Pzn Reactions 4-1
POLYMERIZATION REACTIONS
• Understand the differences between step-growth and chain-growth polymerization reactions.
• Predict the products of polycondensation and polyadditionreactions.
• Explain how reactant ratios and monomer purity may affect the degree of polymerization.
• Understand the initiation, propagation, and termination processes in free-radical, ionic, coordination (Ziegler-Natta), and ring-opening polymerizations.
• Explain how the nature of the reaction catalyst influences polymer structure (stereoregularity and tacticity).
STEP-GROWTH POLYMERIZATION
• Formerly: Condensation polymerization (Carothers, 1931)
• Monomers are difunctional — each has two reactive functional groups
• Chain growth occurs through coupling (condensation, addition) reactions
• [Monomer] decreases rapidly before any high-MW polymer is formed
• Rate of polymerization is highest at outset, decreases as chain ends are consumed long reaction times
Pzn Reactions 4-2
Schematic:
nA + nB —A−B—( )n
The number average degree of polymerization(Xn) is
N0 Number of molecules at startDP = Xn = —— = —————————————————
N Number of molecules at end
The ratio [A]:[B] strongly influences chain lengthThe ratio [A]:[B] strongly influences chain length
Addition: nA + nB —A−B—)n(
Condensation: nA + nB —A−B— + Y)n(
BA A B A B A B A B A B 12/1 12
BA A B A B A B A B A B 12/9 1.3
A B A B A B A B A B A B 12/11 1.1
BA A B A B A B A B A B 12/10 1.2
BA A B A B A B A B A B 12/8 1.5
BA A B A B A B A B A B 12/7 1.7
BA A B A B A B A B A B 12/6 2
BA A B A B A B A A A B 12/5 2.4
BA A B A B A B A B A B 12/4 3
BA A B A B A B A B A B 12/3 4
BA A B A B A B A B A B 12/2 6
A B A B A B A B A B A B 12/12 1
N0/N Xn
Pzn Reactions 4-3
2
4
6
8
10
2 4 6 8 10
Reaction Step Number
Xn
Degree of Polymerization
Representative Step-Growth Reactions
Condensation Reactions
Addition ReactionsPolyurethane
+HO OHCH2)2( CH2OCN NCODABCO
CH2 NHCOO CH2)2( O−C−N
O
H
Polyester (Polyethylene terephthalate/PET)
+ COOHHOOCHO OHCH2)2( + H2OCOOO CH2)2( O−C
O
Polyamide (Nylon-6,6)
H2N NH2CH2)6( COOHHOOC CH2)4(+ HN CH2)6( COCH2)4( + H2OHN−C
O
See also Table 3.2, pp 25-26
Pzn Reactions 4-4
Predicting Molar Mass in Step-Growth Pzns
Since the total number of molecules (N) decreases by 1 for each condensation step,
N0 − N N0 1p = ———— or —— = ———
N0 N 1−p
1Xn = ———
1−p
1Xn = ———
1−pCarothers equation
The extent of reaction (p) isNumber of functional groups reacted
p = —————————————————————Initial number of functional groups
Carothers Equation
50
100
150
0.2 0.4 0.6 0.8
p
Xn
Pzn Reactions 4-5
Step-Growth Reactions: MW Distribution
N0 1Xn = —— = ———
N 1−p
The corresponding weight-average degree of poly-merization is
1+pXw = ———
1−p
The molecular weight distribution (polydispersity)is thereforeXw—— = 1 + pXn
as p 1, MWD 2 for any linear step-growth pzn
Molecular Weight Control
• Lowering the temperature before the reaction is complete can reduce product MW.
Result: a thermally unstable product.
• Alternative: use a non-stoichiometric reactant ratio ([A] < [B]). The less abundant reactant is consumed completely. In terms of functional groups (NA < NB), the reactant ratio is
NAr = —— < 0NB
Substituting gives the general Carothers equation:
nA + nB —A−B—( )n
1 + rXn = ———————
1 + r − 2rp
1 + rXn = ———————
1 + r − 2rp
Pzn Reactions 4-6
Molecular Weight Control (cont.)
