Unraveling the Cause for the Unusual Processing Behavior of Poly(Butylene Succinate). Part 2. The Effect of Synthesis on the Processing Stability of Poly(Butylene Succinate)

Poly(butylene succinate) (PBS) has emerged as a promising bio-based material for packaging applications. However, linear PBS can form branched chain structures during continuous processing, as we have previously shown for commercial samples (see J. Hallstein, et al., J. Appl. Polym. Sci. 2021; 138:e50669). Fumaric acid moieties were identified as "olefinic defects" and thought to lend PBS the susceptibility for branching. This postulate is challenged by new findings we describe in this article: "self-synthesized" PBS without fumaric acid moieties can also form branched polymer chains, as rheological measurements prove. A mechanism is hypothesized which relates the branching susceptibility of the as-prepared PBS to butenyl-terminated PBS chains, as suggested by the correlation between varying reaction conditions, namely PBS synthesis under mixing conditions with different shear stresses, and results from extrusion studies. 1H NMR clearly shows that PBS prepared under mixing with negligible shear stress contains virtually no butenyl-terminated PBS chains, leading to PBS samples not prone to branching. In contrast, PBS prepared under forced conditions bears butenyl end groups and is prone to branching. This work demonstrates the importance of inspecting the origins of non-intended side effects carefully, as polyester syntheses are typically conducted under shearing.