Development of the effective tin(II)-macroinitiators from the ring-opening polymerization of cyclic esters for utilizing in the production of the ultra-high molecular weight polyester

Development of the effective tin(II)-macroinitiators (SnLL and SnCL) was successfully carried out via the ring-opening polymerization (ROP) of l-lactide (LL) and ε-caprolactone (ε-CL) with 4.0 and 8.0 mol% of liquid tin(II) n-butoxide (Sn(OnBu)₂) initiator (SnLL4, SnLL8, SnCL4, and SnCL8) at a temperature of 130 °C. The synthesized tin(II)-macroinitiators were completely characterized by various techniques. The performance of the solid SnCL macroinitiators in the ROP of ε-CL was reported for the first time by using the non-isothermal differential scanning calorimetry (DSC) technique. Non-isothermal kinetics results revealed that the rate of ε-CL polymerization depended on the amount of Sn–O active center presented in macroinitiators. The activation energy (E_a) of the ROP of ε-CL with the prepared SnCL8 was lower than SnCL4. Our synthesized SnLL4 was better than other tin(II)-macroinitiators for small scale production of an ultra-high molecular weight poly(ε-caprolactone) (PCL) with number average molecular weight (M_n) up to 6.3 × 10⁵ g/mol and also showed a higher effectiveness than the conventional initiating system of tin(II) octoate (Sn(Oct)₂) and Sn(Oct)₂/n-butanol (nBuOH) under a similar synthesis temperature and time. The mechanism of the ROP of ε-CL with our macroinitiators was also studied and proposed through the classic coordination–insertion mechanism. The tin(II)-macroinitiators could be considered as the powerful initiator for the manufacturing of the biodegradable polymers applied for the ecofriendly materials that could support the sustainable management of the plastic waste in the future.