1 + rXn = ———————
1 + r − 2rp
1 + rXn = ———————
1 + r − 2rp
1.000r
0.9990.9900.9500.900
10.0p = 0.900
10.09.68.16.8
20.0p = 0.950
19.818.313.410.0
100.0p = 0.990
95.366.828.316.1
1000.0p = 0.999
666.8166.137.618.7
Xn
Molecular Weight Control (cont.)
Producing high-MW step-growth polymers requires
• High conversions (p > 0.98)
• Stoichiometric ratios of functional groups
• High-purity monomers
• No side reactions
Pzn Reactions 4-7
CHAIN-GROWTH POLYMERIZATION
• Formerly: Addition polymerization (Carothers, 1931)
• Polymerization requires an initiator, a substance that starts the reaction
• Chain growth occurs by addition of monomer to a relatively small number of initiation sites (free radicals, anions, cations, transition metal complexes)
• Reaction mixture contains monomer, high polymer, and a low concentration of growing chains
• [Monomer] decreases steadily as polymer is formed
Schematic:
nM —Mn—
Initiation: I + M I−M*
Propagation: I−M* + nM I−Mn−M*
Termination: Reactions depend on M and M*
Initiator
The ratio [initiator]:[monomer]strongly influences chain lengthThe ratio [initiator]:[monomer]strongly influences chain length
Moles of monomer consumedDP = Xn = —————————————————
Moles of chains produced
Pzn Reactions 4-8
AI A A A A A A A A A A A 1 1
8 8AAI A A A A A A A A A A
2 2AAI A A A A A A A A A A3 3AAI A A A A A A A A A A4 4AAI A A A A A A A A A A5 5AAI A A A A A A A A A A6 6AAI A A A A A A A A A A7 7AAI A A A A A A A A A A
12 12AAI A A A A A A A A A A
9 9AAI A A A A A A A A A A10 10AAI A A A A A A A A A A11 11AAI A A A A A A A A A A
AI A A A A A A A A A A A 0
Molcons Xn
2
4
6
8
10
2 4 6 8 10
Reaction Step Number
Xn
Degree of Polymerization
Pzn Reactions 4-9
Representative Chain-Growth Reactions
Free-Radical Vinyl PznsPoly(methyl methacrylate)
K2S2O8
H2C C
CH3
COOCH3
CH2 C
CH3
COOCH3
n
Carbocationic/Anionic Pzns
AlCl3
CH3Cl+H2C C
CH3
CH3
HC C
CH2
CH3
H2CCH2 C
CH3
CH3
98nCH2 CH CH2
CH3
C2n
Butyl Rubber (IIR)
Coordination PznsPolypropylene
TiCl3/AlCl3
Al(C2H5)2ClR2O
H2C C
CH3
H
nCH2 C
CH3
H
CH2 C
CH3
H
CH2 C
CH3
H
CH2 C
CH3
H
See also Table 3.2, pp 25-26
Reactant Compatibilities
Adapted in part from G. Odian, “Principles of Polymerization”, 4th Ed, 2004, p 200; and F.W. Billmeyer, Jr, “Textbook of Polymer Science”, 3rd Ed, 1994, p 83.
*Plus (+) sign = high polymer; minus (−) sign = no reaction or oligomer.
