Synthesis and characterization of poly (l-lactide/ε-caprolactone) statistical copolymers with well resolved chain microstructures

In view of the improved properties of elastomeric copolymers with a random distribution of sequences (randomness coefficient (R) → 1), several synthesis of statistical poly(l-lactide/ε-caprolactone) (PLCL) copolymers at a 75:25 mass feed ratio were conducted evaluating the polymerization conditions (temperatures, times of reaction and comonomer/catalyst (M/C) molar ratios). The mechanisms of reaction of both catalyst employed, SnOct2 and BiSS, differed in the coordination and activation of the monomers leading to different chain microstructures. BiSS was found to be less prone to react with the monomer with higher reactivity, i. e., lactide (LA), than SnOct2, and consequently, the desired random PLCLs (R > 0.85) were obtained. In contrast, the more reactive SnOct2 exhibits a tendency to favor blocky sequences (R < 0.6) that provide more capability to crystallize. The random BiSS-PLCLs showed crystallinity with lower melting temperature (Tm) than moderately blocky SnOct2-copolymers, being both influenced by the average lactide block length (lLA) obtained, a parameter affected by the LA content and R. The Tm values increase from 82 to 125 °C when lLA only rises from 3.3 to 5.4, whereas the corresponding Tm of the SnOct2-PLCLs is in the range of 148-163 °C despite the wide range of lLA (5.4-23.5). Also indicative of the high sensitivity to microstructural chain aspects of the copolymers, the crystallinity degree (xc) of the PLCLs grows in a logarithmic manner shifting quickly from 2.0% to around 20.0% when lLA increases from 3.3 to 5.0. Finally, the adhesion and proliferation of adipose derived stem cells (ADSCs) on some representative PLCLs were successfully performed with no signs of cytotoxicity despite in some cases of the presence of a slight residual amount of toxic tin.

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