Compositional and Stereochemical Analysis of Acrylamide/Vinyl Acetate Copolymers by One- and Two-Dimensional NMR Spectroscopy Munia Mukherjee and S. K. Chatterjee Department of Chemistry, Delhi University, Delhi 110 007, India A. S. Brar* and Kaushik Dutta Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India Received June 15, 1998; Revised Manuscript Received September 1, 1998 ABSTRACT: Acrylamide/vinyl acetate copolymers were prepared by solution polymerization using benzoyl peroxide as the initiator. The copolymer composition was determined from the percent nitrogen in the copolymers. The comonomer reactivity ratios determined using Kelen-Tudos and nonlinear error-in- variable methods are r A ) 9.28 ( 0.6 and rV ) 0.09 ( 0.006. The triad sequence distributions in terms of A- and V-centered triads have been obtained from 13 C{ 1 H} NMR spectra of the copolymers. The methine and the methylene carbon signals were assigned to compositional (triad, tetrads, and pentads) and configurational (triads) sequences. The broad and overlapping 1 H NMR spectra were assigned with the help of 2D inverse-HETCOR and TOCSY experiments. Introduction The macroscopic properties of bulk polymers are influenced fundamentally by their chain level micro- structure which can be investigated by NMR spectroscopy. 1-3 Complicated split resonance signals arising from higher order configurational or composi- tional sequences are usually observed in high-resolution NMR spectra. Two-dimensional NMR, especially the homonuclear (COSY) and heteronuclear (HETCOR) correlation spectroscopies, can be used as reliable techniques for assigning high-resolution spectra. 4-6 Poly(vinyl acetate) and copolymers of vinyl acetate are widely used in water-based paints, adhesives, paper coating, and nonwoven binders. Many workers have reported the sequence distribution in copolymers of vinyl acetate with ethylene, 7 vinyl alcohol, 8 alkyl acrylates, 9,10 etc. To the best of our knowledge, the microstructure of the acrylamide/vinyl acetate copolymer (A/V) system has not been reported. In this paper, we report the polymerization mechanism of A/V copolymers. The reactivity ratios of the comonomers were calculated using the nonlinear error-in-variable (EVM) method. 11 The microstructure in terms of triads sequence distribu- tion was obtained from the 13 C{ 1 H} NMR spectra of the copolymers. The methine carbon of the A unit was assigned to both compositional (triad) and configura- tional (triad) sequences whereas the methine carbon of the V unit showed compositional sequences. The 1 H NMR spectra which consisted of highly overlapping configurational and compositional sequences were com- pletely resolved with the help of 2D NMR spectroscopy. Experimental Section Acrylamide (SRL, India) was recrystallized twice from chloroform. N,N-Dimethylformamide (DMF; Merck, Germany) was distilled and dried. Vinyl acetate (CDH, India) was distilled under reduced pressure and stored below 5 °C. A series of acrylamide/vinyl acetate (A/V) copolymers containing different mole percents of acrylamide in the feed were prepared by solution polymerization using benzoyl peroxide as the initiator at 65 °C. The percent conversion was kept below 6% by precipitating the copolymers in acetone. The copolymers were further dissolved in dimethyl sulfoxide (DMSO), repre- cipitated in acetone, and vacuum-dried. The copolymer composition was calculated from the percent nitrogen of the copolymers. The C, H, and N analyses were done on a Perkin-Elmer 240C elemental analyzer instrument. The 1 H and 13 C{ 1 H} NMR spectra were recorded in DMSO- d6 at 100 °C on a Bruker DPX 300-MHz spectrometer operat- ing at 300.13 and 75.5 MHz, respectively. All of the two- dimensional experiments were carried out on a Bruker DPX 300-MHz spectrometer in DMSO-d 6 at 100 °C. Distortionless enhancement by polarization transfer measurements were carried out using the standard pulse sequence with a J modulation time of 3.7 ms (J CH ) 135 Hz) with a 2-s delay time. A two-dimensional proton-detected 1 H- 13 C hetero- nuclear chemical shift correlation spectrum (inverse-HETCOR) was recorded using the standard pulse sequence. A total of 32 scans were accumulated with a relaxation delay of 2 s for each of the 512t 1 experiments. The 2D homonuclear total correlation spectroscopy (clean-TOCSY) experiment was car- ried out at two mixing times i.e., 4 and 80 ms. A total of 16 scans were accumulated with a relaxation delay of 2 s for each of the 512t 1 experiments. All of the 2D spectra were zero filled to have a 2K × 1K data matrix while processing. All of the curve fittings were done using a nonlinear least-squares deconvoluting program. In all cases regression converged to  2 < 1. Results and Discussion Reactivity Ratio Determination. The composi- tions of acrylamide/vinyl acetate (A/V) copolymers were determined from the percent nitrogen of the copolymers. Table 1 shows the copolymer composition in the feed and in the copolymers along with the percent nitrogen in the copolymers. The copolymer composition data were used to calculate the terminal model reactivity ratios by the Kelen-Tudos 12 (KT) method. The values of the reactivity ratios are r A ) 9.27 ( 1.9 and r V ) 0.09 ( 0.02. These reactivity ratios served as the initial estimate for the calculation of reactivity ratios by the EVM method. The terminal model reactivity ratios determined are r A ) 9.28 ( 0.6 and r V ) 0.09 ( 0.006. Using the van Herk 13 method, the sum of squares of * To whom all correspondence should be addressed. 8455 Macromolecules 1998, 31, 8455-8462 10.1021/ma980940s CCC: $15.00 © 1998 American Chemical Society Published on Web 11/10/1998