Monomer Radical Cationic Anionic
Mechanism*
Ethylene
α-Olefins (≥ C3)
1,1-Dialkylalkenes
1,3-Dienes
Styrene, α-Me styrene
+−
−
++
+−
+
++
−
−
−
++
Vinyl chloride
Tetrafluoroethylene
Acrylate/methacrylate esters
Acrylonitrile, methacrylonitrile
++++
−
−
−
−
−
−
++
Vinyl ethers
Vinyl esters
−
++−
−
−
Coord’n
++−
+++++++−
See also Table 5.1, p 124
Pzn Reactions 4-10
• Alkene, 1,3-diene monomers– Haloethylenes:
– Styrene:
– Acrylic, methacrylic acid derivatives:
• Free radical initiators– Benzoyl peroxide:
– Azobisisobutyronitrile (AIBN):
– K persulfate (peroxydisulfate):
• Thermal, redox, or photochemical initiation
Vinyl Free Radical Polymerization
H2C CH Cl
CH2CH
H2C CH CN C
O
H2C C OCH3
CH3
2NC C(CH3)2 N2 + N2NC C·
CH3
CH3
2∆
C
O
O2
C
O
O·2∆
O3SO2−
OSO3 2 O3SO·2−∆
Initiation
C
O
O2
C
O
O·2∆
CH2C
H
+I·
= I·
CIH
H
C·
H
Propagation
CIH
H
C·
H
CH2C
H
+ CIH
H
C
H
C
H
H
C·
H
Pzn Reactions 4-11
Termination
+ RHChainTransfer
ChainAddition
C
H
H
C
H
C
H
H
C·
H+ ·RC
H
H
C
H
C
H
H
C
H
H
C
H
H
C
H
C
H
H
C·
H
+ C
H
H
C
H
C
H
H
·C
H
+C
H
H
C
H
C
H
H
C
H
H C
H
H
C
H
C
H
C
H
C
H
H
C
H
C
H
H
C
H
C
H
H
C
H
C
H
H
C
H
Molecular Weight Control
• Most monomer is (or should be) consumed by polymer chain growth, not initiation.
• Termination involves reactions that stop polymerization.
• Chain transfer to monomer, solvent, or initiator kills the polymer chain but continues the kinetic chain (new chains can be initiated):
Mi· + X Mi + X·• Chain length is proportional to [M] and 1/[I]½:
• To maximize DP, increase [M]:[I] in the reaction mixture or increase K by the choice of monomer.
[M]Xn = K———
[I]½
[M]Xn = K———
[I]½
Pzn Reactions 4-12
Molecular Weight Control (cont.)
If chain transfer processes to both monomer (M) and solvent (S) are important:
The reciprocal of DP should be a linear function of the ratio of [S] to [M]. The slope would be a function of the rate of chain transfer to solvent.
The reciprocal of DP should be a linear function of the ratio of [S] to [M]. The slope would be a function of the rate of chain transfer to solvent.
If transfer to monomer is not important, then
1 1 [S]—— = —— + CM + CS——Xn Xn0 [M]
1 1 [S]—— = —— + CM + CS——Xn Xn0 [M]
Mayo (Mayo-Walling) eqn
1 1 [S]—— = —— + CS——Xn Xn0 [M]
1 1 [S]—— = —— + CS——Xn Xn0 [M]
Simplified Mayo eqn
where Xn0 is the DP you get with no chain transfer.
Styrene Pzn: Chain Transfer to Solvent
Adapted from R.A. Gregg & F.R. Mayo, 1947 Disc. Faraday Soc., 2, 328-337
5 10 15 20
[S]/[M]
50
100
150
105
——Xn
H
CH2CH3
CH3
CH
CH3
CH3
1/Xn0
Pzn Reactions 4-13
Styrene Pzn: Chain Transfer to Solvent
Mi· + X—H MiH + X·
CH2C ·H
P + CH2C
H
P HX—H + X ·
Function slopes show relative ease of radical (X·) formation:
C ·CH3
CH3
C ·H
CH3
C ·H
H
> >> ·
• Alkene, 1,3-diene monomers– Isobutylene (2-methylpropene):
– Isoprene (2-methyl-1,3-butadiene):
– β-Pinene:
• Lewis acid catalysts– AlCl3– BF3
– (CH3CH2)2AlCl
• Brønsted acid or carbocation-donor co-catalysts– HCl or H2O: BF3 + H2O F3B·OH2
– RCl: AlCl3 + (CH3)3CCl (CH3)3C+AlCl4−
Carbocationic Chain-Growth Pzn
H2C C CH3
CH3
CHH2C C CH2
CH3CH2
CH3
H3C
Pzn Reactions 4-14
Initiation
Propagation
F3B·OH2 F3B·OH− + H+
+ CH2C
CH3
CH3
F3B·OH− + H+ C+H3C
CH3
CH3
F3B·OH−
C+H3C
CH3
CH3
F3B·OH−
+ CH2C
CH3
CH3
CH3C
CH3
CH3
F3B·OH−
CH2
CH3
CH3
C+
Termination
ChainTransferCCH2
CH3
CH3
F3B·OH−
C
CH3
CH3
C+
H
H
+ F3B·OH2CCH2
CH3
CH3
C
CH3
CH3
C
H
Quenching
CCH2
CH3
CH3
F3B·OH−
C
CH3
CH3
C+
H
H
+ ··O RH ··
+ F3B·OH2CCH2
CH3
CH3
C
H
H
C
CH3
CH3
OR
Pzn Reactions 4-15
Molecular Weight Control
• Reactions are extremely rapid controlling heat transfer is important.
