Coordination-insertion Mechanism Research Articles

A new radical initiation for the ring-opening polymerization of tetrahydrofuran with a radical substitution mechanism

A new radical-Lewis acid system has been discovered to achieve the ring-opening polymerization of tetrahydrofuran. Different types of reaction mechanisms were discovered for the initiation and polymerization processes. The proposed initiation mechanism was studied by MALDI-TOF and ESI-MS as a radical substitution mechanism and polymerization was a coordination-insertion mechanism. And polytetrahydrofuran with high molecular weight (Mn = 14.6 × 104 g mol−1) and narrow molecular weight distribution (PDI = 1.44) could be obtained.

Synthesis and catalytic activity of single-site group V alkoxide complexes for the ring-opening polymerization of ε-caprolactone.

The synthesis, characterization, and ring-opening polymerization (ROP) activity of a family of niobium and tantalum alkoxide catalysts was studied. The final catalysts are made in a two-step synthesis, first by reacting the desired homoleptic metal ethoxide with a phenolketoimine ligand to form a series of synthetic intermediates, followed by reaction with catechol to produce a catalytic platform with a single ethoxide initiator. By using two separate ligands, the electronic properties of the catalyst can be tuned, and the molecular weight of the polymer can be increased. It was found that synthetic intermediates adopted a mer geometry both in solution and in the solid state. This mer geometry was retained for the final catechol derivatives, however in one case, where catechol was substituted for 3-methoxycatechol, the molecule adopted a highly distorted fac geometry. Catalytic ROP activity of the synthetic intermediates and final catechol derivatives with ε-caprolactone was studied through a kinetic analysis. In all seven cases studied the reactions proceeded through the expected coordination-insertion mechanism, following pseudo first-order kinetics and increasing in Mn linearly vs. conversion. The single-initiator catechol derivatives increased the Mn by three times compared to that of the three-initiator synthetic intermediates with little decrease in the overall reaction rate. Both the nature of the ligand and metal were found to impact the rate of reaction in these systems. By switching from an electron donating ligand to an electron withdrawing ligand, the rate was found to nearly double. Tantalum species were faster than their niobium counterparts by ∼3 times in the synthetic intermediates and ∼1.5 times in the catechol derivatives. This observed periodicity supports recent literature findings in this area.

Ring-Opening Polymerization of rac-β-Butyrolactone Promoted by New Tetradentate Thioether-Amide Ligand-Type Zinc Complexes.

In this work, thioether-amide ligands featuring a combination of hard amide groups with soft donor groups have been employed to develop new zinc catalysts for the ring-opening polymerization of cyclic esters. All complexes were prepared in high yields through alkane elimination reactions with diethyl zinc and characterized using nuclear magnetic resonance (NMR) spectroscopy. Density functional theory (DFT) characterization provided insight into the parameters that influence catalytic activity, such as steric hindrance at the metal center, Lewis acidity and electronic density of thioether-amide ligands. In the presence of one equivalent of isopropanol, all complexes were active in the ring-opening polymerization of rac-β-butyrolactone. Quantitative conversion of 100 monomer equivalents was achieved within 1 h at 80 °C in a toluene solution. Number-average molecular weights increased linearly with monomer conversion; the values were in optimal agreement with those expected, and polydispersity index values were narrow and relatively constant throughout the course of polymerization. The most active complex was also effective in the ring-opening polymerization of ε-caprolactone and L-lactide. To propose a reliable reaction path, DFT calculations were undertaken. In the first step of the reaction, the acidic proton of the alcohol is transferred to the basic nitrogen atom of the amide ligand coordinated to the zinc ion. This leads to the alcoholysis of the Zn-N bond and the formation of an alcoholate derivative that starts the polymerization. In subsequent steps, the reaction follows the classical coordination-insertion mechanism.

