms and so bonding with the methyl group is minimal so the

cationic active catalyst can be made, analysed and reacted in situ.

The proposed mechanism is;

Figure 2 - Proposed mechanistic cycle for olefin polymerisation

The four steps are as follows;

Initiation formation of the cationic alkyl complex (note the empty coordination site drawnis unlikely to exist as a tetrahedral species, there is likely to be a concerted exchange or a

fluxion in geometry)

In a laboratory the metal bis-chloride is first converted to the metal bis-alkyl with an aluminium

alkyl, which is then able to extract a methyl and activate the catalyst.

In an industrial setting methylaluminoxane (MAOFigure 3 - Methylaluminoxane (MAO))(Figure 3) isinstead used, the preparation involves partial hydrolysis of AlMe3 (which is very air sensitive).

Figure 3 - Methylaluminoxane (MAO)

The structure of MAO is ill-defined, consisting of highly varied oligomers. Typically MAO is used in aratio of over 200:1 with pre-catalyst, however, it has the added benefit of removing water from the


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reaction.

There are then three methods for forming the active catalyst;

1. Oxidative M-C cleavage

2. Protonation

3. Abstraction

This is the most commonly used.

Anions must be chosen carefully as some typical cations are capable of deactivating the

catalyst, some coordinate with the metal, while perfloro anions (BF4-, PF5

-) can undergo F-

abstraction and [BPh4-] can either -coordinate or undergo phenyl abstraction.

Perfluorophenylborate anions are very weakly coordinating and less prone to extraction

than alkyl derivates, examples of these include B(C6F5)3, [B(C6F5)4]- and [MeB(C6F5)3]

-.

The latter of these sometimes displays a B-MeM interaction;

Figure 4 - Illustration of a B-Me--M interaction

This coordination, which disables the catalyst, can be determined by 19F NMR with a change

in the difference between the meta andpara19F resonances (ca. 2.6 ppm to > 3 ppm)

indicative of a B-MeM interaction.

Another common feature that enhances characterisation is that of agostic interactions. Since

these species are d0 it is difficult to achieve an 18e- configuration and as a result these

interactions are common, particularly for organometallic complexes and occasionally for -

C-H and -Si-H bonds. These interactions can be determined by gated coupled (decoupled)13

C NMR, here the decoupler is on during the relaxation delay and off during acquisition,

ensuring a good signal to noise ratio without sabotaging the 13C NMR which depends on

decoupling. The indication of an agostic interaction is a shift in the 1J(CH) coupling constant

from ca. 125 Hz to ca. 200 Hz, or using 29Si NMR a shift from ca. -5 to 5 ppm to ca. -20 ppm.


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These systems are air and water sensitive, again due to the high oxidation state, assuming

no ligand degradation they will form bridging oxo-compounds, deactivating the catalyst.

Suitable solvents for this reaction include chlorobenzene and bromobenzene, coordinating

solvents (e.g. THF, pyridine, ethers) are semi-innocent and may be used while protic

solvents (e.g. CHCl3, DMSO) are non-innocent (they are liable to decomposition and

deactivate the catalyst) and aliphatic hydrocarbons are non-solvating.

Propagation addition of the olefin and extension of the polymer chainThe mechanism for propagation, suggested by Brookhart and Green, is;

Figure 5 - Brookhart-Green mechanism

This mechanism stresses the importance of agostic interactions as they are necessary for holding the

methyl group in position for insertion, this interaction increases the electron density at the metal, in

turn improving the lability of the methyl group.

The d0 configuration means that back-bonding into the * of the olefin is weak, so much so that the

olefin complex cannot be isolated.

Proof of olefin insertion was demonstrated by Eisch et al1

;

1 J. J. Eisch et al.,J. Am. Chem. Soc., 1985, 107, 7219.


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Figure 6 - Crystal structure showing the agostic interaction

This is then unable to further interact as the species stearically prevents more substrate from

approaching the metal, this complex was then isolated as crystals (Figure 6).

