A novel approach to the characterization of end groups in styrene-methyl methacrylate copolymers by pyrolysis-gas chromatography

The end groups of styrene-methyl methacrylate (St-MMA) copolymers polymeriz ed radically with 2,2'-azobisisobutyronitrile (AIBN) as an initiator, which are difficult to characterize even by NMR, were investigated by pyrolysis- gas chromatography. On the resulting pyrograms, characteristic products tha t formed from the end-group moiety due to AIBN, such as 2-cyanopropane, 2-c yanopropen, and various compounds consisting of an isobutyronitrile group a nd a monomer unit, were observed together with those from the main chain, s uch as St and MMA monomers and various dimeric and trimeric products. The r elative abundance between the recombination and disproportionation terminat ion reactions in the copolymerization process was estimated from the relati ve intensities between the characteristic peaks of the end group and those of the main chain. Thus, the estimated abundance for the termination reacti ons suggested that the polymerization process for this particular copolymer system terminated preferentially by recombination rather than by dispropor tionation. Furthermore, the relative abundance between the monomer units ad jacent to the chain-end AIBN residues was estimated on the basis of the pea k intensities of the products consisting of an isobutyronitrile group and e ither monomer unit, which reflected the penultimate neighboring structure o f the end group in the polymer chain. Thus, the observed results suggested that the isobutyronitrile radical formed by the dissociation of AIBN in the initiation reaction was predominantly adjoined by St monomer rather than b y MMA monomer. (C) 2000 John Wiley & Sons, Inc.