Synthesis and catalytic performance of wood cellulose nanofibers grafted with polylactic acid in rare-earth complexes based on tetrazole carboxylic acids

Stannous octanoate [Sn(Oct)2] and 4-dimethylamino pyridine (DMAP) were used to catalyze the synthesis of amphiphilic cellulose-based graft copolymers, but the acute toxicity of tin ions and DMAP prompts the need for the application of less harmful catalysts. Herein, green catalyst complexes 1–3 [M(H0.5L)2(H2O)5]·2(H2O) (M = Sm, 1; M = Nd, 2; M = Eu, 3; H2L = 4-(3-(tetrazol-5-yl)pyridin-5-yl)benzoic acid) were synthesized, and their properties were systematically investigated. Single-crystal X-ray diffraction showed that the complexes possessed a zero-dimensional structure, while the thermogravimetry and scanning electron microscopy results confirmed their stability after heating at 110 °C for 10 h. Using complexes 1–3 and DMAP as the catalysts, CNFs were grafted with l-lactide via homogeneous ring-opening polymerization to form wood cellulose nanofibers grafted with l-lactide (WGLAs), and the effects of the ratio of wood cellulose nanofibers (WCNFs) to l-lactide ([AGU]/[LA]) and catalyst dosage were studied. The polymerization followed the coordination–insertion mechanism. Under comparable reaction conditions, the grafting ratio of WGLA-1 reached 84.7 %, and the grafting ratio of complex 1 was found to be higher than those achieved using DMAP. WGLAs demonstrated good thermal stability without cytotoxicity, and the residual catalysts in the WGLAs exhibited fluorescence characteristics. Overall, amphiphilic cellulose-based materials with fluorescence emission offered a promising modification strategy to prepare high-performance polymer composites for agriculture and biomedical application.

A metal-free coordination–insertion ring-opening polymerization of tetrahydrofuran by the central metalloid bis(pentafluorophenyl)(phenoxy)borane

A new strategy for the metal-free coordination–insertion ring-opening polymerization of tetrahydrofuran by the central metalloid Boron has been first identified. Bis(pentafluorophenyl)(phenoxy)borane was used as a catalyst for the polymerization reaction system. And polytetrahydrofuran with high molecular weight and narrow molecular weight distribution could be obtained. The proposed mechanism was studied by MALDI-TOF, ESI-MS and O-18 isotope labeling analyses as a metal-free coordination insertion mechanism.

Coordination of ε-Caprolactone to a Cationic Niobium(V) Alkoxide Complex: Fundamental Insight into Ring-Opening Polymerization via Coordination-Insertion.

We report three niobium-based initiators for the catalytic ring-opening polymerization (ROP) of ε-caprolactone, exhibiting good activity and molecular weight control. In particular, we have prepared on the gram-scale and fully characterized a monometallic cationic alkoxo-Nb(V) ε-caprolactone adduct representing, to the best of our knowledge, an unprecedented example of a metal complex with an intact lactone monomer and a functional ROP-initiating group simultaneously coordinated at the metal center. At 80 °C, all three systems initiate the immortal solution-state ROP of ε-caprolactone via a coordination-insertion mechanism, which has been confirmed through experimental studies, and is supported by computational data. Natural bond orbital calculations further indicate that polymerization may necessitate isomerization about the metal center between the alkoxide chain and the coordinated monomer. The observations made in this work are expected to inform mechanistic understanding both of amine tris(phenolate)-supported metal alkoxide ROP initiators, including various highly stereoselective systems for the polymerization of lactides and of coordination-insertion-type ROP protocols more broadly.

Photodegradable polar-functionalized polyethylenes.

The degradation of plastics has attracted much attention from the global community. Polyethylenes (PEs), as the most abundant synthetic plastics, are most frequently studied. PE is non-degradable and non-polar because of the sole presence of the pure hydrocarbon components. Concurrent incorporation of both in-chain cleavable and functional groups into the PE chain is an effective pathway to overcome the non-degradable and non-polar issue; however, the method for achieving this pathway remains elusive. Here, we report a strictly non-alternating (>99%) terpolymerization of ethylene with CO and fundamental polar monomers via a coordination-insertion mechanism using late transition metal catalysts, which effectively prevents the formation of undesired chelates originating from both co-monomers under a low CO concentration. High-molecular-weight linear PEs with both in-chain isolated keto (>99%) and main-chain functional groups are prepared. The incorporation of key low-content isolated keto groups makes PEs photodegradable while retaining their desirable bulk material properties, and the introduction of polar functional groups considerably improves their surface properties.

