Polymer Chemistry-7 CUI

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POLYMER CHEMISTRY – 7

COPOLYMERISATION

Shuang CuiImperial College London, UK



Different forms of copolymers

Alternating copolymer

Homopolymer

Periodic copolymer

Random copolymer

Block copolymer

Graft copolymer



Copolymers more than simple mix

Graft copolymer

Block copolymer

Graft copolymers are a special type of branchedcopolymer where the side chains are structurallydistinct from the main chain

Individual chains of a graft copolymer may be

homopolymers or copolymers

More than one kind of block can be made, tri or tetra are also common

Comprise of two or more linked homopolymer subunits.

Block copolymers with two or three distinct blocks are called diblockcopolymers and triblock copolymers, respectively.



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Copolymerisation

Copolymers are formed by simultaneouspolymerisation of two or more differentmonomers.

Copolymers can be produced by both stepcopolymerisation and chain copolymerisation.

The simplest step copolymersiation are thegeneral type ARB + AR’B or RA2 + R’B2 + R’’B2.



Step copolymerisation

ethylene glycol, terephthalicacid, and isophthalicacidpoly(ethylene terephthalate-co-ethyleneisophthalate)

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Statistical copolymers

Most step copolymerisation are taken to high extents of reaction in order to produce copolymers with suitable high

molar masses. A consequence of this is that the overallcompositions of the copolymers obtained correspond tothose of the comonomer mixtures used to prepare them.However, the sequence distribution of the different repeat

units along the copolymer chains is an important factorcontrolling the properties of copolymer and that thedistribution is affected by difference in monomer reactivity.




Block copolymerisationBy using as comonomers, low molar mass prepolymers withterminal functional groups, step copolymerisation can beused to prepare alternating block copolymers.For alternating copolymers prepared by such stepcopolymerisation, it is usual to call the block segments andthe copolymer segmented copolymers .

Soft segmentsAmorphous rubbery phase

Hard segmentsRigid crystalline phase



Chain copolymerisation

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Reaction RateRi1

Ri2R • + M2 RM2 •

ki2

R • + M1 RM1 •ki1Initiation

Propagation

M1• + M2

k12M2•

M2• + M1

k21M1•

M2• + M2

k22

M2•

M1• + M

1

M1•

k11R

11= k

11[M

1•] [M

1]

R12= k12[M1•] [M2]

R21= k21[M2•] [M1]

R22= k22[M2•] [M2]Termination M1• + • M1

k t11P R t11

M1• + • M2k t12

P R t12

M2• + • M2k t22

P R t22




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Rate of consumption of M1and M2

]M][M[+]M][M[=+=]M[

]M][M[+]M][M[=+=]M[

2222211222122

1221111121111

k k R R dt

d

k k R R dt

d

Instant composition of copolymer

d [M1]d [M2]

=k11 [M1 •] [M1] + k21 [M2 •] [M1]

k12 [M1 •] [M2] + k22 [M2 •] [M2]

Steady-state condition to [M 1• ] and [M2

d [M1 •]

d t= 0 d [M2 •]

d t= 0

r1 = k11 / k12

r2 = k22 / k21=

[M1]

[M2]

•r1 [M1] + [M2]

[M1] +r2 [M2]

d [M1]

d [M2]

Mayo-Lewis




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Instantaneous composition of feed and polymer• f1= mole fraction of M1in the feed• f2= mole fraction of M2in the feed• F1= mole fraction of M1in the copolymer•

F2= mole fraction of M2in the copolymer

f 1 = [M1] + [M2][M1]

f 2 = [M1] + [M2][M2] f 1 + f 2 = 1

F1 = d[M1] + d[M2]d[M1] F2 =d[M1] + d[M2]

d[M2] F1 + F2 = 1

F1 =r1 f 12 + 2 f 1f 2 + r2 f 22

r1 f 12 + f 1f 2




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r1= r2= 1, random copolymerF1= f1No preference for homo-polymerisation or copolymerizationr1= r2= 0, alternating copolymerF1= 0.50 < r1, r2< 1, azeotropic copolymerization1 << r1 and r 2<< 1, essentially homopolymer

1 << r1 and 1 << r2 , block copolymerisation

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Reactivity ratios

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Example - ABS

Acrylonitrile Butadiene Styrene

The nitrile groups from neighbouring chains attract each other and bind the chains together,making ABS stronger than pure polystyrene. The styrene gives the plastic a shiny, impervious

surface. The butadiene , a rubbery substance, provides resilience even at low temperatures.

Acrylonitrile : 15 ~ 35%

Butadiene : 5 ~ 30%

Styrene : 40 ~ 60%



Example - SBS

Styrene Butadiene Styrene

styrene styreneButadiene

Polystyrene is a tough hard plastic, and thisgives SBS its durability. Polybutadiene isrubbery, and this gives SBS its rubber-likeproperties. In addition, the polystyrenechains tend to clump together. This givesthe material the ability to retain its shapeafter being stretched.



The Pros and Cons of co-polymer

Advantage

An important process enables to have combined performance

Final material can be designed to cope with specific needs

Using different monomers to compensate individual weaknesses

Disadvantage

The reaction is apparently complex and time consuming

Good chance to have nasty byproducts or even toxins

Mix ratio has to be precise or having “ phase separation ”

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Polymer Chemistry-7 CUI