Theoretical study on the mechanism and kinetics of ring-opening polymerization of cyclic esters initiated by tin(II) n-butoxide

Computational evaluation of zirconocene catalysts for ε-caprolactone cationic ring-opening polymerization

This quantum chemical study presents the ligand effect and a structure-property relationship in the cationic ring-opening polymerization (CROP) of ε-caprolactone using zirconocene catalysts. We first examined the effects of catalyst structure on the initiation and chain propagation steps of the CROP process. A total of 54 catalyst structures were investigated to understand the influence of the ligand structure on the stability of the catalyst-monomer complex and polymerization activity. The properties of the catalysts were analyzed in terms of ancillary ligands, ligand substituents, and bridging units. Calculations showed that the polymerization follows a proposed cationic mechanism, with ring opening occurring via alkyl-bond cleavage. A correlation between complex stability and activation energy was also observed, with ligand substituents dominating in both steps. While the ancillary ligands directly affect the HOMO energy level, the bridges are mainly responsible for the catalyst geometries, resulting in reduced complex stability and higher activation energy for the propagation step. This study contributes to a better understanding of the structural characteristics of zirconocene catalysts, which offers guidance for improving CROP activities in lactone polymerization.

Wood Cellulose Nanofibers Grafted with Poly(ε-caprolactone) Catalyzed by ZnEu-MOF for Functionalization and Surface Modification of PCL Films

Renewable cellulose nanofiber (CNF)-reinforced biodegradable polymers (such as polycaprolactone (PCL)) are used in agriculture, food packaging, and sustained drug release. However, the interfacial incompatibility between hydrophilic CNFs and hydrophobic PCL has limited further application as high-performance biomaterials. In this work, using a novel ZnEu-MOF as the catalyst, graft copolymers (GCL) with CNFs were grafted with poly(ε-caprolactone) (ε-CL) via homogeneous ring-opening polymerization (ROP), and used as strengthening/toughening nanofillers for PCL to fabricate light composite films (LCFs). The results showed that the ZnEu-MOF ([ZnEu(L)₂(HL)(H₂O)_0.39(CH₃OH)_0.61]·H₂O, H₂L is 5-(1H-imidazol-1-yl)-1,3-benzenedicarboxylic acids) was an efficient catalyst, with low toxicity, good stability, and fluorescence emissions, and the GCL could efficiently promote the dispersion of CNFs and improve the compatibility of the CNFs and PCL. Due to the synergistic effect of the ZnEu-MOF and CNFs, considerable improvements in the mechanical properties and high-intensity fluorescence were obtained in the LCFs. The 4 wt% GCL provided the LCF with the highest strength and elastic modulus, which increased by 247.75% and 109.94% compared to CNF/PCL, respectively, showing the best elongation at break of 917%, which was 33-fold higher than CNF/PCL. Therefore, the ZnEu-MOF represented a novel bifunctional material for ROP reactions and offered a promising modification strategy for preparing high-performance polymer composites for agriculture and biomedical applications.

Development of an Antimicrobial-Coated Absorbable Monofilament Suture from a Medical-Grade Poly(l-lactide-co-ε-caprolactone) Copolymer

In this study, a medical-grade poly(l-lactide-co-ε-caprolactone) (PLC) copolymer with a monomer ratio of l-lactide (L) to ε-caprolactone (C) of 70:30 mol % for use as an absorbable surgical suture was synthesized via ring-opening polymerization (ROP) using a novel soluble liquid tin(II) n-butoxide (Sn(OnC4H9)2) as an initiator. In fiber fabrication, the process included copolymer melt extrusion with a minimal draw followed by sequential controlled hot-drawing and fixed-annealing steps to obtain oriented semicrystalline fibers with improved mechanical strength. For healing enhancement, the fiber was dip-coated with "levofloxacin" by adding the drug into a solution mixture of acetone, poly(ε-caprolactone) (PCL), and calcium stearate (CaSt) in the ratio of acetone/PCL/CaSt = 100:1% w/v:0.1% w/v. The tensile strength of the coated fiber was found to be increased to ∼400 MPa, which is comparable with that of commercial polydioxanone (PDS II) of a similar size. Finally, the efficiency of the drug-coated fiber regarding its controlled drug release and antimicrobial activity was investigated, and the results showed that the coated fiber was able to release the drug continuously for as long as 30 days. For fiber antimicrobial activity, it was found that a concentration of 1 mg/mL was sufficient to inhibit the growth of Staphylococcus aureus (MRSA), Escherichia coli O157:H7, and Pseudomonas aeruginosa, giving a clear inhibition zone range of 20-24 mm for 90 days. Cytotoxicity testing of the drug-coated fibers showed a %viability of more than 70%, indicating that they were nontoxic.

