Question

For each of the following, determine the number average sequence length for both monomers and draw approximate structures of the copolymers assuming you use 50% monomer 1 and 50% monomer 2 as feed in the polymerizations.

(a) maleic anhydride / methyl acrylate Data: r1=0.02 r2=2.5 r1r2=0.07

(b) butadiene / styrene Data: r1=1.40 r2=0.78 r1r2=1.1

(c) styrene / vinyl acetate Data: r1=55 r2=0.01 r1r2=0.55

Answer 1

(b)

НаС CH H2 H2 H2 H2 CH H H2 НС CH H2 H2 CH НС CH HC CH НС CH CH нС HCS НС: CH CH HC H CH нС C H

Styrene-butadiene or styrene-butadiene rubber (SBR). Its the most commonly used rubber in tyres.

(a)

H f-GH Men COOCH

MALEIC ANHYDRIDE - METHYL ACRYLATE COPOLYMER

(c)

Sr = Styrene, VA = Vinyl Acetate

It is known that, in the Sr–VA pair, a significant superiority in the reactivity of the first co-monomer (r_Sr = 55 and r_VA = 0.01) appears, which is why statements that formation of the co-polymer is impossible in this system can quite often be found in the literature.

r_Sr = 55 and r_VA = 0.01

This means that either radical has a much greater preference to add styrene than vinyl acetate. The first polymer formed consists mainly of styrene. This also means a faster depletion of styrene in the feed. As polymerization proceeds, styrene is essentially used up and the last polymer formed consists mostly of vinyl acetate. However, at 100% conversion, the overall polymer composition must reflect the initial composition of the feed. It follows that the copolymers generally have a heterogeneous composition except in very special cases.

When one r is < 1 and the other r >1 then, the two monomers have a tendency to polymerize consecutively; the first polymer is composed predominantly of the more reactive monomer; the other monomer polymerizes only after almost all of the more reactive monomer has been exhausted. The case of styrene–vinyl acetate exemplifies this type of copolymerization.

1.0 II 0.8 0.6 F1 0.4 0.2 III 0.8 0.2 04 0.6 1.0 f1

Graph showing the variation of the instantaneous copolymer composition F1 with monomer composition f1.

Case ii is styrene-vinyl acetate.

The range of feed composition that gives co-polymers with appreciable quantities of both monomers is small for Case II where the differences in reactivity ratios is large.