Ring-opening polymerization of ε-caprolactone initiated by heteropolyacid

In this work, phosphotungstic acid (PTA) as a novel initiator was reported for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). It was found that PTA was an efficient initiator. ROP of ε-CL can be readily initiated by PTA at room temperature and form poly(ε-caprolactone) with narrow molecular weight distribution. Polymerization mechanism study indicates that the polymerization proceeds via acyl-oxygen bond cleavage.     

Calcium methoxide initiated ring-opening polymerization of ε-caprolactone and L-lactide

Summary A commercial calcium dimethoxide and an in-situ generated calcium methoxide prepared from bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide] and methanol, were investigated as initiators for the ring-opening polymerization of ε-caprolactone and L-lactide. Commercial calcium dimethoxide initiated rapid ε-caprolactone polymerization at 120°C in bulk to give quantitatively a polymer with a polydispersity index around 1.3. Significant racemization was observed for L-lactide polymerization. The In-situ formed calcium methoxide promoted the solution polymerization of both ε-caprolactone and L-lactide to high conversion at room temperature over a short time period, yielding the corresponding polyesters with narrow molecular weight distribution. NMR spectra showed that the poly(L-lactide) isolated had a purely isotactic microstructure. The initiator efficiency could be tuned by varying the molar ratio of methanol and bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide]. Received: 11 August 2000/Revised version: 21 December 2000/Accepted: 3 January 2001     

Ethyl magnesium bromide as an efficient anionic initiator for controlled polymerization of ε-caprolactone

The ring-opening polymerization of ε-caprolactone initiated by ethyl magnesium bromide was studied. Because ring-opening polymerization is extremely fast in the monomer phase, we focused on solution polymerization in toluene. The solution polymerization of ε-caprolactone was characterized by the linear dependence of degree of conversion on polymerization time, which we described by a first-order kinetic equation. The molar mass, determined by SEC using a light scattering detector, was in agreement with that calculated from the degree of conversion attained and the ratio of ε-caprolactone to ethyl magnesium bromide. On this basis, we propose a mechanism for the effect of ethyl magnesium bromide on ε-caprolactone polymerization and provide evidence for this mechanism by isolating a novel stable complex of magnesium bromide with ε-caprolactone.     

Preparation of single-site tin(IV) compounds and their use in the polymerization of ε-caprolactone

Butyltin(IV) carboxylate compounds were obtained by reactions of butyltrichlorotin(IV) with potassium pivalate, perfluoroheptanoate, methacrylate, 2,6-pyridinedicarboxylate, and phthalate. The synthesized complexes were fully characterized by nuclear magnetic resonance (¹H-, ¹³C-NMR), Fourier transform infrared (FTIR), mass spectroscopies (MS) and elemental analysis. These tin complexes were used as catalysts for the ring opening polymerization of ε-caplolactone and the conversion of monomers to polymers was completed in just 1 h. The structures of polymers were characterized by a combination of spectroscopic techniques (NMR, FTIR, MS), differential scanning calorimeter (DSC) and gel permeation chromatography. In this study, the ε-caplolactone polymers with different average molecular weights between 5000 and 40,000 Da having a regular structure were obtained.     

A novel approach to RE–OR bond from in situ reaction of rare earth triflates and sodium alkoxides: A versatile catalyst for living ring-opening polymerization of ε-caprolactone

A series of rare earth triflates (RE(OTf)₃, RE = Sc, Y and Lu) were used for the first time as moisture-stable precursors to generate rare earth alkoxide complexes through an in situ reaction with sodium alkoxides (NaOR) in tetrahydrofuran. ¹H NMR and ¹³C NMR results confirmed the fast ligands exchange process and the formation of rare earth–oxygen (RE–OR) bond. The in situ formed catalysts displayed high reactivity toward living ring-opening polymerization (ROP) of ε-caprolactone (CL). For instance, Lu(OTf)₃/sodium isopropoxide (NaOⁱPr)-catalyzed ROP of CL with the [CL]₀/[NaOⁱPr]₀/[Lu(OTf)₃]₀ feeding ratio of 300/3/1 produced poly(ε-caprolactone) (PCL) with controlled molecular weight (M_n,exp = 11.9 kDa vs M_n,theo = 11.8 kDa) and narrow polydispersity (PDI) of 1.08 within 3 min at 25 °C. The kinetic studies and chain extension confirmed the controlled/living nature for the Lu(OTf)₃/NaOⁱPr-catalyzed ROP of CL. In addition, end-functionalized PCLs bearing vinyl or alkynyl group with narrow PDIs were obtained by using functional sodium alkoxides in the presence of Lu(OTf)₃. ¹H NMR and MALDI-ToF MS analyses of the obtained PCLs clearly indicated the presence of the residue of OR groups at the chain ends. A coordination–insertion polymerization mechanism was proposed including a fast ligand exchange between Lu(OTf)₃ and NaOR giving the respective lutetium alkoxide complexes, and a CL insertion into RE–OR bond via acyl-oxygen cleavage.     

