Branching Enhances Multiple Pathways to Inhomogeneities in Polyester Gels from Radical Ring-Opening Polymerization

Polymer-network gels synthesized via radical ring-opening polymerization of cyclic ketene acetals (CKAs) display an intrinsic tendency to form highly branched networks, and the density of branching can be controlled by the monomer conversion and the identity of the cross-linker. This ability offers a design feature, as branching not only affects the local mesh scale but also propagates into mesoscopic network inhomogeneity. In this work, we investigate these effects using bulk polymerized poly-2-methylene-1,3,6-trioxocane gels that are cross-linked with either an additional bis-functional CKA or a divinyl ether in a concentration range of [CL] = 2–4 mol %. We use multiple light scattering techniques (SLS/DLS) in combination with multiple quantum NMR (MQ NMR) to characterize the gel structure on multiple length scales. Static light scattering reveals that the CKA-cross-linked gels bear two distinct static correlation lengths (ξ1 ≈ 10 nm; ξ2 ≈ 80 nm), whereas divinyl ether cross-linked variants only exhibit unimodal spatial inhomogeneity (ξ1 ≈ 10 nm). Dynamic light scattering substantiates these findings, resulting in two cross-linker concentration-dependent apparent diffusion coefficients in CKA-only gels of Dfast = 2.8–3.4 × 10–6 cm2 s–1 and Dslow = 4–8 × 10–10 cm2 s–1. MQ NMR indicates a significantly low fraction of non-elastic defects. The presence of residual anisotropy in a significant fraction of methyl groups was confirmed with site-selective MAS MQ NMR, thereby suggesting that most of the branches are short-chain branches. These complementary results indicate that bis-functional CKA cross-linkers can actively contribute to the network topology and introduce a second scaling of inhomogeneity into a polymer-network gel.