Mimicking (Linear) Low-Density Polyethylenes Using Modified Polymacrolactones

Pepels, Mark PF; Koeken, Ronald AC; van der Linden, Sjoerd JJ; Heise, Andreas; Duchateau, Rob
Macromolecules, 2015
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This paper presents a new approach toward the introduction of both short- (SCB) and long-chain branching (LCB) in polyethylene-like polyesters via the ring-opening polymerization of macrolactones. Macrolactones containing an alkyl (S1) or alcohol (S2) branch were obtained using radical thiol–ene chemistry of ambrettolide (Amb). Kinetic studies revealed the need for an excess of thiol to achieve a high conversion of the double bond. Even though homopolymerization of the three monomers Amb, S1, and S2 revealed comparable reactivities, the molecular weight buildup during polymerization of S2 differs drastically from that of Amb and S1. Instead of the linear increase of Mn with conversion observed for Amb and S1, the molecular weight buildup for the ring-opening polymerization of S2 resembles that of a step-growth polymerization—slow buildup at low and moderate conversion followed by a rapid increase in molecular weight at high conversions. This disparity was attributed to the possibility of S2 to function as both an initiator and a monomer, leading to oligomers during the first part of the reaction that are subsequently connected to each other at the final stage of the reaction. Copolymerization of pentadecalactone (PDL) with various ratios of Amb, S1, and S2 in bulk led to the associated random copolymers containing double bonds, short-chain branches, and long-chain branches. The trans-double bonds in poly(PDL-co-Amb) are included in the crystal lattice, leading to a slight decrease in the melting temperature, melting enthalpy and yield stress, while up to 20 double bonds/1000 backbone atoms the crystallinity and lamellar thickness remain similar to those of polypentadecalactone. In contrast, SCBs are fully excluded from the crystal lattice, leading to a more significant decrease in melting temperature and enthalpy as well as crystallinity and lamellar thickness with increasing branching density. The stiffness of these SCB-copolymers exponentially decreases as a function of branching content, effectively changing the mechanical behavior from semicrystalline to elastomeric. The LCB-containing polymers show an even larger linear decrease in melting temperature with increasing branching density than their SCB equivalents, likely due to the particular topology of the polymers consisting of a brush to a hyperbranched structure. However, a rapid decrease of molecular weight as was observed upon increasing the S2 content is also likely to play a role. The observed low molecular weight can be ascribed to both the fact that (macrocycles of) S2 can function as initiator, effectively increasing the amount of polymer chains, and the change of molecular weight buildup.

KEYWORDS: Crystal size, Polymers