• Chain transfer to monomer is often significant.
• DP is independent of initiator concentration.
• For a typical exothermic polymerization, the reaction rate increases as the temperature is reduced.
• Net outcome:
DP increases at lower temperature
1 Ct—— = —— + CMXn [M]
1 Ct—— = —— + CMXn [M] Simplified Mayo equation
• Alkene, 1,3-diene monomers– 1,3-butadiene:
– Styrene:
– Methyl methacrylate:
– Vinyl pyridine:
• Catalysts: Lewis bases, organometallics– NH2
−
– CH3CH2CH2CH2Li (n-butyllithium)
–
Anionic Chain-Growth Polymerization
Na+·−
CHH2C CH CH2
CH CH2
CH CH2N
C
O
H2C C OCH3
H3C
Pzn Reactions 4-16
Initiation
CC4H9
H
H
C− Li+H
CC4H9
H
H
C
H
C
H
H
C− Li+H
CH2C
H
+LiC4H9
Propagation
CH2C
H
+CC4H9
H
H
C− Li+H
Molecular Weight Control
• Anionic polymerizations lack inherent termination processes (ion-pair rearrangements, anion-metal cation reactions).
• Monomer molecules only react with the anionic end groups on the propagating polymer chains.
• Polymer chains continue to grow as long as additional monomer is added. These chains are
Living Polymers
Pzn Reactions 4-17
Molecular Weight Control (cont)
[M]0DP = Xn = p———[I]0
where p is the fraction of monomer converted.
At complete conversion, DP is the monomer:initiator ratio at the outset of reaction:
[M]0DP = Xn = ———[I]0
[M]0DP = Xn = ———[I]0
Xw 1—— = 1 + ——Xn Xn
Xw 1—— = 1 + ——Xn Xn
The MWD (polydispersity) is therefore :
The MWD of a living anionic polymer will be extremely narrow, tending toward a value of 1 as Xn increases.
The MWD of a living anionic polymer will be extremely narrow, tending toward a value of 1 as Xn increases.
Anionic Pzn as a Route to Block Copolymers
• Living polymerization yields polymer chains with reactive end groups:
R—SSS—S−Li+
• These living chains can react with a second monomer to extend the chain and create a final product with unique chemical and physical properties:
R—SSS—S−Li+ + nB R—SSSS—BBB—B−Li+
• Block copolymers comprise two or more homopolymer subunits. These are referred to as n-block copolymers.
Commercial block copolymers include SIS (styrene-isoprene-styrene) and SBS (styrene-butadiene-styrene) triblocks.
Pzn Reactions 4-18
Controlling Stereochemistry
• Natural polymers have a high level of structural regularity:
• Synthetic polymers often contain many structures:
1,2-addition
C CCH3H
CH2H2CMj
Mi
3,4-addition
C CCH3H
CH2CH2Mi
Mj
trans-1,4-addition
C CCH3
H CH2
CH2Mi
Mj
cis-1,4-addition
C CCH3H
CH2CH2Mi Mj
C CCH3H
CH2H2C1
23
4
C CH
CH2
CH3
CH2
C CH CH3
CH2 CH2
C CH CH3
CH2 CH2
C CCH3H
CH2CH2
cis-1,4-Polyisoprene (NR)
STEREOCHEMISTRYSTEREOCHEMISTRYProperties that relate to a
molecule’s 3-dimensional structure.Properties that relate to a
molecule’s 3-dimensional structure.
