Styrene-methacrylic anhydride reactivity ratios

Mathematical procedures for calculating structural features of cyclocopolymers and of copolymers derived from them are proposed and are used in studies on the 1H-NMR spectra of styrene-methyl methacrylate copolymers derived from styrene-(methacrylic anhydride) copolymers. Reactivity ratios and cycliza-tion constants for styrene-methacrylic anhydride copolymerization were determined from structural features of the derived styrene-methyl methacrylate copolymers. The amount of uncyclized methacrylic anhydride units present in styrene-methacrylic anhydride copolymers having high styrene contents is considerably less than that predicted by these copolymerization parameters. The methoxy proton resonances of the derived copolymers are more intense in the highest field methoxy proton resonance area than would be expected if such resonance were due only to cosyndiotactic SMS triads. Possible explanations for these discrepancies are proposed. 

Several theoretical treatments of cyclocopolymerization have been reported previously (8-11). These relate the compositions of cyclocopolymers to monomer feed concentrations and appropriate rate constant ratios. To our knowledge, procedures for calculating sequence distributions for either cyclocopolymers or for copolymers derived from them have not been developed previously. In this paper we show that procedures for calculating sequence distributions of terpolymers can be used for this purpose. Most previous studies on styrene-methacrylic anhydride copolymerizations (10,12,13) have shown that a high proportion of the methacrylic anhydride units are cyclized in these polymers. Cyclization constants were determined from monomer feed concentrations and the content of uncyclized methacrylic anhydride units in the copolymers. These studies invoked simplifying assumptions that enabled the conventional copolymer equation to be used in determinations of monomer reactivity ratios for this copolymerization system. 

Smets, et al. (12) noted that the compositions of styrene-methacrylic anhydride copolymers prepared from given styrene-methacrylic anhydride mixtures were independent of the amount of solvent present in the system and concluded that ri and r3 in the kinetic scheme outlined above must be equal. This conclusion, which was also accepted by Baines and Bevington (13), enabled reactivity ratios for this copolymerization system to be calculated by use of the standard copolymer equation. Unfortunately, this is not a valid conclusion the results in Table II show that for conditions similar to those employed in the previous work, the styrene contents of the copolymers are imperceptibly affected by dilution of the system, even when r3 is five times greater or less than ri. 

Due to the difficulty of working with styrene-methacrylic anhydride copolymers, we have elected to determine reactivity ratios and cyclization constants from the compositions and structures of styrene-MMA copolymers derived from these copolymers. As is discussed in the experimental section it is possible to measure the styrene contents and the proportions of methoxy proton resonance occurring in three different areas (designated A, B and C) from the 1H-NMR spectra of S/MMA copolymers. The proportions of methoxy proton resonance observed in the A (F ), B (Fjj) and C (Fc) areas obey the following relationships in conventional styrene-MMA copolymers (6 7). 

Reactivity Ratios and Cyclization Constants Estimated for Styrene-Methacrylic Anhydride Copolymerization... 

The reactivity ratios for styrene-aryl methacrylate copolymerizations [79 — 27] differ significantly from those for the styrene-MMA system, so that copolymers derived from the aryl methacrylate copolymers should have different structures (sequence distributions) than conventional styrene-MMA copolymers of equivalent composition. In the system used to prepare styrene-methacrylic acid copolymers [75], the monomer reactivity ratios are comparable to those of the styrene-MMA system, but the stereochemical structure of the conventional copolymers and of those derived from the methacrylic acid copolymers might be expected to differ. In addition, change of the copolymerization solvent can alter the reactivity ratios for the styrene-methacrylic acid system. Finally, styrene-MMA copolymers derived from styrene-methacrylic anhydride copolymers [22] were expected to have especially interesting structures. The tendency of the anhydride units to become incorporated into the copolymers as cyclic units is very high and there is a great tendency for styrene and cyclic anhydride units in the co-... 

These equations are similar to those assumed for the reactivity ratio determination. In contrast to what has been observed for conventional styrene-MMA copolymers, however, these equations indicate that a substantial proportion of the (SMM+MMS)-type resonance appears to occur in the C-area. The proportion of methoxy resonance observed in the C-area, in fact, exceeds P(SMS) by a substantial amount for many of the copolymers. This can be due to the assumption of an inadequate model for the copolymerization reaction, to the use of incorrect reactivity ratios and cyclization constants for the calculations or to an inadequate understanding of the methoxy proton resonance patterns of S/MMA copolymers. It is possible that intramolecular reactions between propagating radicals and uncyclized methacrylic anhydride units present on propagating chains result in the formation of macrocycles. Failure to account for the formation of macrocycles would result in overestimation of rc and rc and in underestimation of the proportions of MMA units in SMS triads in the derived S./MMA copolymers. This might account for the results obtained. An alternate possibility is that a high proportion (>50%) of the M-M placements in the copolymers studied in this work can be expected to have meso placements (], J2), whereas only a small proportion of such placements ( 20%) are meso in conventional S/MMA copolymers. Studies with molecular models (20) have indicated that the methoxy protons on MMA units centered in structures such as the following can experience appreciable shielding by next nearest styrene units. 

Table 2.13 (Section 2.16.5) gives the reactivity ratios for free-radical copolymerization of styrene with (a) butadiene, (b) methyl methacrylate, (c) methyl acrylate, (d) acrylonitrile, (e) maleic anhydride, (f) vinyl chloride and (g) vinyl acetate. For each of these copolymerizations calculate ... 

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