Pulsed Laser Copolymerization of Styrene and Maleic Anhydride

Abstract

Pulsed laser polymerization (PLP) has been done for the free radical copolymerization of styrene (monomer 1) and maleic anhydride (monomer 2) at 25,35, and 50 OC in butanone (methyl ethyl ketone (MEK) and acetonitrile (Acn) using AIBN as photoinitiator. The mean values for the propagation rate constant, (k,), were determined and found to be independent of solvent. Copolymer compositions were measured as a function of comonomer ratio in an isothermal continuous stirred reactor using MEK as solvent over a temperature range from 60 to 140 C. The combination of these data and the data from PLP experiments were used in model discrimination by multivariate nonlinear regression. The best description of this copolymerization is obtained from the penultimate unit model. No convergence could be reached in a multivariate nonlinear regression of the combined data set to the complex participation model. The mean k, is well described by the Mayo-Lewis model, where copolymer composition shows some deviation from the model. Numerical inspection reveals that invoking complex participation leads to a good fit of the copolymer composition data but very bad agreement with the mean k, data. The comonomer pair showed several peculiarities compared to systems described in the literature so far. Extremely high molecular weight copolymer was formed during the PLP experiments, the amount being dependent, among others, on solvent. Furthermore, large amounts of oligomers, i.e., dimers and trimers, are formed under PLP conditions. Two possible explanations for the anomalies of this comonomer pair are discussed. First, diradical initiation could account for the observed phenomena. Laser pulses are thought to excite electron donor-acceptor complexes, resulting in diradicals, which may initiate copolymerization. Second, dark polymerization might give rise to formation of high molecular weight copolymer. Due to significant or even total light absorption, part of the monomer solution is unaffected by the laser pulses.