Catalysts for the ring-opening polymerization of ε-caprolactone and l-lactide and the mechanistic study

Two novel magnesium aryloxides have been prepared and their catalytic activities toward ring-opening polymerization (ROP) of ε-caprolactone and l-lactide have been investigated. The reaction of 2,2′-(2-methoxybenzylidene)-bis(4,6-di(1-methyl-1-phenylethyl)phenol) (MEMPEP-H₂) (1) and 2,2′-methylene-bis(4,6-di(1-methyl-1-phenylethyl)phenol) (MMPEP-H₂) with ⁿBu₂Mg yield dimeric magnesium complexes [Mg(μ-MEMPEP)(THF)]₂ (2) and [Mg(μ-MMPEP)(THF)]₂ (3), respectively. Catalytic studies of complexes 2 and 3 illustrate that both 2 and 3 are good catalysts in ε-caprolactone and l-lactide polymerization. Theoretical study of the ROP mechanism of ε-caprolactone catalyzed by 2 demonstrates that the initiator, benzyl alcohol, is activated by the formation of a hydrogen bond with the phenoxy oxygen of MEMPEP²⁻ ligand.     

Natural weathering of spruce wood chemically modified by re-used ε-caprolactone solution

Increasing environmental pressures over the last few years have led to attention for non-biocide treatments in the wood protection field. The ε-caprolactone modification of wood by substitution or blocking of hydroxyl groups with hydrophobic poly(ε-caprolactone) (PCL) is one of the novel modification methods developed in recent years. In this study, ring-opening polymerisation of ε-caprolactone in wood cell walls was evaluated by the third and sixth re-use of recovered monomer and by oven-curing method. Spruce samples were modified by re-used ε-caprolactone and exposed to the natural weathering agents for 12 months. After weathering, the colour change, surface roughness measurements, and macroscopic and ultra-microscopic observations revealed that the modified wood had better surface properties than reference wood. Fourier-transform infrared (FTIR) analysis proved that PCL could be found on the weathered surface up to the sixth month, but very little amount was detected on the surfaces with a longer weathering period. The results clearly showed that the efficiency of PCL modification with re-use of the monomer solution was sufficient during the initial weathering periods, but efficiency was reduced after a prolonged exposure period.     

Brønsted acid-free controlled polymerization of tetrahydrofuran catalyzed by recyclable rare earth triflates in the presence of epoxides

We report a series of novel, highly efficient, hydrolytically stable and recyclable Lewis acid rare earth triflate catalysts for the living/controlled bulk ring-opening polymerization (ROP) of tetrahydrofuran (THF) in the presence of epoxides. The rare earth triflates [RE(OTf)₃] include that of Sc, Y, La, Nd, Dy and Lu. Epoxides include propylene oxide (PO), cyclohexene oxide (CHO) or styrene oxide (SO). Especially RE(OTf)₃/PO shows high activities, producing PTHF with up to 62 percent conversion and relatively narrow molecular weight distributions (MWDs) (as low as 1.14). Molecular weights (MWs) can be readily controlled from 1.7 to 56.1 kDa by changing monomer to catalyst molar ratios. The Sc(OTf)₃/PO catalyst gives an α,ω-dihydroxyl telechelic PTHF with MWs close to calculated values and MWDs below 1.2. The rare earth catalysts are easily recovered by simple water extractions. For instance, Sc(OTf)₃ can be used at least six times without an obvious decrease of catalytic activity. A polymerization mechanism involving the first two propagation steps of the alkyltetrahydrofuranium cation is proposed to account for the induction periods observed.     