Ring-opening polymerization of cyclic esters mediated by zinc complexes coordinated with benzotriazo-based imino-phenoxy ligands

A series of zinc complexes coordinated with benzotriazo-based imino-phenoxy ligands (ZnII(BTPIP)) has been developed for the mediation of ring-opening polymerization (ROP) of ε−caprolactone (ε−CL) and L−lactide (L−LA). In the presence of ZnII(BTPIP), the polymerization generated the PCL with the molecular weights in the range of 9600–17700 g mol−1 and narrow dispersity (Ð) around 1.08 to 1.33 as well as the PLA showing a molecular weight as 12300–14400 g mol−1 with narrow molecular weight distribution (Ð < 1.11). Furthermore, the living characters of the linearly increased molecular weight with monomer conversion and the synthesis of the tri-block copolymer of PEG-b-PCL-b-PLA were achieved. The substituent effect of ZnII(BTPIP) in the ROPs of ε−CL and L−LA was rationalized by the DFT calculations based on the coordination-insertion mechanism.

Efficient and well-controlled ring opening polymerization of biobased ethylene brassylate by α-diimine FeCl3 catalysts via a coordination-insertion mechanism.

A highly efficient late-transition metal based catalytic system of α-diimine FeCl3 for well-controlled ring opening polymerization of a cheap and biobased macrolactone, ethylene brassylate (EB), is described herein. Proceeding via a coordination-insertion mechanism, such a catalytic system is capable of demonstrating unprecedented higher activities than previously reported organocatalysts or main-group metal based catalysts. Moreover, benefiting from the bulky nature of the α-diimine ligands, transesterification side reactions can be greatly suppressed, allowing the polymerization to proceed in a well-controlled living manner, as revealed from detailed kinetic studies. Additionally, such a catalytic system was also workable for ring opening copolymerization of EB and ε-caprolactone (ε-CL), giving the desired random copolymers with various compositions.

Bimetallic polymerization of lactide with binaphthol-derived bis-heteroscorpionate dizinc and dimagnesium complexes.

Discrete bimetallic catalysts often provide enhanced reactivity and selectivity in lactone polymerization, making metal-metal cooperativity an important design principle for new catalyst development. However, the poor modularity of binucleating ligands limits structure-reactivity analysis and optimization. This report describes a modular, binucleating bis(pyrazolyl)alkane ligand series (1-R) bridged by a chiral binaphthol unit, prepared by nucleophile-catalyzed condensation between a dialdehyde and a bis(pyrazolyl)methanone. A bis(ethylzinc) complex was characterized by single-crystal X-ray diffraction, but in situ complexation with Zn(HMDS)2 and Mg(HMDS)2 provided more active catalysts for lactide polymerization (HMDS- = hexamethyldisilazide). Structure-reactivity studies identified complexes of 1-Me2 as the most active, and these catalysts show significant enhancements in rate compared to their monometallic analogues. Kinetic analysis resulted in first-order dependence on both mono- and bimetallic catalysts, suggesting metal-metal cooperativity as the basis for this rate enhancement. End-group analysis and low dispersity implicate a coordination-insertion mechanism through an alkoxide. Despite rapid transesterification observed by MALDI, we still demonstrated controlled polymerization in the block copolymerization of ε-caprolactone and L-lactide. Although we observed rate differences in the polymerization of L-lactide by opposite enantiomer catalysts, we did not observe catalyst-directed stereoselectivity in the polymerization of rac- or meso-lactide.

Synthesis and catalytic performance of banana cellulose nanofibres grafted with poly(ε-caprolactone) in a novel two-dimensional zinc(II) metal-organic framework

In this study, banana cellulose nanofibres (BNCFs) were prepared using waste banana stems. A novel two-dimensional (2D) Zn-MOF was synthesized using 5-(1-hydro-imidazolyl) isophthalic acid as the ligand. The results of the X-ray diffraction, thermogravimetry, and scanning electron microscopy analyses confirmed the stability of Zn-MOF. Thus, Zn-MOF was used to catalyze BNCFs grafted with poly(ε-caprolactone) (ε-CL) (BGCL) by homogeneous ring-opening polymerization in ionic liquid. The grafting ratio of the BGCL prepared by Zn-MOF reached 84.12 %, which was 10.03% higher than that catalyzed by 4-dimethylamino pyridine which was the state-of-the-art catalyst for this reaction. The banana cellulose nanofibres grafted with ε-CL reactions followed the coordination-insertion mechanism, which was confirmed by the theoretical calculations using the Materials Studio software. The cytotoxicity results indicated that BGCL was not only non-toxic to Hela cells but also promoted Hela cell growth within a certain concentration range. Therefore, this study demonstrated the promising potential of BGCL for biomedical applications, and the high efficiency, non-toxicity, and stability of Zn-MOF as the catalyst. Further, this will provide a theoretical basis for the application of MOFs as catalysts in polymerization reactions.

