Radiation‐initiated copolymerization of styrene with unsaturated esters

The kinetics of the γ‐ray‐initiated, gel‐forming copolymerization of styrene with the unsaturated ester containing 1.5 double bonds per molecule of number‐average molecular weight of 890 (derived from maleic anhydride, phthalic anhydride, and propylene glycol) have been studied with initial ester concentrations ranging from 65% to 2.4%. From experimentally determined gel fractions, rates of disappearance of styrene and ester unsaturation (measured up to complete conversion of ester unsaturation), and the degree of polymerization of the growing chains, a model is suggested based on a free radical copolymerization, assuming a single effective propagation and termination rate constant ( k p and k t , respectively), and bimolecular chain termination except between a pair of immobile gel radicals. Three ester concentration ranges were considered. At high ester concentrations (≥ 50%), styrene and ester enter the gel at a constant rate (molar ratio of styrene:ester double bond of 2:1), precluding application of conventional copolymer composition theory. The rate at which overall unsaturation, α, disappears, is given by d α / dt = ( k p 2 / k t ) M 2 ° where M 2 ° is the initial molar concentration of ester unsaturation. The rate at which styrene, α 1 , disappears is given by d α1 / dt = 2 / 3 ( k p 2 / k t ) M 2 °( M 1 ° + M 2 °)/ M 1 °. The rate at which styrene is consumed in the dilute esterstyrene solutions (⩽4.6%), where M 2 ° is negligibly small, is given by d α1 / dt = k p ( k 1 /2 k t ) 1/2 . Polymerization rates of the intermediate ester concentration ranges (10% to 30%) are higher than predicted from the simple model. An empirical rate expression is presented which describes these systems to 40% conversion of styrene: d α1 / dt = ( k p 2 / k t ) M 2 ° {[(1 + m )( r 1 + M )]/( r 1 + 2M )}, where M is the ratio of ester unsaturation to styrene in the starting mixture and r 1 is the reactivity ratio for styrene, defined in the usual manner. For initial conversions of ester unsaturation (to 10%) d α / dt = ( k p 2 / k t ) M 2 °.