Controlling Stereochemistry (cont.)
• Polymer structures depend on– Monomer structure and conformation (cisoid vs. transoid)
– Nature of active unit structure (free radical vs. anion)– Stability of active unit structure (cis-trans isomerization in 1,4-
addition)
Conditions
Free-radical, −20°C 0.01 0.90 0.05 0.04
Free-radical, 100°C 0.23 0.66 0.05 0.06
Anionic, RLi, 30°C 0.94 0.01 0.00 0.05
cis-1,4 trans-1,4 1,2 3,4
IR Structure, mol fraction
C CCH3H
CH2H2CC C
CH2H
CH3H2C
Pzn Reactions 4-19
Controlling Stereochemistry (cont.)
C CH
CH2
CH3
CH2P Li+
C CCH3H
CH2H2C −
C CH
CH2
CH3
CH2PLi+
C CCH3H
CH2H2C −
C CCH2H
CH3H2CC C
H
CH2
CH2
CH3P
·
Free-radical
C CCH2H
CH3H2C
C CH
CH2
CH3
CH2P Li+−
Anionic
1,4-trans addition
1,4-cis addition
1,4-cis addition
·C CCH2H
CH3CH2
C CH
CH2
CH2
CH3P
C CH
CH2
CH2
CH3P
·C CCH3H
CH2CH2
• Alkene, cycloalkene, 1,3-diene monomers– Propylene (Propene):
– 1,3-Butadiene:
– 5-Ethylidene-2-norbornene:
• Group 4B-8B transition metal catalysts– TiCl4– VCl4– Ti, Zr metallocenes
• Group 1A-3A metal co-catalysts/activators– (CH3CH2)3 Al (with TiCl4)
– [Al(CH3)O]n (with titanocene dichloride)
Ziegler-Natta Catalysis:Coordination Chain-Growth Polymerization
H2C CH2 CH3
CHH2C CH CH2
CH CH3
ClCl
Ti
Pzn Reactions 4-20
Ziegler-Natta Catalysts
• Active catalyst produced by reaction of transition metal component with Group 1A-3A co-catalyst:
TiCl4 + (C2H5)3Al Cl3Ti−C2H5 + (C2H5)2AlCl
• The concentration of active catalyst [C*p] is (0.01-10)n% of [transition metal component].
• Termination reactions are relatively unimportant. – DP tends to be high– Reactions must be quenched by active H sources (H2):
Cl3Ti−Mn−C2H5 + [H] Cl3Ti−H + H−Mn−C2H5
HH
CH3H
CC
Kinetics of Coordination Polymerization (cont.)
Adapted in part from E.J. Arlman, Jr, & P. Cossee, 1964 J. Catal., 3, 99-104.
Specificstereochemistry
Ti
Cl
ClCl
H3CH
Et
CH2
CCl
EtTi Cl
Cl
ClCl
CH3H
C
HH C
EtTi Cl
Cl
ClCl
Pzn Reactions 4-21
Stereochemistry of Polymerization
• The stereochemistry of polymer chain propagation controls the relative stereochemistry (tacticity) of the polymer.
• In ionic polymerizations, high stereoregularity can be achieved if there is strong coordination of the counter-ion with the terminal unit on the growing chain. (Promoted by low reaction T, polar substituent groups)
• Stereoregularity influences chain-chain interactions, chain stiffness, crystallizability.
