Uncyclized methacrylic anhydride

In the cyclopolymerization of methacrylic anhydride(MA) as a typical 1,6-diene, five- and six-membered ring anhydride structures can be formed, respectively corresponding to intramolecular hh and ht addition of uncyclized radical. The IR spectrum of poly-MA has been tentatively compared earlier with that of poly-MA prepared by dehydration of poly(methacrylic acid)(37) no five-membered cyclized anhydrides were detected, any absorption at higher frequency due to the strain of a five-membered ring being absent. 

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. 

The 1H-NMR spectra of the copolymers in DMSO solution were recorded and the ratios (X) of uncyclized methacrylic anhydride units to styrene units in the copolymers were determined from the relative intensities of the resonances at 6=5.72, 6.15 and 6.5-7.5 ppm. 

Figure 1. Olefinic proton resonances of styrene-methacrylic anhydride copolymers containing both uncycttzed methacrylic anhydride units and absorbed monomer (top), and uncyclized methacrylic anhydride units but little absorbed monomer (bottom). This sample was prepared by reacting a styrene-methacrylic acid copolymer with methacrylic anhydride.

H-NMR studies. Varian A-60 and HR-100 NMR spectrometers were used to measure the 1H-NMR spectra of styrene-methacrylic anhydride copolymers in DMSO-dg solution at 90° and of the derived styrene-methyl methacrylate copolymers in CCli, and C6D6 solution at 75-80°C. Solvent resonances interfered with composition determinations in the case of styrene-methacrylic anhydride copolymers, but the ratio of uncyclized methacrylic anhydride to styrene units (X) could be measured from the relative intensities of resonances observed at 6=5.72 and 6.15 ppm (olefinic protons) and at 6.5-7.5 ppm (aromatic protons). The compositions of the derived styrene/-MMA copolymers were calculated from the proportion of aromatic proton resonance observed in the spectra of copolymers dissolved in CClm as was described previously (6). Letting Y represent the ratio of styrene to MMA units in the derived copolymers, the compositions of the parent styrene-methacrylic anhydride copolymers were calculated as follows ... 

Mole fraction of uncyclized methacrylic anhydride units... 

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). 

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). 

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... 

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. 

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