Titanium Complexes of Salicylbenzoxazole and Salicylbenzothiazole Ligands for the Ring-Opening Polymerization of ε-Caprolactone and Substituted ε-Caprolactones and Their Copolymerizations.

Two series of titanium complexes, including salicylbenzoxazole titanium complexes (1-4) and salicylbenzothiazole titanium complexes (5-8), were successfully synthesized and characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction crystallography (for 2 and 5). The 1H NMR spectra of complexes 7 and 8 reveal fluxional behavior in solution at room temperature, and the activation parameters were determined by lineshape analysis of variable-temperature (VT) NMR spectra in toluene-d8: for 7, ΔH⧧ = 73.0 ± 1.8 kJ mol-1, ΔS⧧ = 22.1 ± 5.5 J mol-1 K-1; for 8, ΔH⧧ = 73.7 ± 1.2 kJ mol-1, ΔS⧧ = 20.3 ± 3.8 J mol-1 K-1. The positive values of ΔS⧧ suggested that the isomerization occurred via a dissociative mechanism. All complexes were active initiators for the ring-opening polymerization of ε-caprolactone (ε-CL) and three substituted ε-CLs: γ-methyl-ε-caprolactone (γMeCL), γ-ethyl-ε-caprolactone (γEtCL), and γ-phenyl-ε-caprolactone (γPhCL). Of all complexes, complex 5 was found to be the most active initiator in this study. The copolymerizations between ε-CL and three substituted ε-CLs produced completely random copolymers. The polymerization was proposed to proceed via a dissociative coordination-insertion mechanism. The catalytic activity of the salicylbenzoxazole titanium complex was lower than that of its closely related salicylbenzothiazole titanium congener. Additionally, DFT calculations unveiled that the ligand decoordination step and the less steric congestion at the titanium center in the salicylbenzothiazole titanium complexes were the key factors in enhancing the catalytic rate.

Determination of the activation parameters for the ring-opening polymerization of ε-caprolactone initiated by Sn(II) and Zn(II) chlorides using the fast technique of DSC

The kinetics of the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) initiated by 1.0 mol% of tin(II) chloride (SnCl2) and zinc(II) chloride (ZnCl2) was completely investigated by the differential scanning calorimetry (DSC) technique. The fast method to determine the activation parameters such as activation enthalpy (∆H≠) and activation entropy (∆S≠) was successfully developed basing on the transition state theory and isoconversional method. From Friedman isoconversional method, the values of activation energy (Ea) for the ROP of ε-CL initiated by SnCl2 (36.2 ± 1.5 to 40.8 ± 1.5 kJ/mol) were lower than ZnCl2 (51.9 ± 2.8 to 62.3 ± 2.8 kJ/mol). From Starink isoconversional method, the Ea values for the ROP of ε-CL initiated by SnCl2 (32.3 ± 1.1 to 35.8 ± 1.1 kJ/mol) were also lower than ZnCl2 (59.2 ± 0.8 to 62.1 ± 0.8 kJ/mol). By using the compensation parameters, the reconstructed monomer conversion function (f(α)) was fitted with the Avrami-Erofeev nucleation model (A2, f(α) = 2(1-α)[-ln(1-α)]1/2). The values of frequency factor (A) for the ROP of ε-CL with SnCl2 and ZnCl2 were 4.3 × 103to 1.6 × 104 min−1 and 2.1 × 105 to 5.3 × 105 min−1, respectively. From the developed rate equation, the values of ∆H≠ and ∆S≠ could be conveniently determined from dynamic heating. The obtained ∆H≠ values for the ROP of ε-CL initiated by SnCl2 and ZnCl2 were similar to the Ea values. The ∆S≠ values for the transition state of the ROP of ε-CL initiated by SnCl2 (-152.0 to -161.7 J/mol.K) were lower than ZnCl2 (-129.6 to -153.1 J/mol.K). The kinetics and the stability of transition state information demonstrated that the reactivity of SnCl2 in the ROP of ε-CL was higher than ZnCl2. The mechanism of the ROP of ε-CL with SnCl2 and ZnCl2 was proposed through a classical coordination-insertion mechanism. From poly(ε-caprolactone) (PCL) synthesis via solvent-free polymerization, SnCl2 acted as the highly effective initiator than ZnCl2. The highest number average molecular weight (Mn) of PCLs obtained from the SnCl2 and ZnCl2 initiator were 1.8 × 105 g/mol and 3.6 × 104 g/mol, respectively. The effectiveness of the slow ZnCl2 initiator in the production of high molecular weight and %yield of PCL could be improved by increasing the synthesis temperature. These SnCl2 and ZnCl2 compounds acted as a powerful initiator for the synthesis of PCL. The obtained results from this work were useful for understanding the transition state formulation in the ROP of other cyclic esters with various initiating systems that still in our attention and would be described in our future work.