Novel Poly(Methylenelactide-g-L-Lactide) Graft Copolymers Synthesized by a Combination of Vinyl Addition and Ring-Opening Polymerizations

In this work, a novel poly (methylenelactide-g-L-lactide), P(MLA-g-LLA) graft copolymer was synthesized from poly(methylenelactide) (PMLA) and L-lactide (LLA) using 0.03 mol% liquid tin(II) n-butoxide (Sn(OnBu)2) as an initiator by a combination of vinyl addition and ring-opening polymerization (ROP) at 120 °C for 72 h. Proton and carbon-13 nuclear magnetic resonance spectroscopy (1H- and 13C-NMR) and Fourier-transform infrared spectroscopy (FT-IR) confirmed the grafted structure of P(MLA-g-LLA). The P(MLA-g-LLA) melting temperatures (Tm) range of 144-164 °C, which was lower than that of PLA (170-180 °C), while the thermal decomposition temperature (Td) of around 314-335 °C was higher than that of PLA (approx. 300 °C). These results indicated that the grafting reaction could widen the melt processing range of PLA and in doing so increase PLA's thermal stability during melt processing. The graft copolymers were obtained with weight-average molecular weights (M¯w) = 4200-11,000 g mol-1 and a narrow dispersity (Đ = 1.1-1.4).

DFT Analysis of Organotin Catalytic Mechanisms in Dehydration Esterification Reactions for Terephthalic Acid and 2,2,4,4-Tetramethyl-1,3-cyclobutanediol

Polyesters synthesized from 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) and terephthalic acid (TPA) are improved alternatives to toxic polycarbonates based on bisphenol A. In this work, we use ωB97X-D/LANL2DZdp calculations, in the presence of a benzaldehyde polarizable continuum model solvent, to show that esterification of TMCD and TPA will reduce and subsequently dehydrate a dimethyl tin oxide catalyst, becoming ligands on the now four-coordinate complex. This reaction then proceeds most plausibly by an intramolecular acyl-transfer mechanism from the tin complex, aided by a coordinated proton donor such as hydronium. These findings are a key first step in understanding polyester synthesis and avoiding undesirable side reactions during production.

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

Ring-opening polymerization (ROP) of cyclic esters (lactones, lactides, cyclic carbonates and phosphates) is an effective tool to synthesize biocompatible and biodegradable polymers. Metal complexes effectively catalyze ROP, a remarkable diversity of the ROP mechanisms prompted the use of density functional theory (DFT) methods for simulation and visualization of the ROP pathways. Optimization of the molecular structures of the key reaction intermediates and transition states has allowed to explain the values of catalytic activities and stereocontrol events. DFT computation data sets might be viewed as a sound basis for the design of novel ROP catalysts and cyclic substrates, for the creation of new types of homo- and copolymers with promising properties. In this review, we summarized the results of DFT modeling of coordination ROP of cyclic esters. The importance to understand the difference between initiation and propagation stages, to consider the possibility of polymer-catalyst coordination, to figure out the key transition states, and other aspects of DFT simulation and visualization of ROP have been also discussed in our review.

Data on electronic structures for the study of ligand effects on the zirconocene-mediated trimethylene carbonate polymerization

The data presented in this paper are related to the research article entitled “Effect of ligand structure in the trimethylene carbonate polymerization by cationic zirconocene catalysts: A “naked model” DFT study” (Jitonnom and Meelua, 2017) [1]. In this data article, we present 3D molecular information of 29 zirconocene catalysts that differ in electronic and steric properties. The data contains all cationic species along the initiation and first propagation step of the polymerization, which are provided in a PDB format that can be used for further studies.

Cationic ring-opening polymerization of cyclic carbonates and lactones by group 4 metallocenes: A theoretical study on mechanism and ring-strain effects

Group 4 metallocene-mediated cationic ring-opening polymerizations of a series of lactones and cyclic carbonates, with different ring sizes ([Formula: see text]–8) have been theoretically studied. Using the “naked cation” approach in combination with density functional theory, the activated chain-end mechanism and the influence of transition metals, solvent and monomer ring size on the polymerizability were explored in detail. The results showed that the cationic metallocene–monomer complex, [catalyst][monomer][Formula: see text], is formed, generating cationic (carbocation ion) species responsible for polymer chain growth. We found that poor polymerizability of five-membered lactone and six-membered ring carbonate depends not only on the nature of the monomer ring size but also the relative stability of the complex, which was found to correlate well with the ring strain. Subsequently, several propagation steps take place through an SN2 reaction which involves ring opening of an active monomer, via alkyl–oxygen bond cleavage. Based on the computed activation energies of all metallocene systems, the first propagation was found to be the rate-determining step of the overall propagation and the hafnocene was found to be most active with the energy barrier of 17.6[Formula: see text]kcal/mol, followed by zirconocene (18.6[Formula: see text]kcal/mol) and titanocene (19.5[Formula: see text]kcal/mol), respectively. The mechanistic study may be applicable to the cationic ROP of lactides and other related monomers.