Termination the processes that halt propagation, e.g. the release of the polymer chainduring reformation of the active catalyst

Figure 7 - The termination process

Where the R group is the polymer chain, once released a new olefin may approach and the process

begin anew.

Chain transfertransfer of the polymer chain to the aluminium co-catalyst, often due toresidual AlMe3 found in MAO

Figure 8 - Chain transfer

Polymer Characterisation

The two techniques used most often are mass spectrometry and gel permeation chromatography

(GPC).

Mass spectrometry will indicate which masses are present without providing information on the

abundance of each (a larger peak indicates a better flying capability, not a greater amount).

GPC, a form of size exclusion chromatography, indicates the relative abundance of each molecular

weight by retention time (calibrated against polystyrene standards). Distortions in the

chromatogram peaks, e.g. shoulders, suggest different forms of catalyst (i.e. 4 shoulders would infer4 forms of active catalyst), and so GPC offers more information that mass spectrometry.


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Stereospecific Olefin Polymerisation

For all but the simplest of-olefins there is a chiral carbon with an R group attached, relative

orientation of these R groups in the polymer chain controls the properties of the polymer and so

control of this orientation, or tacticity, is necessary when designing catalysts. The different types of

tacticity are isotactic(Figure 9), syndiotactic(Figure 10) and atactic(Figure 11).

Figure 9 - Isotactic orientation (regular)

Isotacticity is very difficult to achieve and most isotactic polymers are in fact very slightly atactic. A

chain may be hemiisotactic if every other repeat unit has randomised insertion.

Figure 10 - Syndiotactic orientation (alternating)

Figure 11 - Atactic orientation (random)

These polymers are, despite containing a large number of chiral carbons, not optically active as they

possess symmetry however poly(-olefins) do display optical rotation. In solution the polymer

chains form helices, and for polymers longer than poly(propylene) this helix is unable to unwind. This

does mean that optical activity is not a method for measuring the tacticity of a polymer as it is due to

the size of the polymer chain and not the orientation of the individual chiral centres.

To determine the tacticity of a polymer 13C NMR (Figure 13) is utilised. By locating and identifying

the peaks associated with certain pentads the orientation of monomer units can be determined. The

length of chain with five R groups (Figure 12), which forms the pentad, can be described by the

relationship between neighbouring R groups using the letters r and m, where r represents a racrelationship and m a meso relationship. These relationships effect the shift of the observed peaks


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Figure 12 - Example pentads with m/r notation

If a pentad is symmetrical it will have the same description as its mirror image (i.e. rmmr will rotate

around the central carbon to give rmmr).

The most indicative resonance in the 13C NMR is that of the first carbon in the polymer R-group, C3.

Figure 13 - An example

13

C NMR of poly(1-hexene)

2

with pentads (in m/r notation) identified

Stereocontrol in polymerisation is achieved either with chain end control, where the growing

polymer chain determines the stereochemistry, or by enantiomorphic site control at the ligand using

a chiral catalyst. In chain end control any erroneous insertion (i.e. any insertion of a wrongly

orientated molecule) will propagate throughout the polymer, whereas with enantiomorphic site

control an error will not propagate as the catalyst still prefers a certain configuration upon the next

addition of monomer unit to the complex.

In the polymerisation of-olefins there is a preference for the R-group to be at as great a distance as

possible from the growing polymer chain (Figure 14). There are, however, other factors to consider;

2 T. Asakura, M. Demura and Y. Nishiyama, Macromolecules 1991, 24, 2334.


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in a metallocene catalyst the polymer chain is able to move side to side, and so the chirality of both

sites (the cis coordination sites of the metal) must be considered for full stereocontrol. A rotation of

the cyclopentadienyl rings will also remove selectivity

Figure 14 - Illustration of the preferred, stearically controlled, orientation of an incoming monomer unit

The solution is to link the two rings, creating an ansa-metallocene (Figure 15).