Reversible coordinative chain transfer polymerization of isoprene and copolymerization with ε-caprolactone by neodymium-based catalyst

Coordinative chain transfer polymerization (CCTP) of isoprene was investigated by using the typical Ziegler–Natta catalytic system [Nd(Oi-Pr)₃/Al(i-Bu)₂H/Me₂SiCl₂] with Al(i-Bu)₂H as cocatalyst and chain transfer agent (CTA). The catalyst system exhibited high catalytic efficiency for the reversible CCTP of isoprene and yielded 6–8 polymer chains per Nd atom due to the high chain transfer ability of Al(i-Bu)₂H. The narrow molecular weight distribution (M_w/M_n = 1.22–1.45) of the polymers, the good linear relationship between the M_n and yield of the polymer, and the feasible seeding polymerization of isoprene indicated the living natures of the catalyst species. Moreover, the living Nd-polyisoprene active species could further initiate the ring-opening polymerization of polar monomer (ε-caprolactone) to afford an amphiphilic block copolymer consisting of cis-1,4-polyisoprene and poly(ε-caprolactone) with controllable molecular weight and narrow molecular weight distribution.     

Large-scale microwave-assisted ring-opening polymerization of ɛ-caprolactone

To probe into the industrialization possibility of microwave-assisted ring-opening polymerization (ROP) of ?-caprolactone (?-CL), it was performed with monomer mass from 750 to 2450 g using a self-designed industrial microwave oven (6000 W) catalyzed by stannous octanoate (Sn(Oct)₂) (M/Cat = 1000). The large-scale ROP of ?-CL proceeded smoothly at various microwave power levels (850, 1700, and 2550 W) and produced poly(?-caprolactone) (PCL) with weight-average molar mass (M_w) from 66,000 to 12,2000 g/mol within 40 min in a yield over 93%. The structure of PCL was characterized by ¹H and ¹³C NMR, FT-IR, DSC and WAXD. We propose that the microwave ROP of ?-CL is a potential technology to prepare PCL commercially.     

MgO/SnO2/WO3 as catalysts for synthesis of ε-caprolactone over oxidation of cyclohexanone with peracetic acid

Different Mg/Sn/W mixed oxides prepared by precipitation were used as catalysts in the Baeyer–Villiger oxidation of cyclohexanone with a mixture of 50% hydrogen peroxide and acetic acid as oxidant. The Mg/Sn/W oxide obtained by precipitation from NH₃·H₂O was found to be the catalyst providing the highest yield of ε-caprolactone and initial catalytic activity among all samples.     

Ring-opening polymerization of ?-caprolactone by base catalyst for synthesis of grafted polysilsesquioxane

The polysilsesquioxane having amino and phenyl groups (APSQ) was prepared from the corresponding trimethoxysilanes by co-condensation under basic conditions. The amino group on APSQ was utilized as an initiator for graft polymerization of ?-caprolactone (CL). The ring-opening polymerization of CL, in the presence of a catalytic amount of the base prepared from triazabicyclodecene, proceeded effectively to afford the polysilsesquioxane having poly(CL) as the graft chains (GrPSQ). In the grafting, no formation of cross-linked product was observed. The grafted poly(CL) component in GrPSQ showed improved durability for heat as an advantage of the hybrid polymer containing polysiloxane structure.     

Oxidative dehydrogenation of propane over a series of low‐temperature rare earth orthovanadate catalysts prepared by the nitrate method

A series of high‐purity rare earth orthovanadates were prepared by the nitrate method and found to be effective low‐temperature catalysts for the oxydehydrogenation of propane at 320°C, at which no reactions occurred over the catalysts reported in the literature, and, thus, may be of practical significance. The catalytic performances of LnVO₄ (Ln = Y, Ce–Yb) at 500°C were much better than those of rare earth orthovanadate catalysts and also slightly exceeded that of magnesium orthovanadate Mg₃(VO₄)₂ reported in the literature. LnVO₄ (Ln = Y, Ce–Yb) materials were tetragonal active phases which could stabilize the existence of active sites for the oxydehydrogenation of propane. Some catalysts with a certain amount of LnVO₃ reduced from LnVO₄ (Ln = Ho–Yb) under reaction atmosphere exhibited better redox properties and catalytic performances possibly due to the existence of biphasic catalytic synergy. LaVO₄ was a monoclinic unstable active phase, although its bulk structure did not change after reaction. The remarkable deactivation of the LaVO₄ catalyst was probably due to that LaVO₄ could not stabilize the existence of surface active sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Aluminum- and calcium-incorporated MCM-41-type silica as supports for the immobilization of titanium(IV) isopropoxide in ring-opening polymerization of l-lactide and ε-caprolactone

