Synthesis of Polyester-Polystyrene Hybrid Gel by PET-RAFT Polymerization in “RAFT Gel” and the Role of Dynamic Covalent Bonding in Its Self-Healing Properties

We describe photoinduced electron/energy transfer reversible addition-fragmentation chain transfer (PET-RAFT) radical polymerization of styrene mediated by a gelatinous agent (“RAFT gel”). The resulting polymer possesses self-healing properties via dynamic covalent bonding. First, we synthesized poly[(butylene thiomalate)-co-(butylene adipate)] by chemoselective ternary polycondensation of thiomalic acid, adipic acid, and 1,4-butanediol. Next, we prepared gels in which cross-linking points are trithiocarbonates via the reaction of 1,1′-thiocarbonyldiimidazole with the pendant mercapto group in the parent polyesters. We named this type of gel, "RAFT gel" because the cross-linking points of the gel also act as RAFT agents. After swelling of this gel by a styrene monomer, we polymerized the monomer inside the gel by a PET-RAFT polymerization procedure triggered by irradiation with blue LED light for several hours catalyzed by tris(2-phenylpyridinato)iridium (III) [Ir(ppy)₃] as a photoredox catalyst. A previously reported method initiated by 2,2′-azobis(isobutyronitrile) reported a poor grafting efficiency (ca. 50%), but it was dramatically improved (>99%) here by using the PET-RAFT technique. We also found that the polymerization rate using RAFT gel was faster than using a synthesized low-molecular-weight trithiocarbonate model RAFT agent, TTC-I. After a two-step degradation of the prepared gel via aminolysis and subsequent saponification, we characterized the number-averaged molecular weight of the branched poly(styrene)s. Additionally, we performed compression tests in order to evaluate the gel’s self-healing properties via dynamic covalent bonds, targeting a new type of photosensitive glue for poly(styrene) foams.