Effect of Temperature on Polystyrene Tacticity Through Para Aromatic Carbon Splitting in 13C NMR Spectroscopy

Stereoregularity and microstructure of polystyrene (PS) were investigated by split para carbon of the benzene ring demonstrated by 13C nuclear magnetic resonance (NMR) spectroscopy. Polystyrene was synthesized via bulk thermal polymerizations at 50, 150 and 250°C at low conversions. Assignment of all the peaks of para carbon in a triad sequence was achieved. Bernoullian statistics was applied for three PS samples and by comparing the experimental and theoretical results it is found that with increasing polymerization temperature the probability of meso is augmented. Furthermore, with increasing NMR acquisition temperature from 20 to 65°C, triad sequences were converted to pentad sequences which follow 1st-order Markov statistics. Finally, it is shown that different deuterated solvents and their mixtures affect peak resolution and splitting of syndiotactic and isotactic sequences. To investigate the effect of deuterated solvent, deuterated benzene and tetrahydrofuran (THF-d8) were also used and the spectra of polystyrene para aromatic carbon of benzene ring in these solvents were compared. There are three peak regions in CDCl3 at 20°C while, in THFd8, syndiotactic region is almost separated and isotactic region overlaps with the atactic region, and in C6D6 both syndiotactic and isotactic regions overlap. As a result, motions of racemic rich segments in THF-d8 are higher than those of meso and conversely the mobility of racemic rich segments in CDCl3 is lower than that of meso rich segments. In summary, in THF-d8 the upfield regions and in CDCl3 the downfield regions show better splitting.