Solvent-free rac-lactide polymerization: Controlled organocatalytic synthesis and real-time dielectric monitoring

A controlled solvent-free ring-opening polymerization of rac-lactide (rac-LA) was investigated under bulk conditions using benzyl alcohol (BnOH) as initiator and limited loadings (up to 0.2 mol%) of the organocatalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). The reaction was performed at 403 K, i.e., just above the melting point of rac-LA, where a homogeneous melt is maintained while avoiding crystallization that hampers polymerization at lower temperatures. Three systems were investigated: [rac-LA] 0 /[BnOH] 0 /[TBD] 0 = 50:1:0.1, 100:1:0.2, and the catalyst-free reference 50:1. The TBD-catalyzed polymerizations reached nearly quantitative monomer conversion (∼95%) within 6 h for 50:1:0.1 and ∼96% conversion after 30 h for 100:1:0.2, and yielded poly(D,L-lactide) (PLA) with number-average molecular weights ( ) up to ∼15,000 g mol -1 and low-to-moderate dispersities ( Đ = 1.3–1.5). The close agreement between experimental and theoretical molecular weights supports the controlled character of the process despite the limited catalyst loading. Chain extension of the obtained PLA with ε-caprolactone afforded a higher-molecular-weight copolymer with similarly low dispersity and molecular weight consistent with the theoretical prediction, confirming preserved chain-end fidelity, pseudo-living character, and the potential for further tuning through copolymerization with other monomers to access materials with tailored properties. Simultaneously, dielectric data collected during polymerization established broadband dielectric spectroscopy (BDS) as an in situ platform for real-time monitoring of the process. Analysis of the static permittivity and temperature-dependent dielectric loss spectra enabled simultaneous tracking of monomer conversion, polymerization kinetics, segmental dynamics, and evolution throughout the reaction. The study thus combines an efficient low-TBD route to controlled bulk PLA synthesis with a dielectric methodology that captures, within a single experiment, the evolution of reaction kinetics, conversion, segmental dynamics, and molecular weight during polymer formation.