Chapter 3 Free Radical Copolymerization

1 Introduction

1.1 Concept Homopolymerization —— polymerization of one kind of monomer; Copolymerization (addition polymerization) —— polymerization of more than one kind of monomer; New kinds of copolymer can be synthesized by copolymerization of different combination of various monomers. Copolymer generated from same monomers can have different compositions and topologies.

Examples Styrene Styrene + Acrylonitrile Styrene + Butadiene Styrene + Acrylonitrile+ Butadiene PS SAN SBR ABS Brittle, transparent, poor solvent toleration High impact, good solvent toleration High elasticity Engineering plastics Maleic anhydride Styrene + maleic anhydride No polymer SMA

1.2 Type of copolymer also obtained by other methods Normally prepared by other methods

1.3 Nomenclature of copolymer poly(ethylene-co-propylene) 乙烯-丙烯共聚物 poly(ethylene-r-propylene) 乙烯-丙烯无规共聚物 poly(ethylene-b-propylene) 乙烯-丙烯嵌段共聚物 poly(ethylene-g-propylene) 乙烯-丙烯接枝共聚物 poly(ethylene-a-styrene) 乙烯-苯乙烯交替共聚物

Key topics in copolymerization -ABABBAABABBAAABABA- Composition distribution the relative amounts of each monomer unit incorporated into the copolymer, A% or B% Sequence distribution the way in which these monomer units are arranged within a polymer chain; AA or AB

2. Copolymer composition

Essential Point The composition of a copolymer does not necessarily match the composition of the comonomers used in the reaction ( feed ratio 投料比). Ethylene-vinyl acetate copolymer vinyl acetate % property 10 ~ 20 plastic 30 ~ 40 adhesive 50 rubber

Possible propagation sequences in free-radical copolymerization

Assumptions in copolymerization the rate of the propagation be chain-length-independent. only the terminal unit of the polymer radical can affect its reactivity no side reactions, such as depropagation, monomer partitioning, and various forms of complex formation

Reactions in copolymerization

2.1 Copolymer composition equation The overall rates of consumption of monomers [M1],[M2] then follow these expressions: ignore the initiation The expression for instantaneous copolymer composition (F1/F2) molar fraction of M1 molar fraction of M2

Steady-state assumption

Mayo-Lewis Equation

Reactivity ratio （竞聚率） The ratio of rate constant of homopolymerization to copolymerization 1 homopolymerizaiton copolymerizaiton r Special cases: r1=0 and r2=0 Each comonomer prefers to react with the other. Perfectly alternating copolymer. r1 > 1 and r2 > 1 Each comonomer prefers to react with others of its own kind. Tendency to form block copolymers. r1 * r2=1 There is no preference due to the chain ends. Random incorporation of comonomers; “Ideal” copolymerization.

Examples Neither monomer homopolymerizes very well. Forms almost perfectly alternating copolymer of high MW. Alternating copolymerization is common in free radical reactions when one monomer has electron withdrawing groups and the other has electron donating groups.

Influnece of polymerization mechanism on r St(M1)-MMA(M2) Radical ： r1=0.52, r2=0.46; Cationic： r1=10, r2=0.1; Ionic: r1=0.1, r2=6;

Copolymer equation in terms of mole fraction Copolymer composition depends on not only the monomer feed, but the reactivity ratio as well.

2.2 Copolymer composition curve Variation of reactivity ratio r1= 0 or k11= 0 only copolymn. r1<1 or k11<k12 tend to co- than homopolymn. r1>1 or k11>k12 tend to homo- than copolymn. r1= or k11>>k12 only homopolymn. 1 r1 F is the function of both r and f

Typical copolymer composition curves 1. Ideal copolymerization (r1*r2=1) 0.1 0.2 0.5 1 2 5 10 r1>1 r1<1 butadiene r1 = 1.39, styrene r2 = 0.78

A. r1=r2=1 B. r1<1, r2>1 C. r1>1, r2<1 azeotropic copolymerization 恒比共聚 copolymer composition equals to monomer feed A. r1=r2=1 VDC (偏氯乙烯)- MMA, r1 = 1.0, r2 = 1.0 Acrylamide(丙烯酰胺)-丙烯酸甘油酯, r1 = 1.0, r2 = 1.0 Ethylene-VAc r1 = 1.07, r2 = 1.08 B. r1<1, r2>1 C. r1>1, r2<1