Aluminum- and calcium-incorporated MCM-41-type silica with various Al/Si and Ca/Si ratios were evaluated as catalytic supports for ring-opening polymerization (ROP) of l-lactide and ε-caprolactone. The catalytic centers were generated by grafting titanium(IV) isopropoxide onto the support. All prepared heterogeneous catalysts better restrained the ROP of lactide than the homogeneous analog, titanium(IV) isopropoxide. Compared to siliceous MCM-41, the incorporation of aluminum or calcium in MCM-41 framework improved the molecular weight of the polymers although it lessened the polymerization rate. The more acidic Al-MCM-41 support appeared to be more favorable than the more basic Ca-MCM-41 support for the ROP reactions.     

Polymerization temperature effects on the properties of l-lactide and ε-caprolactone copolymers

Summary The large difference in reactivity of L-lactide and -caprolactone in ring opening polymerization with stannous octoate, leads to the formation of copolymers with blocky structures. By varying the polymerization temperature, copolymers with different average sequence lengths and molecular weights can be synthesized. It is shown that the average monomer sequence length has a large effect on the thermal and mechanical properties of these copolymers.     

Synthesis of block copolymers of ε-caprolactone and lactide in the presence of lithium t-butoxide

Summary High molecular mass copolymers with microphase separation can be obtained in block copolymerization of -caprolactone with L,L- and D,L-lactide carried out in the presence of lithium t-butoxide. A kind of solvent applied strongly affects the molecular mass and polydispersity index of obtained copolymers.The study has been supported by the research program (Project 4-1743) of the Committee of Scientific Research in Poland.     

Effects of alkylaluminum as cocatalyst on the active center distribution of 1-hexene polymerization with MgCl2-supported Ziegler–Natta catalysts

Effect of AlEt₃ on 1-hexene polymerization with MgCl₂-supported Ziegler–Natta catalyst was studied. Complete activation of active centers (C*) producing medium to low molecular weight polymer requires relatively higher Al/Ti than C* producing high molecular weight polymer. Raising Al/Ti from 30 to 300 caused continuous shifting of the active center distribution from centers of smaller number, fast chain propagation and slow chain transfers to centers of larger number, slow chain propagation and fast chain transfers. Chain transfer with AlEt₃ is more evident in C* producing lower molecular weight polymer.     

Bis(cyclopentadienyl) lanthanide benzylthiolate: synthesis,molecular structure and catalytic property for ε-caprolactone –polymerization

Reaction of LnCl₃ first with three equivalent of C₅H₅Na in THF, then with one equivalent of benzyl mercaptan, led to complexes of [(C₅H₅)₂Ln(SCH₂Ph)]₂ (Ln?=?Sm(1), Yb(2)), being characterized by infrared spectra, elemental analyses and X-ray crystallography for 2. Complex 2 is a dimer with two thiolate ligands as bridging groups in eight coordinate. The Yb-S(benzyl) bonds in 2 (2.703(19) and 2.719(2) Å, respectively) were longer than the Yb-S(aryl) bonds (about 2.640 Å) in analogous complexes. The catalytic property for the polymerization of ε-caprolactone by 1 and 2 was studied. Similar experiment was also made with [(C₅H₅)₂Ln(SPh-p-CH₃)(THF)]₂ (Ln?=?Sm(3), Yb(4)) for comparison. It was found that complex 2 showed the activity best, and the activity decreased in the order of 2?>?3?>?1?>?4. When [ε-CL]₀/[Ln] was 500 and the polymerization temperature was 35°C, complex 2 catalyzed the polymerization in living character, which could not be achieved by lanthanide arylthiolates such as 3 and 4.     

Supported Ziegler–Natta catalyst for ethylene polymerization: Novel data on the effect of polymerization temperature on the number of active centers and propagation rate constant

The number of active centers at ethylene polymerization over highly active supported catalyst TiCl₄/MgCl₂ + AlEt₃ was determined at various polymerization temperatures. It was found that the increase in polymerization temperature in the range of 40–80 °C increases the number of active centers. The data on reversible changes in the polymerization rate with temperature in a single experiment give grounds for supposing that alteration of the number of active centers with temperature is a reversible process. The propagation rate constants and the activation energy of the propagation reaction (4.0 kcal mol^− 1) were calculated; the latter value being considerably lower than the effective activation energy of polymerization (16.5 kcal mol^− 1) due to the increase in C_P with rising temperature.