Structure and dynamics of poly(ethylene-co-1,5-hexadiene) as studied by solid state ¹³C NMR and quantum chemical calculations

Poly(ethylene-co-1,5-hexadiene) with 1,5-hexadiene (HD) contents of 1.8, 8.1, 9.7 and 20.3% was prepared by copolymerization of ethylene and HD involving intermolecular cyclization. Higher-order structures and dynamics of these samples were studied by solid state NMR and quantum chemistry. The ¹³C solid state NMR spectra and ¹³C spin-lattice relaxation time (T₁) of the samples were measured. The observed ¹³C CP/MAS and PST/MAS NMR spectra for all samples were decomposed into six peaks. The cyclopentane units incorporated in the main chain of polyethylene affected not only the crystalline structure but also the noncrystalline structure. This causes a trans-rich conformation in the noncrystalline region. Even in the melt-quenched samples, incorporation of the cyclopentane structure into the polyethylene chain suppresses the increase in the gauche structure in the noncrystalline region. Based on the ¹³C chemical shift of the methylene carbon, the low cyclopentane content sample assumes an orthorhombic crystal structure, and the high cyclopentane content samples assume a hexagonal-type chain packing. ¹³C spin-lattice relaxation times show that the crystalline region of the low cyclopentane content sample has two regions with different mobility, although the high cyclopentane content samples have only one region with a high mobility for each peak. Furthermore, quantum chemical calculations for the ¹³C NMR shieldings were carried out for precise assignment of the peaks.