Titanium ONN-(phenolate) Alkoxide Complexes: Unique Reaction Kinetics for Ring-Opening Polymerization of Cyclic Esters.

The synthesis, characterization, and polymerization kinetics of four new titanium ONN-(phenolate) alkoxide catalysts were studied. Each catalyst is fluxional at room temperature, suggesting the ligand amine arm may be labile, but adopts a fac geometry in solution at low temperature (223 K) and in the solid state. All catalysts are active for the ring-opening polymerization of both ε-caprolactone (CL) and rac-lactide (LA). GPC analysis indicates that the well-known coordination-insertion mechanism is being followed. However, whereas the typical first-order dependence on monomer concentration is observed in CL, an unexpected zeroth-order dependence is observed with LA. This suggests that, in the case of LA, catalyst saturation occurs and a Michaelis-Menten model can be used to explain the kinetics. An initial mechanism is discussed within this model that proposes CL polymerization proceeds by a 7-coordinate intermediate, whereas LA polymerization adopts a 6-coordinate intermediate, facilitated by the ligand amine arm. Attempts to isolate catalyst-monomer intermediates are ongoing.

Fluorinated β‐Ketoiminate Zinc Complexes: Synthesis, Structure and Catalytic Activity in Ring Opening Polymerization of Lactide

AbstractComplexes LZnR (L=C6F5NC(CF3)C(H)C(CF3)O; R=Me 1; Et 2) and L2Zn(thf)2 (3) were synthesized and analyzed by NMR (1H, 13C, 19F) and IR spectroscopy, elemental analysis, and single crystal X‐ray diffraction. Complexes 1 and 2 are dinuclear in the solid state but monomeric in toluene solution according to diffusion‐ordered spectroscopy (DOSY‐NMR). They showed poor activity in the ring opening polymerization (ROP) of lactide (LA) but moderate activity in the presence of benzyl alcohol (BnOH), yielding polymers with high number average molecular weight (Mn) and moderately controlled molecular weight distribution (PDI). Homonuclear‐decoupled 1H NMR analysis of polylactic acid (PLA) obtained from rac‐LA showed isotactic enrichment of the polymer microstructure, and kinetic studies of the ROP of L‐LA with complex 2 showed a first order dependence of the monomer concentration. Analyses of low molecular weight polymers by 1H NMR and MALDI‐ToF mass spectrometry demonstrated the coordination‐insertion mechanism (CIM).

Coordination Insertion Mechanism of Ring‐Opening Polymerization of Lactide Catalyzed by Stannous Octoate†

Main observation and conclusionRing‐opening polymerization (ROP) of cyclic esters in the presence of stannous octoate (Sn(Oct)2) is the main way to obtain biodegradable aliphatic polyesters, an important family of biodegradable polymers which have been widely used and still rapidly developed in the fields of biomedical polymers and environment‐friendly materials. The underlying mechanism is thought via a coordination‐insertion way, but the pathway is still open owing to the absence of direct experimental evidence. Herein, we inquire this issue through density functional theory (DFT) calculations. According to our DFT calculations and the following Curtin‐Hammett evaluation, the carbonyl oxygen has a significant advantage over the ester oxygen, and thus the ring is opened mainly through pathway A instead of pathway B. The stannous octoate is identified as a catalyst rather than an initiator. We eventually summarize the main stages during the whole polymerization of lactide.

Ring-opening polymerization of ε-caprolactone initiated by tin(II) octoate/n-hexanol: DSC isoconversional kinetics analysis and polymer synthesis

