Kinetics of Branching Reactions

The information available up to 1965 on the polymerization kinetics of vinyl acetate has been reviewed by Lindemann 184% who collected data on the transfer coefficients with monomer and polymer, which may be denoted as Clm and Clp respectively. These are the ratios of rate constants for attack on monomer or polymer (per monomer unit) to the propagation rate constant they are the 

The published figures for Ctm are in general fairly consistent values at 60° C in the range 2-3 x 10 4 have been found by several workers (142,143,179,184, 186). There is a bigger spread in the values reported for Clp, from about 1.2 to 7.0 x 10 4 In some treatments of the problem it is assumed that all terminal double bonds, produced by transfer to monomer, can copolymerize, but that their reactivity is some fixed fraction K of the reactivity of the double bonds of the monomer K is found to be about 0.8 at 60° C. Graessley (143) has assumed as an alternative that only a fraction h of these double bonds can copolymerize at all (presumably those formed by transfer at a monomer acetate group), and that these have the same reactivity as the monomer h was found to be 0.76 at 60° C and 0.88 at 72° C. 

The amount of branching at the acetate group (whether in monomer or polymer) estimated from the change in MW or [ ] produced by hydrolysis and reacetylation was found to be about 70% of the total (143) the changes in M , Mw, and [ /] indicated 63%, 75%, and 70%, respectively. However, unless this fraction is the same for transfer with both monomer and polymer, which appears from the results of Nozakura and co-workers mentioned below not to be the case, this fraction will vary with conversion. 

It is usually assumed that the transfer coefficients, which are the ratios of rate constants for the transfer reactions concerned to that for the propagation reaction, are independent of conversion in estimating them from the dependence of MW averages or [rj] on conversion, this assumption is made implicitly. It is known (187) that the rate constants for propagation and termination vary with the conversion, especially at low temperatures the termination reaction, which 

Nagasubramanian, Saito, and Graessley (44) have carried out further experimental work on branched PVAc, using the known kinetic constants to 

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