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Highly branched poly( N -isopropylacrylamide) (PNIPAM) compounds were prepared by copolymerization of 3 H -imidazole-4-carbodithioic acid 4-vinylbenzyl ester, 1, with N -isopropylacrylamide (NIPAM) using reversible addition−fragmentation chain transfer (RAFT) polymerization. The polymerizations proceeded well with few side reactions. An increase in the content of 1 in the monomer feed appears to increase the number of branch chains, and at the same time no evidence was obtained for the presence of substituted acrylamide chain ends that may potentially result via elimination of the dithioate group. The polymer products show a clear tendency to increased molecular weight as the extent of conversion of monomer increases, while size exclusion chromatography (SEC) profiles indicate a complex distribution of molecular weights compared to linear polymers obtained with a non-RAFT carboxylate monomer. Both NMR and viscometry indicate that, as expected, increasing the amount of 1 in the feed has the effect of increasing the degree of branching in the final product. This increase in branching reduces the intrinsic viscosity of the solutions of the highly branched polymers compared to similar linear polymers. Poly( N -isopropylacrylamide) displays a lower critical solution temperature (LCST) in aqueous solutions, and cloud point data indicate a clear effect of chain architecture on the temperature at which this transition occurs. Thus, a set of linear analogous copolymers have LCST's that, for equivalent mole fractions of imidazole content, are higher than the similar highly branched polymers. However, on complexation of copper by the imidazole groups the LCSTs of the linear and highly branched sets cannot be differentiated.
Carter et al. (Sun,) studied this question.