ABSTRACT The kinetics of ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) initiated by 1.0, 1.5 and 2.0 mol% of stannous(II) octoate/n-hexanol (Sn(Oct)2/n-HexOH) wase successfully studied by non-isothermal differential scanning calorimetry (DSC) at heating rates of 5, 10, 15 and 20 °C/min. The DSC polymerization kinetic parameters of ε-CL were calculated using differential (Friedman) and integral isoconversional methods (Kissinger-Akahira-Sunose, KAS). The average activation energy (Ea ) values obtained from Friedman and KAS methods were in the range of 64.9–70.5 kJ/mol and 64.9–80.4 kJ/mol, respectively. The values of frequency factor (A) were determined from model fitting method using Avrami-Erofeev reaction model. The average values of A for the ROP of ε-CL initiated by 1.0, 1.5 and 2.0 mol% of Sn(Oct)2/n-HexOH (1:2) were 7.3x107, 2.8x106 and 1.2x106 min−1, respectively. From kinetics studied, the polymerization rate of ε-CL increased with increasing initiator concentration. The performance of Sn(Oct)2/n-HexOH in the synthesis of poly(ε-caprolactone) (PCL) was investigated by bulk polymerization at temperatures of 140, 160 and 180 °C. Sn(Oct)2/n-HexOH (1:2) could produce high number average molecular weight (= 9.0 × 104 g/mol) and %yield (89%) of PCL in a short period of time at Sn(Oct)2 concentration of 0.1 mol% and temperature of 160°C. The mechanism of the ROP of ε-CL with Sn(Oct)2/n-HexOH was proposed through the coordination-insertion mechanism.

DFT study of lactide ring-opening polymerizations by aluminium trialkoxides: Understanding the effects of monomer, alkoxide substituent, solvent and metal

The initiation of the ring-opening polymerization (ROP) of l-lactide, d-lactide and meso-lactide (namely, LLA, DLA and mLA, respectively) initiated by aluminium trialkoxide complex with formulae Al(OR)3(R = Me, iPr and sBu) are investigated using DFT calculations. The results show that all Al(OR)3/lactide systems initiate through a coordination-insertion mechanism, involving the rate-limiting “pendulum” transition state. The reactivities of the lactides are predicted to increase as mLA < DLA < LLA. The rate for the LA ROP initiation was slightly slower in polar solvents. Metal substitutions show a reactivity trend (Al < Sc < Y) with different reaction steps.

Undiscovered Potential: Ge Catalysts for Lactide Polymerization.

Polylactide (PLA) is a high potential bioplastic that can replace oil‐based plastics in a number of applications. To date, in spite of its known toxicity, a tin catalyst is used on industrial scale which should be replaced by a benign catalyst in the long run. Germanium is known to be unharmful while having similar properties as tin. Only few germylene catalysts are known so far and none has shown the potential for industrial application. We herein present Ge complexes in combination with zinc and copper, which show amazingly high polymerization activities for lactide in bulk at 150 °C. By systematical variation of the complex structure, proven by single‐crystal XRD and DFT calculations, structure–property relationships are found regarding the polymerization activity. Even in the presence of zinc and copper, germanium acts as the active site for polymerizing probably through the coordination–insertion mechanism to high molar mass polymers.

Influence of butyl group of tin chloride initiators on the non-isothermal DSC ring-opening polymerization of ε-caprolactone: The studies of kinetics, mechanism and polymer synthesis

The kinetics of ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) initiated by tin(II) chloride (SnCl2), dibutyltin(IV) dichloride (Bu2SnCl2) and tributyltin(IV) chloride (Bu3SnCl) was successfully investigated by non-isothermal DSC technique. The steric effect of butyl group of these tin chloride initiators on the catalytic performance of the ROP of ε-CL was studied by isoconversional kinetics methods. From isoconversional kinetics analysis by both Friedman and Kissinger-Akahira-Sunose (KAS) methods, the activation energy (Ea) values for the ROP of ε-CL initiated by SnCl2 were lower than Bu2SnCl2 and Bu3SnCl, respectively. The average values of frequency factor (A) for the ROP of ε-CL with SnCl2, Bu2SnCl2 and Bu3SnCl are 2.05 × 104, 1.34 × 108 and 2.64 × 108 min-1, respectively. The kinetics results clearly demonstrated that the reactivity of SnCl2 > Bu2SnCl2 > Bu3SnCl in the ROP of ε-CL. The effectiveness of all tin chloride initiators in the synthesis of poly(ε-caprolactone) (PCL) was investigated via bulk polymerization. The highly reactive SnCl2 produced high number average molecular weight PCL (M¯n = 3.5 × 104 – 1.1 × 105) in a short polymerization time. The increasing of butyl group in tin chloride initiators reduced the effectiveness in the synthesis of high molecular weight PCL. Furthermore, the mechanism of the ROP of ε-CL with tin chloride initiators was also studied and proposed through the coordination-insertion mechanism.