Analytical evaluation of stereoregularity in poly(2-N-carbazolylethyl methacrylate) by two-dimensional NMR spectroscopy

Poly(2-N-carbazolylethyl methacrylate) with varying degree of tacticities was synthesized by anionic and free radical polymerization. 2,2′Azobisisobutyronitrile (AIBN) and t-C4H9MgBr were used as initiators for free-radical and anionic polymerizations, respectively. Molecular weight distribution was determined by gel permeation chromatography (GPC). The samples obtained were analyzed for their stereochemical structure and tacticity utilizing various 1D (1H, 13C{1H}, DEPT) and 2D{HSQC, TOCSY, HMBC} NMR techniques. Signal assignments to different carbon resonances were done using 13C{1H} NMR in conjunction with DEPT-135 spectrum. The peak splittings for α-methyl carbon resonances were used to achieve tacticity assignments up to the pentad level. β-Methylene carbon of the main-chain showed configurational sensitivity up to the tetrad level. 2D heteronuclear single quantum coherence (HSQC) spectrum along with total correlation spectroscopy (TOCSY) further, helped in making complete spectral assignments and strengthening them. The 2D heteronuclear multiple bond coherence (HMBC) studies proved highly beneficial in resolving the configurational sensitivity of the non-protonated carbons viz., the quaternary and carbonyl carbon up to pentad level. The mechanism involved in the anionic polymerization of 2-N-carbazolylethyl methacrylate by the initiator t-C4H9MgBr was also investigated with the help of various NMR techniques. The validity of these assignments has been tested and confirmed by calculations using statistical relationships. The glass transition temperature (Tg) of poly(2-N-carbazolylethyl methacrylate) was found to be lower than the traditional photoconductive polymers which meant an easier processing.