Synthesis and Characterization of Hyperbranched Polyacrylamide Using Semibatch Reversible Addition−Fragmentation Chain Transfer (RAFT) Polymerization

Hyperbranched polyacrylamides (b-PAM) were synthesized using a semibatch RAFT copolymerization of acrylamide (AM) and N,N ′-methylenebis(acrylamide) (BisAM) with continuous feeding of BisAM. Small amounts of chain transfer agent (CTA), 3-benzyltrithiocarbonyl propionic acid, with 1/20 to 1/50 molar ratios to BisAM were used to control gelation during the polymerization. Influences of BisAM addition rate, CTA to initiator molar ratio, and AM concentration were also systematically examined. Hyperbranched structures were analyzed by a triple-detector GPC and NMR measurement. The b-PAM polymers had branching densities from 7.2 to 11.7 branches per 1000 carbons, with contraction factors g′ of 0.295−0.416 and g of 0.201−0.290, weight-average molecular weights of 5.63 × 105 to 1.28 × 106 g/mol, and polydispersity indexes of 4.7−8.6. It was found that intramolecular cyclization was significant in the polymerization and a substantial number of cyclic structures were formed in the b-PAM samples. Fractionation of GPC traces having the same numbers of primary chains elucidated that branch points were randomly distributed along primary chain backbones. The semibatch process with combined use of BisAM and CTA provided an effective approach to prepare the b-PAM samples at high conversion under low CTA usage.