Free radical copolymerisation of N-(4-hydroxy phenyl) maleimide with vinyl monomers:: solvent and penultimate-unit effects

N-(4-hydroxy phenyl) maleimide (HPM) was copolymerized with butyl acrylate (BuA), methyl methacrylate (MMA) and styrene in dimethyl formamide (DMF) and dioxane. The terminal model reactivity ratios were estimated using the method of Kelen-Tudos. In all cases, the penultimate model reactivity ratios were estimated by the Barson-Fenn method and by a computer-assisted non-linear regression analysis. The copolymerization behaviour was found to be dependent both on the electronic nature of the comonomer and on the solvent medium for a given monomer pair. The terminal model reactivity ratios revealed the statistical nature of copolymerization between HPM and BuA. The r(1) (HPM = M-1) value near zero and r(2) value close to unity indicated the tendency for MMA to form its sequences between HPM molecules in the copolymer at lower concentrations of the latter in the feed and to form alternating copolymer in HPM-rich feeds. The electron-rich styrene formed nearly alternating copolymer with HPM at all practical feed compositions, as evidenced from near-zero values of both r(1) and r(2). On changing the solvent from dioxane (THF in the case of styrene) to DMF, the apparent reactivity of HPM increased in the case of BuA and decreased in cases of both MMA and styrene. The increased reactivity of HPM in DMF in the former case can be due to possible aggregation of this monomer and its association with the propagating radical through hydrogen bonding. This association is interrupted in dioxane due to reciprocal H-bonding of HPM with this solvent. The enhanced reactivity in DMF can also be caused by the DMF-stabilized polarized and hence more electrophilic structure of HPM monomer or radical which has reduced preference for the electron-deficient BuA or its radical. This polarized structure has enhanced preference for the electron-rich MMA and styrene or their corresponding radicals which accounts for the reduced reactivity in these cases. The same polarized structure of HPM caused penultimate-unit effects for radicals terminated with MMA or styrene in DMF, although no such effect was observed in the case of BuA. In DMF, the reactivities of these radicals towards their corresponding monomers were several-fold enhanced when the penultimate group was HPM. In the cases of both MMA and styrene copolymerized with HPM in DMF, reasonable feed-copolymer composition profiles could be predicted only by the penultimate model. The monomer sequence distribution in the copolymers were calculated by the statistical method taking into account the penultimate-unit effect. The calculation showed that the penultimate-unit effect caused HPM-styrene system to deviate from the otherwise expected alternating copolymerization which caused some styrene diads in the polymer chain. (C) 1999 Elsevier Science Ltd. All rights reserved.