Figure 15 - An example of an ansa-metallocene

The chirality of the polymer is then dependent on the chirality of the metallocene catalyst and the

active catalyst species, according to Ewens symmetry rules.

Figure 16 - The result of different chiralities, based on symmetry, of metallocenes. The C2V and Cs (achiral) symmetries

provide atactic polymer, the C2 symmetry provides isotactic polymer, the Cs (prochiral) provides syndiotactic polymer

and the C1 symmetry gives hemi-isotactic polymer.

Post-Metallocene Catalysts

Although metallocene catalysts are almost ideal they are very difficult to research, this is due to

years of aggressive patent applications. Patents will often include several variations, including

different metals or modified moieties, beyond the scope of the work actually performed. This leaves

very few available systems for research.


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This has led to work on post-metallocene catalysts, these use alternate supporting ligands and allow

for more variability as well as a higher degree of control.

The metals used in post-metallocene systems are the group III and IV metals, the lanthanides,

vanadium, chromium, iron and nickel. Nickel provides the best catalysis however nickel allergies are

quite common, and so polymers with embedded nickel catalyst are not desirable for large sections

of industry.

The first class of post-metallocene catalyst is the half-sandwich(Figure 17), using a single Cp ring

held in a constrained geometry. These systems are considerably less effective than bis-Cp systems,

however.

Figure 17 - Example 'half-sandwich' complexes

Group IV catalysts are often based on amido (NR2) ligands (Figure 18) in conjunction with cis-halides.

Figure 18 - Example amido complexes

Group III and lanthanide catalysts, usually in the +3 oxidation state, are adapted to different ligand

types compared to group IV, however, they are also susceptible to several unique challenges.

Particularly in the case of lanthanides a lot of the tuning can be achieved with the selection of the

metal, however, as the lanthanides get smaller across the group the activity decreases as thermal

stability increases and so a trade-off must be achieved.

The redox stability of lanthanides are also important, europium and ytterbium have a very high 3rd

ionisation energy (~2400 kJ mol-1) while cerium and gadolinium have very low 4th ionisation energies

compared to other lanthanides, and so maintaining a 3+ oxidation state is also a challenge.

Group III catalysts should be active without ionisation as they are isoelectronic with the active

cationic group IV catalysts (Figure 19), however, using a similar system to bis-Cp, they have yet to

materialise as a competitive alternative.


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Figure 19 - Isoelectronic scandium and cationic titanium complexes

Homoleptic alkyl catalysts (Figure 20) using lanthanides have emerged as incredibly active catalysts

for ethylene polymerisation with an increase at the metal ionic radii leading to an increase in activity

(Figure 21).

Figure 20 - Structure and synthesis of homoleptic alkyl catalyst

Figure 21 - Data showing the trend in ionic radii:activity3

Another group of complexes used in ethylene polymerisation are amidinate complexes (Figure 22).

Here the importance of balancing stability with activity can be observed (Figure 23) with activity

peaking at yttrium (1.04 ). This peak is different for any given ligand system.

3 S. Arndt, T. P. Spaniol, and J. Okuda,Angew. Chem. Int. Ed., 2003, 42, 5075


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Figure 22 - Synthesis of an amidinate complex

4

Figure 23 - Data for amidinate complexes of different metals

N3 donor ligands have also been reported (Figure 24).

Figure 24 - Example N3 donor catalysts56

4 S. Bambirra, M. W. Bouwkamp, A. Meetsma, and B. Hessen,J. Am. Chem. Soc., 2004, 126, 9182.


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Stereoselectivity using post-metallocenes is considerably understudied though some attempts have

been made using trisoxalane ligands.

Polylactic Acid

There is a desire to move away from poly(-olefins) as the starting materials are derived from crude

oil, a limited resource with negative ecological effects. Polymers based on biomass show somepotential, being less reliant on hydrocarbon resources and biodegradable they have some preferable

qualities, however, there remains debate about the efficiency and the amount of land required to

grow the crops from which the monomers are then taken.

