Synthesis of high‐molecular‐weight poly(L‐lactide) initiated with tin 2‐ethylhexanoate

Abstract The bulk polymerization of L , L ‐dilactide was studied as a function of polymerization temperature ( T p ), time and concentration of catalyst (tin 2‐ethylhexanoate). Poly( L ‐lactide) (PLLA), with the highest value of intrinsic viscosity (η = 13 dl · g −1 ; M̄ v ≈ 1 · 10 6 ) and heat of fusion (Δ H m = 64,7 J · g −1 ), was synthesized at a low catalyst concentration (0,015 wt.‐%) and at the lowest T p studied (100°C), just above the melting point of L , L ‐dilactide (98°C). The ceiling temperature of PLLA was found to be 275°C, as deduced from an M̄ v max. − T p curve. The M̄ w /M̄ n ratios of as‐polymerized PLLA samples ranged from 2 to 3. Fractions of PLLA with M̄ v max. were already present at 50% conversion. The experimental results support a proposed nonionic insertion polymerization mechanism. Polymerization at 100–140°C resulted in early crystallization of PLLA leading to a rather untangled polymer and microporous (pores up to 100 nm) sample texture.