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