Polylactic acid (PLA), from starch, is one of the most studied biomass polymers. Uses are limited as

PLA has a low glass transition temperature, Tg, the temperature at which the polymer becomes

brittle and likely to disintegrate.

Figure 25 - The polylactic acid cycle

For the ring opening polymerisation (ROP) step (Figure 26) a tin catalyst is, in industry, almost

exclusively used. Tin is subject to safety regulations however, and although usage currently complies

with these laws they are changeable, and so research must be done to identify replacement

catalysts if PLA is to continue to be manufactured. Most research is done using d0 group II/III/IV

metals, although aluminium is also used (Figure 27), in order to minimise discolouring.

5 S. Hajela, W. P. Schaefer, J. E. Bercaw,J. Organomet. Chem., 1997, 532, 45.

6 S. C. Lawrence, B. D. Ward, S. R. Dubberley, C. M. Kozak, P. Mountford, Chem. Commun., 2003, 2880.


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Figure 26 - Mechanism of ROP

Figure 27 - Examples of aluminium catalysts for PLA synthesis7

Stereoselective Polymerisation of Lactic Acid and Tacticity

There are 3 possible isomers of lactide when formed from lactic acid; D-lactide, L-lactide and meso-

lactide. Meso-lactide can be separated leaving D/L-lactide, or rac-lactide, to then be used inpolymerisation.

Figure 28 - The different isomers of lactide

Unlike with -olefins the chiral carbon here is not at the point of insertion and tacticity is determined

entirely by the chirality of the monomer. Enantiopure, (RR) or (SS), monomer will result in an

isotactic polymer but is a far too expensive solution and so catalysts are chosen that will polymerise

only one enantiomer in the racemic mixture, it is possible to use racemic catalysts to achieve a

crystalline mixture of R-PLA and S-PLA.

7 A. D. Schwarz, Z. Chu, P. Mountford, Organometallics, 2010, 29, 1246.


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Tacticity in PLA systems is slightly more complicated, heterotactic (alternating RR/SS monomer units)

and atactic are still as before, but isotactic polymer may now, in order of decreasing control, be

considered chiral (i.e. PLLA and PDLA formed as separate chains), diblock (i.e. all of one monomer is

polymerised followed by the other) or multiblock (i.e. blocks of various length of each monomer are

incorporated in turn).

Figure 29 - Different types of tacticity (atactic omitted) in polylactides

PLA is itself chiral and so tacticity can be determined by optical rotation however NMR, both`13

C andhomonuclear decoupled 1H modes (this prevents coupling of the methyl and methine protons

allowing analysis of tetrad units that describe the tacticity present), is preferred. Similar to -olefins

the NMR shift depends on the orientation of L and D units, with i describing an iso relationship and

s a syndio relationship.

The tacticity of PLA is often defined as the probability of meso (Pm) or racemic (Pr) enchainment. If

Pr=Pm=0.5 the polymer is atactic, Pr=1 is heterotactic and Pm is isotactic.

Metathesis

From the Greek for transposition, metathesis describes a bimolecular reaction involving the

exchange of chemical bonds between the two reactants, for example the metal halide/alkyl

exchange using MeLi to generate an active catalyst for olefin polymerisation is described as a halide

metathesis, similarly catalysts for PLA synthesis can be prepared by -bond metathesis.

Ring opening metathesis polymerisation (ROMP) is a common approach to synthesising

functionalised polymers, in particular olefinic groups that can be subsequently used in cross-linking.

By using strained, cyclic olefins the reaction is thermodynamically preferable, for example the

polymerisation of norbornene.


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Figure 30 - Poly(norbornene) synthesis

Figure 31 - General mechanism for ROMP reactions

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CH2402 - Polymerisation Catalysis

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