Characteristics of ideal copolymerization r1*r2=1 There is no preference due to the chain ends. Random incorporation of comonomers. "Ideal" copolymerization

St/MMA initiated by BPO/AlR3 Typical copolymer composition curves 2. Alternative copolymerization (r1=r2=0) St/MMA initiated by BPO/AlR3 Each comonomer prefers to react with the other. Perfectly alternating copolymer. copolymer composition independent on monomer feed If r2=0 and r1  0 [M2]>>[M1] [M2]~[M1]

Typical copolymer composition curves 3. Others r1<1 and r2 <1 r1>1 and r2 <1 r1>1 and r2 >1 block

Summary r1 > 1, r2 < 1 close to random r1 r2 = 1 random r1 = r2 = 0 alternative r1 < 1, r2 < 1 random r1 > 1, r2 > 1 block or mixture of homopolymer r1 > 1, r2 < 1 close to random

Azeotropic composition 恒比组成 Definition: To get solution, it should be that r1 and r2>1 or r1 and r2<1

Independent on feed ratio

2.3 Copolymer composition with conversion r1 > 1，r2 < 1 r1 < 1，r2 > 1   f1 or F1 conversion f1 or F1 conversion

r1< 1，r2 < 1 r1>1，r2>1 conversion conversion f1 or F1 f1

Copolymer composition with conversion Since f varies with conversion, F1 also varies with conversion. Therefore, the average copolymer composition should be obtained by integration Monomer before polym. Polymerized monomer Monomer after polym.

Average copolymer composition M0=1 M F1=f1=0.484 St-MMA f10=0.8, f20=0.2, r1=0.53, r2=0.56

Controlling copolymer composition By adjusting feed ratio When the f10 is close to azeotropic point, the copolymer composition slightly varies with conversion (<90%). When the f10 is far from azeotropic point, the copolymer composition greatly varies with conversion. Adding relatively active comonomer during polymerization. 苯乙烯-反丁烯二酸二乙酯共聚物瞬时组成与转化率的关系(r1=0.30, r2=0.07)

Controlling copolymer composition By controlling monomer conversion St-Bd copolymer composition with conversion Bd-AN copolymer composition with conversion

2.4 Copolymer microstructure and composition distribution -ABABBAABABBAAABABA- 1 2 3 4 An N(An)/W(An) % AAA AAB ABA ABB BAB BBB 1 4 3 Chain-length distribution Monomer sequence distribution

Monomer distribution and polymer propertiy A：random copolymer average of both polymer A and B ….AAABBABBAABBBABAAB… B：block copolymer mixture of polymer A and B …AAAAAAAABBBBBBBBBBBAAAAAAAAAAA…..

Chain-length distribution by Probability For formation of M1M1 from M1· so The probability to find block M2(M1)xM2 The probability to find block M1(M2)xM1

Number-fraction distribution Weight-fraction distribution

Number-average chain length On the other hand,

When r1 , r2 >>1, and [M1]~[M2] Special examples When r1 = r2 = 0 When r1 , r2 >>1, and [M1]~[M2]

Derivation of copolymer composition equation Terminal effect vs penultimate effect M1M1 vs M2M1 Depolymerization effect For copolymerization involving α-methyl styrene

3. Multi-component copolymerization

Ethylene-propylene-vinylidene norbornene Terpolymerization Three initiation reactions Nine propagation reactions Six termination reactions Six reactivity ratios Ethylene-propylene-vinylidene norbornene

4. Experimental evaluation of reactivity ratios

4.1 Method for measurement of reactivity ratio By fitting copolymer composition curve with various r1 and r2 Since the F is not sensitive to the variation of r, this method is not so good

Line cross Plot r2vs r1 with different [M1]/[M2] and F1/F2

Fineman-Ross method

4.2 Variation of reactivity ratio Effect of temperature Active energy of homopolymerization Active energy of copolymerization Since E11 and E12 are low, their difference is also low. So the reactivity ratio varies slightly with temperature. In the case of r1<1, indicating E11 > E12, as temperature increases, k11 increase more faster than k12, so r1 increases and approach to unit. In the case of r1>1, as as temperature increases, r1 decreases and approach to unit.

