Methyl methacrylate cotacticity

Pham, Q.T. The Cotacticity of (Acrylonitrile-Methyl-Methacrylate) Copolymer by NMR Spectroscopy. 4, 119-128 (1971). 

A study of the NMR spectra of (acrylonitrile-methyl methacrylate) copolymers (PAM), with very low percentages in M monomer, allows a quantitative determination of AMA and MMA or AMM triad sequences from the methoxy resonances. The resolution of the complex pattern of the a-methyl resonances of isolated M units has been attempted. If our assignments are correct, the analysis of the cotactic pentad sequences (with a central M unit) has revealed that the configurations of the copolymer chain do not follow the Bernouillian or the first order Markoffian statistics. 

As a technique for the analysis of sequence distribution in copolymers, high-resolution NMR spectroscopy is particularly useful when the spectral resolution is sufficient to resolve the resonances of the specific sequences. A number of copolymer structural problems can be elucidated by using NMR spectroscopy. The composition of the copolymer can be quantitatively determined. The detection of compositional dyads can be used to determine the distribution of composition, that is, whether the sample is a mixture of homopolymers, a block copolymer, an alternating copolymer, or a random copolymer (see Chapter 1). If resonances are resolved due of the triad sequences of the copolymer, sequence distributions can be determined, and the mechanism of the copolymerization can be tested in terms of Bemoullian, first-order Markov, second-order Markov, or non-Markovian statistics. In rare circumstances, the tactic nature of the copolymer can be determined if distinguishable syndio- and isotactic n-ad resonances are resolved. Such an analysis has been carried out for copolymers of methyl methacrylate-methacrylic acid, for which the a-CHs resonances of all 20 triads have been assigned and have been used to determine the cotacticity of the copolymer [22]. 

Gerken and Ritchey [190] reported the C-NMR of series of acrylonitrile-methyl methacrylate (A/M) copolymers and determined the copolymer microstructure. They reported the configuration probabilities of A/M copolymers and Bernoullian statistics were found to describe the monomer configuration, a result in conflict with the conclusions of Pham [191]. Gerken and Ritchey [190] assigned the resonance lines of carbonyl and nitrile carbons to different triad cotactic sequences on the assumption that M and A monomer units have the same sensitivity to the tacticity of an adjacent A monomer unit. Also they did not calculate cotactic triad fractions from C-NMR spectra and did not compare experimental cotactic triad fractions with those expected from configurational probabilities have been made. 

Kapur and Brar [192] report the C-NMR spectra of a series of acrylonitrile-methyl methacrylate copolymers. The primary structure, including monomer composition, monomer sequence distribution and triad tacticity of A/M copolymers were determined on the basis of C[ H]-NMR analysis and compared with the calculated fractions. The resonance of carbonyl and nitrile carbons were assigned to different cotactic triads by considering the possible electronic interactions between the central monomer unit and its immediate neighbours. 

This study has been extended to copolymers of a-fluorostyrene with methyl acrylate because of our interest in the cotacticity of statistical and alternating copolymers of styrene and substituted styrenes with acrylates and methacrylates. The present paper covers the 300 MHz 20 MHz C- and 282 MHz F-NMR studies of... 

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