Methylene carbon resonance polystyrene

T.K. Chen, T.A. Gerken, and H.J. Harwood, Methylene carbon resonance spectra of eplmerized isotactic polystyrene,... 

Several different tetrad assignments have been proposed for the methylene carbon resonances. Except for the mmm assignment, which could be made from the spectr m of isotactic polystyrene, the assignments are based on efforts to fit the observed resonance intensities to calculated stereosequence distributions, assuming a particular statistical model, coupled with the requirement that "necessary n-add relationships" were held in all cases. One group [73] was led to conclude that polystyrene contains about 30 percent meso dyads and that 1st order Markoffian statistics are necessary to characterize the stereosequence distributions. Randall [74], however, concluded that polystyrene contains about 55 percent meso dyads and that Bernoullian statistics are adequate to characterize the tetrad distributions. These groups make totally different assignments for the methylene carbon resonances. 

The C-1 aromatic carbon resonances of polystyrene are even more complex than the methylene carbon resonances. Randall reports the resolution of as many as 20 separate resonances and declined to attempt structure assignments. Matsuzaki and coworkers [75] made pentad assignments, assuming a Bernoullian distribution of stereosequences (a== 0.3). These differed from assignments made by Inoue, Nishioka, and Chujo [73] who assumed 1st order Markoffian statistics (m=0.3). Model compound studies [77] tend to support the Matsuzaki assignments, but a convincing interpretation of the C-spectrum of polystyrene is yet to appear. 

The C-13 spectrum of polystyrene, shown In Figures 2 and 3, contains two regions where stereochemical Information can be extracted. There are nine methylene resonances and at least 20-22 aromatic quaternary carbon resonances. No other carbons In polystyrene e lblt a configurational sensitivity. Tentative assignments have been made for the methylene carbons based on an assumed Bernoulllan behavior (IJ). 

The resonances of methylene carbons and of the methine carbons of MA units (6 =45-32 ppm are partially overlapped but are sufficiently well defined that assignments may be made based on dyad distributions and on MA-centered triad fractions calculated for the copolymers and on assignments made previously for polyMA and polyaFS The three sharp signals observed between 6(,=46-32 ppm in Figures 7A, 7B and 8A are attributed to methine carbons because the resonances of such carbons are narrower than those of methylene carbons in the spectra of polystyrene and poly(alkyl... 

A momber of authors have reported C-spectra of polystyrene. The methylene and quaternary aromatic (c-1) carbon atom resonances are rich in stereosequence information, but they are difficult to interpret. 

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