Effect of Pressure Same as temperature. Effect of solvent Effect of pH

5. Monomer structure and the copolymerization reactivity

5.1 Relative reactivity of monomer With respect to same radical, the monomer reactivity can be evaluated. To compare the reactivity of AN (M1), VAc (M2), St (M3) The reactivity of VAc is one fourth of that of AN The reactivity of St is 25 times of that of AN The reactivity of St is 100 times of that of VAc

The monomer reactivity decreases in the order of : St>AN>VAc (100: 4:1) So the 1/r1 can be applied to estimate the monomer reactivity. The larger the reciprocal of r1, the higher the reactivity of M1 compared to M2.

5.2 Relative reactivity of radical With respect to same monomer, the radical reactivity can be evaluated. To compare the reactivity of St· and VAc· k11 = 145 L/mols k22 = 2300 L/mols The reactivity of VAc· is 1586 times of that of St·

General rules Monomer with low reactivity (VAc) generates corresponding radical with high reactivity(VAc) Monomer with high reactivity (St) generates corresponding radical with low reactivity(St). The rate of copolymerization is mainly determined by the reactivity of the radical

5.3 Influence of substitute on the monomer/radical reactivity From Table 3-8 in the textbook Monomer reactivity Radical reactivity The substitute group affects reactivity of radical stronger than monomer

Resonance effect For CH2=CHX Resonance effect: strong weak Monomer reactivity: high low Radical reactivity: low high

Reaction tendency： RS· + M < RS· + MS < R· + M < R· + MS S: With resonance effect RS· + MS(•)< R· + M(•) Homopolymerization of monomer without resonance effect is faster than monomer with resonance effect     RS· + M(•) < < R· + MS (•) Copolymerization of M-M or Ms-Ms is better than M-Ms Reaction tendency： RS· + M < RS· + MS < R· + M < R· + MS

Can VAc and St be well copolymerized? Good! Poor!

Polarity effects Monomer with electron-withdrawing substitute has tendency to copolymerize with monomer with electron-donating substitute, in favor of forming alternative copolymer

Charge-transfer interaction Homopolymerization of the charge-transfer complex or donor-acceptor complex

Steric effect no influence on mono- or 1,1-disubstituted vinyl monomer 1,2-disubstituted monomer can be copolymerized with monomer with less steric effect. Steric effect plays a more important role in homopolymerization than copolymerization.

6. Q-e concept

Q-e scheme To simplify the process of determine reactivity ratios, a semiempirical relationship has been developed that expresses the ratios in terms of constants that are characteristic of each monomer but independent of comonomer. P1 and Q2 are measures of the reactivity of radical and monomer related to resonance stabilization, e1 and e2 are measures of the polarity of the radical and monomer

Assuming the e value of monomer and radical are same, Styrene was chosen as the standard and assigned value of Q=1.00 and e= - 0.80. Value of Q increase with increasing resonance stabilization, while e value become less negative as groups attached to the double bond become more electron attracting. The bigger the (e1-e2), the smaller the r1*r2.

Application of Q-e equation Anticipate the reactivity ratio of monomer by Q-e equation. Compare the reactivity of monomer by Q value: High Q value --- high reactivity Compare the polarity of monomer by e: With electron-withdrawing group, +e; With electron-donating group, -e; Anticipate the copolymerization results by Q-e value Copolymerization is difficult for monomer pair with big Q Copolymerization is easy and becomes ideal when Q and e values of monomer are close alternative copolymerization occurs when monomer pair with big e

7. Copolymerization rate

Chemically controlled termination Diffusion controlled termination

kp of copolymerization

Summary Nomenclature of copolymer Reactivity ratio and copolymer composition equation Copolymer composition curve Copolymer composition with conversion Control of copolymer composition Copolymer composition distribution The reactivity of monomer and radical Q-e equation