Renewable and Degradable Triblock Copolymers Produced via Metal-Free Polymerizations: From Low Sticky Pressure-Sensitive Adhesive to Soft Superelastomer

A series of thermoplastic elastomer (TPE) systems with an ABA-type triblock structure, derived from renewable resources, were prepared using an eco-friendly approach, subsequently developed to demonstrate industrial applications ranging from pressure-sensitive adhesive (PSA) to elastomer, and structurally broken by degradation process. First, alpha,omega-dihydroxy poly(delta-hexalactone)s (PHLs) as a rubbery block (B), which could be derived from vegetable-oil, were precisely synthesized with target Mn values of 30 and 60 kg mol-1 for desirable viscoelastic performance, using metal-free ring-opening polymerization (ROP) with an organic base catalyst. The end-hydroxyl groups of the PHLs were completely esterified with a chain-transfer agent (CTA). Second, the resulting macro-CTAs were initiated via reversible addition-fragmentation chain-transfer (RAFT) polymerization of lignin-based guaiacol methacrylate (GM) for a hard side block (A). Finally, poly(guaiacol methacrylate) (PGM)-PHL-PGM triblock copolymers were prepared with fPGM of 0.21 and 0.30. The clearly defined molecular structures resulted in controlled block sizes and a microphase-separated structure. The PGM-PHL-PGM(5-30-5) prepared without a functional additive showed low tack PSA performance based on the viscoelastic window, including a peel adhesion of 0.48 N cm-1 and a tack force of 0.06 N, comparable to those of commercial removable/repositionable tapes. PGM-PHL-PGM(15-60-15) exhibited features of a soft superelastomer, with an elongation at break (epsilon b) of >1500%, a tensile modulus of 2.07 MPa, and an ultimate strength at break of 3.09 MPa. Degradation of the PGM-PHL-PGM triblocks could be attributed to the hydrolysis of the poly(ester) PHL blocks up to 91-94% and the catalyst-free depolymerization of the RAFT-synthesized PGM blocks up to 56-64%.