Ethylene brassylate, a macrolactone derived from castor oil, can undergo ring-opening polymerization (ROP) to yield poly(ethylene brassylate) (PEB), a fully bio-based polyester. In this study, PEB was synthesized using reactive extrusion (REx), a technique that employs conventional melt processing equipment to facilitate chemical reactions, eliminating the requirement for organic solvents. Lipase B from Candida antarctica (CALB) was selected as a biocatalyst for ROP and was supported on microcrystalline cellulose by self-assembling in water. This approach served a dual purpose: (i) preserving enzymatic activity during melt processing at 90 °C and (ii) producing cellulose-PEB composites in situ. The presence of the cellulose support was essential for polymerization and complete monomer conversion, which was not achieved using the free enzyme. Furthermore, the role of enzymes for the degradation of PEB was investigated. PEB was susceptible to depolymerization into monomers and oligomers when incubated in a CALB-containing solution. Additionally, degradation was observed when CALB was embedded within the polymer matrix during polymerization. Beyond enzymatic degradation, industrial composting was explored as an alternative end-of-life strategy, demonstrating PEB disintegration (93% mass loss) within 90 days. On the basis of these findings, the ongoing research has the aim of embedding enzymes within biodegradable thermoplastic polymers to enhance their degradation in controlled environments, such as industrial composting. To enable enzyme incorporation via melt processing, bio-based materials such as lignocellulose could play a crucial role in preserving the enzymatic activity and ensuring polymer biodegradability.