Cyclized methacrylic anhydride units copolymers

In the copolymerization of styrene (S) with methacrylic anhydride (Anh), three structures are incorporated into the resulting copolymers styrene units (S), uncyclized methacrylic anhydride units (U), and cyclized methacrylic anhydride units (C). 

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. 

For simplicity, possible reactions involving U units will be ignored. This is partially justified by the fact that the copolymerizations were only allowed to proceed to low conversions. In accordance with previous treatments of this copolymerization process, the following reactions and reactivity ratios are believed to be Important for determining copolymer compositions and structures. S refers to a styryl radical, A- refers to an uncyclized methacrylic anhydride radical and B refers to a completely cyclized methacrylic anhydride radical. 

The results of such calculations need to be "translated to obtain the relative concentrations of styrene units, Fs, cyclized metha-crylic anhydride units, Fc, and uncyclized methacrylic anhydride units, Fu, present In styrene-methacrylic anhydride copolymers. 

The copolymerization of methacrylic anhydride with styrene has been investigated by several groups. With the exception of Smets, et al. (12) it has been reported that the methacrylic anhydride units in the polymers are almost completely cyclized. This is also in accord with our experience. It proved difficult to separate unpolymerized methacrylic anhydride from the copolymers, and considerable effort was made to remove unreacted monomer from the polymers. 1H-NMR spectroscopy proved to be an effective method for distinguishing uncyclized methacrylic anhydride units present on the polymers from adsorbed monomer (see Experimental). 

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. 

In many samples, no resonances due to uncyclized anhydride units were detected. This was particularly true of copolymers with high styrene contents. Table I lists the compositions determined for styrene-methacrylic anhydride copolymers prepared at 40°. The styrene contents of the copolymers are in good agreement with those reported by Smets, et al., for copolymers prepared from comparable monomer ratios, but where the anhydride concentration was 2M. However, Smets, et al. report that 30-50 percent of the anhydride units were uncyclized in their copolymers and it appears that the extent of cyclization is better than 90 percent, generally about... 

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