Influence of chain transfer agent on the cross-linking of poly(n-butyl methaerylate-co-N-methylol acrylamide) latex particles and films

The effects of the concentration of carbon tetrabromide (CBr4) chain transfer agent on the cross-linking of films formed from a ca. 44 wt % n-butyl methacrylate and N-methylol acrylamide copolymer latex were studied. The use of CBr4 greatly decreased the amount of gel in the polymer obtained after the emulsion polymerization process. Gel content measurements in toluene show that there is a limiting value of the gel fraction after thermal curing, which depends on the amount of CBr4. Calculations using the equations developed by Miller and Macosko show that this is not because of insufficient N-methylol acrylamide cross-linking agent in the copolymer. In fact, only ca. 10% of the N-methylol acrylamide groups on the polymer chains seem to result in cross-links and contribute to the gel content. This could be due to the constraint on movement limiting the ability to meet and react with the hydroxyl groups on other chains, once the polymer molecule becomes incorporated in the cross-linked network. The thermally cured copolymer film swells to a much lower extent than expected from statistical calculations. This indicates a nonrandom distribution of N-methylol acrylamide groups in the polymer particles obtained by the monomer-starved semibatch process, giving a copolymer film with a high degree of cross-linking at the interparticle boundaries and lower values in the interior of the particles.