Synthesis of poly(N-isopropylacrylamide) copolymer containing anhydride and imide comonomers – A theoretical study on reversal of LCST

The stimuli sensitive copolymer NIPAM-co-MI was prepared by copolymerizing NIPAM (N-isopropylacrylamide) with varying concentrations of maleimide (MI). The copolymer showed the same ratio of the monomeric components as that of the initial monomer feed ratio, with the two components arranged in the chain in a purely random sequence. Interestingly, the lower critical solution temperature (LCST) of NIPAM-co-MI was found to decrease with increase in MI loading in the copolymer. This behavior was in drastic contrast to the LCST behavior of a similar copolymer NIPAM-co-MA of NIPAM and maleic anhydride (MA) where the LCST showed an increase with increase in the MA concentration. A theoretical interpretation of the contrasting LCST behavior of both NIPAM-co-MI and NIPAM-co-MA was obtained by quantum mechanical (QM) modeling on small structural units of the polymers as well as molecular dynamic (MD) simulations at LCST and above the LCST on 50-unit oligomer model of the polymers. The QM models showed that the MI based polymer is more inclined towards bend structure, higher hydration, and higher intramolecular hydrogen bond formation between its monomer units when compared to those of the MA based polymer. The results of the large scale MD simulation was in complete support of the QM results as it showed the formation of a more folded and highly hydrated NIPAM-co-MI than NIPAM-co-MA.