NMR studies of polymer solutions. III. Conformational transition study of oligomeric polyethylene in mixed solvents

The intramolecular structure of oligomeric polyethylene (PE) in solvent mixtures of α-chloronaphthalene and carbon tetrachloride, α-chloronaphthalene and deuterated octane, and α-chloronaphthalene and deuterated hexadecane was studied at 35°C. by a high-resolution nuclear magnetic resonance technique. It was clearly shown that the n-alkanes show no detectable selective solvent absorption in these systems. The conformational structure (P_p), which was formed in “bulky” aromatic solvents when the chain length was greater than 16, was significantly destroyed by the presence of carbon tetrachloride, octane, or hexadecane in the above-mentioned solvent mixtures. Therefore, the “bulky” aromatic solvents, such as α-chloronaphthalene and 9-chloroanthracene, can be classified as P_p-structure-promoting solvents, in which the PE remains in the P_p conformation. In contrast, carbon tetrachloride and linear alkanes are P_m-structure-promoting solvents, in which the PE does not exhibit any P_p structure but is in the disordered P_m conformation. It is speculated that the P_p structure is caused by a “chain-fold” mechanism.