The study, led by Watcharee Funfuenha and Wanich Limwanich of Rajamangala University of Technology Lanna, together with colleagues at Chiang Mai University, centered on a deceptively simple idea. Instead of using liquid titanium(IV) n-butoxide directly as an initiator, the researchers first allowed it to ring-open ε-caprolactone at 130 degrees Celsius with no solvent present, producing short living PCL chains that remain capped with reactive titanium-oxygen active centers. Three versions of these macroinitiators, dubbed TiCL2, TiCL4, and TiCL8, were prepared using 2, 4, and 8 mole percent of the titanium alkoxide respectively. Because the concentration of titanium determines how many chains grow from each metal center, the chain length and molecular weight of the macroinitiators increased as the titanium content decreased, giving the team a tunable family of initiators with precisely controlled architectures.

What distinguishes these materials from conventional initiators is that they are not merely catalysts but genuine macromolecular species, carrying living poly(ε-caprolactone) segments that can themselves seed further polymerization. The concept echoes earlier work with tin(II) macroinitiators, but titanium offers a compelling advantage: titanium compounds are generally regarded as low-toxicity and environmentally benign compared with organotin species, which have faced increasing regulatory scrutiny. By pre-forming the titanium-PCL adduct, the researchers create a species in which the reactive Ti-O bond sits at the end of an organic chain, ideally positioned to coordinate incoming monomer and insert it into the growing polymer, the classic coordination-insertion mechanism that underpins much of modern lactone polymerization chemistry.

To understand exactly how these macroinitiators behave, the team turned to differential scanning calorimetry, a technique more familiar for measuring melting points than for probing reaction kinetics. By monitoring the heat released as the polymerization proceeds under controlled heating rates of 4, 6, 8, and 10 degrees Celsius per minute, they were able, for the first time, to extract non-isothermal kinetic parameters for ε-caprolactone polymerization initiated by titanium macroinitiators. The approach follows recommendations from the ICTAC Kinetics Committee, the international body that sets standards for thermal analysis, lending rigor to the analysis. Intriguingly, only the most titanium-rich variant, TiCL8, proved effective at promoting the ring-opening polymerization under these dynamic heating conditions, suggesting that a minimum density of active centers is required for the reaction to proceed at an appreciable rate.

From the calorimetry data, the researchers applied the Kissinger-Akahira-Sunose isoconversional method, a well-established approach for extracting activation energies from experiments conducted at multiple heating rates. The results revealed a clear concentration dependence: the activation energy fell from 73.5 plus or minus 2.9 kilojoules per mole at 10 weight percent TiCL8 to 64.8 plus or minus 1.1 kilojoules per mole at 20 weight percent. In practical terms, a lower activation energy means the reaction proceeds more readily, requiring less thermal pushing to get over the energetic barrier. The frequency factor, determined by fitting an n-order reaction model to the data, followed the same trend, confirming that higher macroinitiator loadings genuinely accelerate the polymerization rather than merely shifting it within the experimental window.

The team went further, computing the activation enthalpy and activation entropy of the process through Eyring-type transition state analysis. At higher TiCL8 concentrations, the transition state formation was facilitated, but the activation entropy became more negative, indicating a more ordered activated complex. This is chemically intuitive: when many titanium centers coordinate monomer molecules simultaneously, the arrangement demands a greater degree of molecular organization than a sparse distribution of active sites would require. Together with a proposed coordination-insertion mechanism, in which the monomer coordinates to the titanium center before inserting into the Ti-O bond, these thermodynamic parameters paint a coherent picture of a reaction whose speed and efficiency can be dialed simply by adjusting the initiator architecture and loading.

Kinetics on a small scale, however, only matters if the chemistry survives scale-up. To test this, the researchers performed larger-scale polymerizations under the same solvent-free conditions using conventional heating. The outcome was striking: high molecular weight PCL was obtained from all three macroinitiators, with weight-average molecular weights reaching 7.18 by ten to the fourth, 7.81 by ten to the fourth, and 6.88 by ten to the fourth grams per mole for TiCL2, TiCL4, and TiCL8 respectively. Molecular weight in this range corresponds to robust, mechanically useful material suitable for film blowing, melt processing, and biomedical fabrication. Achieving it without any solvent not only eliminates a major source of waste and cost but also simplifies purification, since no residual reaction medium needs to be stripped from the product.

The green credentials of the process were then pushed further with microwave irradiation, an energy-transfer method that heats reaction mixtures volumetrically rather than through the vessel walls. Microwave-assisted polymerization sharply reduced reaction times and, crucially, rescued the performance of the less reactive macroinitiators. Under microwave conditions, TiCL2 and TiCL4, which had lagged behind in conventional runs, delivered PCL with weight-average molecular weights of 6.08 by ten to the fourth and 6.23 by ten to the fourth grams per mole respectively. This suggests that the energy profile of microwave heating preferentially benefits systems with fewer, less accessible active centers, potentially by promoting faster and more uniform activation of the Ti-O bonds. For manufacturers weighing energy costs against product quality, the ability to reach high molecular weights quickly at moderate power inputs could prove decisive.

The broader significance of the work lies in its convergence of several green chemistry principles into a single, practical platform. The monomer, ε-caprolactone, derives from a ring-strained cyclic ester that polymerizes without releasing small molecules, making the process atom-economical. The initiator is based on titanium, an abundant and comparatively harmless metal. The reaction proceeds in bulk, with no solvent, at a moderate 130 degrees Celsius, and can be accelerated further by microwave energy rather than prolonged heating. Each of these choices independently reduces the environmental footprint of PCL production; combined, they point toward a manufacturing route that could compete with conventional processes on cost while outperforming them on sustainability metrics.

There remain challenges ahead. The concentration dependence of the kinetics means that process designers must carefully balance initiator loading against molecular weight targets, since the macroinitiator chain length and the density of active centers pull in opposite directions. Understanding how residual titanium content affects the properties and biocompatibility of the final polymer will also be essential for medical applications. Nevertheless, by coupling rigorous non-isothermal kinetic analysis with demonstration-scale synthesis, the Thai team has provided exactly the kind of quantitative foundation that translates laboratory chemistry into industrial practice. As demand for biodegradable polyesters continues to climb amid global pressure on plastic waste, solvent-free titanium-catalyzed routes to PCL may well move from the pages of Polymer Bulletin into the factories of the circular economy.

Subject of Research: Solvent-free ring-opening polymerization of ε-caprolactone using titanium(IV) macroinitiators to produce biodegradable polycaprolactone

Article Title: Development of new titanium(IV)-macroinitiators towards the green and facile ring-opening polymerization of ε-caprolactone

Article References: Funfuenha, W., Cheechana, N., Meepowpan, P., Punyodom, W., & Limwanich, W. (2026). Development of new titanium(IV)-macroinitiators towards the green and facile ring-opening polymerization of ε-caprolactone. Polymer Bulletin, 83(12), Article 645. https://doi.org/10.1007/s00289-026-06707-9

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06707-9

Keywords: polycaprolactone, ring-opening polymerization, titanium(IV) macroinitiator, epsilon-caprolactone, biodegradable polyester, solvent-free synthesis, microwave-assisted polymerization, DSC kinetics, coordination-insertion mechanism, green chemistry, activation energy, polymer bulletin