Styrene-methacrylic anhydride units

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 ... 

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). 

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. 

Since A- and B- entities can both become MMA-units in the derived copolymers, the calculation of sequence probabilities is more difficult than for the case of styrene-methacrylic anhydride copolymers there are often many combinations of A-, B- and S- entities that can yield a given sequence of styrene and MMA units. For example, an MMM triad can result from the following sequences ... 

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. 

Figure 3. Mole fraction of styrene units in styrene-methyl methacrylate copolymers %S (S/MMA) derived from styrene-methacrylic anhydride copolymers, as a function of the molar percentage of styrene (% S,) in the monomer mixture used to prepare the styrene-methacrylic anhydride copolymers. The solid line was calculated using rt = r, = 0.19, r, = 0.11, rc = 37 and rc = 7.7. Key O, 60°C polymerization and , 40°C polymerization.

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-... 

When the difference in the solubility parameters is greater than 2 hildebrand units, as in the case with methyl methacrylate and the styrene-maleic anhydride macroradical, the rate of formation of block... 

Side-chain polymers are usually prepared in the form of copolymers with NLO-active moieties attached on the backbone via flexible spacers such as methylene units. Examples are copolymers of methyl methacrylate and chromophore-substituted methacrylate monomers [53,61-63], poly(styrene-fy -acrylic acid ester) [54], and alternating styrene-maleic-anhydride copolymer [64], and there are many others. 

A special case of asymmetric enantiomer-differentiating polymerization is the isoselective copolymerization of optically active 3-methyl-1-pentene with racemic 3,7-dimethyl-1-octene by TiCl4 and diisobutylzinc [Ciardelli et al., 1969]. The copolymer is optically active with respect to both comonomer units as the incorporated optically active 3-methyl-l-pentene directs the preferential entry of only one enantiomer of the racemic monomer. The directing effect of a chiral center in one monomer unit on the second monomer, referred to as asymmetric induction, is also observed in radical and ionic copolymerizations. The radical copolymerization of optically active a-methylbenzyl methacrylate with maleic anhydride yields a copolymer that is optically active even after hydrolytic cleavage of the optically active a-methylbenzyl group from the polymer [Kurokawa and Minoura, 1979]. Similar results were obtained in the copolymerizations of mono- and di-/-menthyl fumarate and (—)-3-(P-styryloxy)menthane with styrene [Kurokawa et al., 1982],... 

Clearly, the relative reactivity of a monomer does depend upon the nature of the radical that is attacking it. Maleic anhydride is much more reactive than stilbene toward radicals ending in a stilbene unit, and stilbene is much more reactive than maleic anhydride toward the other kind of radical. (Indeed, these two compounds, individually, undergo self-polymerization only with extreme dilhculty.) A more modest—and more typical- tendency toward alternation is shown by styrene and methyl methacrylate. Here, toward either radical (- M ) the opposite monomer (M2) is about twice as reactive as the same monomer (Mj). 

PolyCACN) has a rigid chain structure yet can form excimers with alternate units along the chain (8), or by stacking in a helical conformation. Excimer formation has been reported for alternate copolymers of ACN with styrene (9) and for ACN with maleic anhydride CIO). The situation is different for 2-vinylnaphthalene since alternating copolymers of 2VN with methyl methacrylate or methacrylic acid did not form excimers, yet random copolymers of the same systems showed excimer fluorescence Cll). Only random copolymers of ACN were prepared in this work. 

The UV absorption in the 260 nm region is frequently used to evaluate styrene content in styrene-based polymers (2, 2, 3, 4, 5, 6, 7). Calibration curves for polystyrene solutions are usually based on the assumptions that the UV absorption of the copolymer depends only on the total concentration of phenyl rings, and the same linear relationship between optical density and styrene concentration that is valid for polystyrene holds also for its copolymers. These assumptions are quite often incorrect and have caused sizable errors in the analysis of several statistical copolymers. For example, anomalous patterns of UV spectra are given by random copolymers of styrene and acrylonitrile (8), styrene and butadiene (8), styrene and maleic anhydride (8), and styrene and methyl methacrylate (9, 10, 11). Indeed, the co-monomer unit can exert a marked influence on the position of the band maxima and/or the extinction... 

The copolymers obtained by radical copolymerization of maleic anhydride (MA) with acrylic or vinyl comonomers, and the maleic add copolymers, generally obtained by the hydrolysis of the maleic anhydride copolymers (Figure 10.1), can be called maleic copolymers. They were intensively studied from a theoretical perspective, but also for their applications [1-3]. Copolymers ofMA with electron-donating comonomers, such as styrene, vinyl acetate, N-vinyl pyrrolidone, and methyl vinyl ether, have an alternant structure [ 1 ], but when MA is copolymerized with electron-acceptor comonomers like methyl methacrylate, acrylonitrile, statistic copolymers are obtained [1,2]. MA units from the copolymers are very reactive active agents with amine or hydroxyl groups... 

Montaudo [11, 13, 14] also described a new method for fully characterising copolymers, which is based on off-line SEC-NMR and SEC-MALDI. It was applied to the analysis of random copolymers reacted at high conversions. The method involves fractionation of the copolymers by size exclusion chromatography, analysis of the fractions by NMR and MALDI mass spectroscopy and derivation of bivariate distribution of composition of the fractions. These copolymers include copolymers containing units of methyl methacrylate, butyl acrylate, styrene and maleic anhydride. Perspectives and limitations of the technique are also considered. 

Most monomers are above their free-radical ceiling temperature and only a single monomer unit is often added onto the polymer backbone. This is notably the case with maleic anhydride, acrylic acid, and methyl methacrylate. Styrene is an exception and one graft polymerize long polystyrene chains onto polyolefins [223]. 

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