TACTICITYThe relative stereochemistry of adjacent chiral centers within a macromolecule
TACTICITYThe relative stereochemistry of adjacent chiral centers within a macromolecule
ATACTICrandom configurations
ATACTICrandom configurations
C C C C C C C C C
H H H
H
H
HHH
HH
RR HH
R
R
R H
C
H
H
C
H
H
C
H
R
Pzn Reactions 4-22
ISOTACTICidentical configurations
ISOTACTICidentical configurations
C C C C C C C C C
H R H
H
R
HHH
HH
HH HH
R
H
R R
C C
H
HH
R
C
H
H
SYNDIOTACTICalternating configurations
SYNDIOTACTICalternating configurations
C C C C C C C C C
H H H
H
R
RHH
HH
HH HH
H
R
R R
C C
H
RH
H
C
H
H
Pzn Reactions 4-23
Stereoselective Polymerizations
Et2O·BF3 in propane, −80 to −60°C isotacticIsoC4 vinyl ether
PropyleneVCl4-Et3Al + PhOCH3 in toluene, −78°C syndiotactic
TiCl4-Et3Al in heptane, 50°C isotactic
Me methacrylaten-C4H9Li in THF, −78°C syndiotactic
n-C4H9Li in toluene, −78 to 0°C isotactic
ConditionsMonomer Structure
Adapted from G. Odian, “Principles of Polymerization”, 4th Ed, 2004, p 640.
Property AtacticPolypropylene
IsotacticPolypropylene
SyndiotacticPolypropylene
—— 160-171 130-160Melting point, °CCrystallinity, % 0 55-65 50-75
Comparing Step-Growth & Chain–Growth Pzn
Adapted from ExxonMobil Chemical “Polymer Technology Course” 2009.
Step-Growth Chain-Growth
Monomer addition
Reaction rate
MW growth
Process type
Examples
Any two species can combine (monomer orpolymer)
Rapid addition to small number of active centers
Much slower than chain growth Rapid and constant
Increases continuously as monomers react
High MW present even at low conversion
Batch Continuous, semi-batch
Nylon, PETE BR, IR, SBR, IIR, EPM, EPDM
Pzn Reactions 4-24
RING-OPENING POLYMERIZATION
• Chain-growth reactions. Proceed by relieving– Strain in small-ring monomers– Steric crowding in large-ring monomers
• Reactions are catalyzed by – Free radicals– Anions– Cations– Transition metal-carbene complexes
C
(C6H11)3PH
Ru
(C6H11)3P
Cl
ClExample:
(CH2)x
HC
C H
HC
RHC
HC
M
(CH2)x
HC
C H
n
(CH2)x
HC
C H
HC
RHC
HC
M+
(CH2)x
CHRHCHC
M
HCCHn+1
RING-OPENING PZN (cont.)
• Polymerizations follow two general pathways:– Ring opening of cyclic monomers: ethers (epoxides),
esters (lactones), amides (lactams), etc.
– Ring-opening metathesis polymerization (ROMP) of olefins: cycloalkenes, bicycloalkenes
n
Y
(CH2)x
nY(CH2)x Y = O, COO, CONH
METATHESISThe exchange or transposition of
bonds between two reactants
METATHESISThe exchange or transposition of
bonds between two reactants
Pzn Reactions 4-25
Representative Ring-Opening Pzns
Ring-Opening Pzn of EpoxidesPolyethylene glycol (PEG) from ethylene oxide
ROMP of BicycloalkenesPolynorbornene Rubber (PNR)
H2C
H2CO
HOCH2CH2OH
KOHn HOCH2CH2 OCH2CH2O CH2CH2OH
n−1
Ring-Opening Pzn of LactamsNylon-6 from ε-caprolactam
CH2
CH2
CNH
O
H2C
H2C
H2 C260°C
N2n C
O
CH2CH2CH2CH2CH2n
NH
RuCl3/HCl
C4H9OHn C
H
HC
n
≡CH n
CH
COPOLYMERIZATION
• Simultaneous polymerization of two or more mono-mers yields copolymers with unique properties unlike those of homopolymers from each monomer.
• Copolymers with irregular (random) monomer sequences can have rubber-like properties because of reduced crystallinity.
