From polyethylene to polyester : influence of ester groups on the physical properties

Aliphatic long-chain polyesters (ALCPEs) were synthesized using the ring-opening metathesis copolymerization of ambrettolide and cis-cyclooctene followed by exhaustive hydrogenation, yielding saturated ALCPEs with methylene-to-ester ratios (M/E) varying from 15 to 223 and 8 (polyethylene), of which the ester groups were pseudo randomly distributed over the backbone of the polymer. The melting temperature of these ALCPEs showed an inverse proportional trend with respect to the amount of ester groups in the polymer structure, ranging from 132.1 °C (M/E = 8) to 91.5 °C (M/E = 15), of which the former is comparable to the melting temperature of high-density polyethylene. The crystallinity and the orthorhombic unit cell of the polymers did not significantly change with increasing M/E. Solid-state NMR was used to show the uniform partitioning of the ester groups over the crystalline and amorphous phase. Even though the lamellar thickness showed a decrease with increasing amount of ester groups, this did only partly explain the decrease in melting temperature. The main factor determining the decrease in melting temperature is the inclusion of ester groups in the crystal lattice, which causes the crystal lattice to be less stable.