~(13)C NMR STUDY OF THE STEREOCHEMICAL CONFIGURATION OF PMMA PRODUCED BY RADIATION-INDUCED POLYMERIZATION

A ~(13)C NMR method is developed to study the stereochemical configuration ofpolymethylmethacrylate(PMMA) produced by ~(60)Co γ--radiation--induced polymeriza--tion. Seven ~(13)C NMR lines are observed for the carbonyl carbon resonance withexcellent resolution at ambient temperature and a field of 20 MHz. A systematic investigation of the effects of radiation on the NMR spectra ofthe tactic sample shows that the spectrum of the skeletal carbon and that of theα--CH_3 are well resolved. The most striking aspect of the--resonance area isthat it is clearly resolved into pentads. The triad and pentad distribution can becorrelated with the equations of 2~2(mm) (γγ)/(mγ)~2=1. and 4~4 (1/2m~2-(1/2)m+1)~4(mmmm) (γγγγ)/(mmmγ+…+mγγγ)=1. Bernoullian statistics is applicable tothe description of the stereochemical configuration by a single parameter P_m=0.209. From the data of the triad distribution, the average sequence length ?, themeso linkage sequence length ?_m and the racemic linkage sequence length ?_γ arecalculated to be 1.27, 1.26 and 5.78 respectively. It is also observed that m--arran--gement is favored over γ--arrangement, which might be attributed to the differencein the activation free energy in the formation step.