• Appropriate reaction conditions can provide a range of tailored structures:– Random (statistical) −ABBABAABABBAABAB−– Alternating −ABABABABABABABAB−– Block −AAAABBBBAAAABBBB−– Graft −AAAAAAAAAAAAAAAA−
BBBB BBBB
Pzn Reactions 4-26
Composition Control
• Monomers differ in their ability to copolymerize.
• The relative rates of monomer reaction are a function of the monomer concentrations ([M1],[M2]) and the monomer reactivity ratios (rn):
where kij is the rate constant for the reaction of monomer i with monomer j.
• Reactivity ratios determine copolymer composition and structure.
k11r1 = ——k12
k22r2 = ——k21
Free Radical Pzn: Reactant Ratios
Adapted from G. Odian, “Principles of Polymerization”, 4th Ed, 2004, pp 491-492.
r2Monomer 1 T, °Cr1 Monomer 2
Ethylene7
1.4
20
130
0
0.79
Acrylonitrile
Vinyl acetate
0.01 60
1.4 500.58
0.020Styrene 600.29
55 Vinyl acetate
1,3-Butadiene
Acrylonitrile
1.4 50.78
1.5Me acrylate 500.84
1,3-Butadiene
Acrylonitrile
Vinyl acetate 0.03 606.4
Pzn Reactions 4-27
Composition Control (cont.)
Limiting (or idealized) reactivity ratios:
• r1 = r2 = 0 Neither chains end adds its own monomer. Results: 1. a perfectly alternating copolymer and 2. an azeotropic copolymer composition (the ratio of monomers in the feed = the ratio of monomer units in the copolymer).
• r1 = r2 = ∞ Each chain adds only its own monomer. Result: a mixture of homopolymers.
• r1r2 = 1 or r1 = r2 = 1 All chain ends add either monomer with equal probability in an ideal copolymerization. Result: a random copolymer.
Composition Control (cont.)
• In most cases ri is not exactly 0, 1, or ∞; polymers tend toward a particular composition and type of structure.
• Comonomer sequences can be inferred from r1r2 values:– r1r2 < 1 Tendency toward alternating structure– r1r2 > 1 Tendency toward block structure– r1r2 >> 1 Tendency toward homopolymer structure
• When r1 >> 1 >> r2 there is copolymer composition drift.– At low conversions, the copolymer is enriched in the more
reactive monomer.– At high conversions, the copolymer is enriched in the less
reactive monomer that remains.
• Composition drift can be reduced by delayed or (semi-batch) continuous addition of the more reactive monomer.
Pzn Reactions 4-28
Composition Control (cont.)
In terms of the mole fraction (ƒ) of each monomer in the co-monomer mixture,
[M1]ƒ1 = ——————[M1] + [M2]
[M2]ƒ2 = 1 − ƒ1 = ——————[M1] + [M2]
the copolymer equation, expressed as the mole fraction (F) of each monomer unit in the copolymer chain, is:
r1ƒ12 + ƒ1ƒ2F1 = ————————————
r1ƒ12 + 2ƒ1ƒ2 + r2ƒ2
2
F2 = 1 − F1
These equations can be used to calculate the relationship of reactant feed composition to instantaneous polymer composition for various reactant ratios.
Copolymer Composition: Impact of r1r2
0.2
0.4
0.6
0.8
0.2 0.4 0.6 0.8
ƒ1
F1
compositiondrift
Styrene Vinyl Cl
17 0.02
r1r2 = 0.34
alternating
random
StilbeneMaleic
Anydride
0.03 0.03
r1r2 = 0.0009
Styrene Butadiene
0.78 1.39
r1r2 = 1.1
Polymerization 4-29
Exercise
The graph shows the feed and product compositions for the free-radical reaction of styrene (monomer 1)and acrylonitrile (monomer 2).
What feed composition do you need to get a copolymer that is 50% styrene?
a) About 55% styreneb) About 55% acrylonitrilec) About 25% styrene
What kind of copolymer will you get?
a) One that tends to have an alternating structure.
b) One that is enriched in styrene.c) One that tends to be a mixture of
copolymers.
0.2
0.4
0.6
0.8
0.2 0.4 0.6 0.8ƒ1
F1