Ring-opening polymerization of lactide initiated by magnesium and zinc alkoxides

A mono methylether Salen-type ligand, SalenMe-H (1) is prepared in a one flask reaction by condensation of trans-1,2-diaminocyclohexane with 2-methoxybenzenaldehyde and followed by the addition of 2,4-di-tert-butylsalicylaldehyde. Further reaction of 1 with Mg(OBn)₂ in THF produces a magnesium alkoxide, [(SalenMe)Mg(OBn)]₂ (2). Compound 1 reacts with ZnEt₂ yields monomeric complex (SalenMe)ZnEt (3), which further reacts with 1 molar equiv of benzyl alcohol giving [(SalenMe)Zn(OBn)]₂ (4). Experimental results show that complexes 2 and 4 efficiently initiate the ring-opening polymerization of l-lactide and rac-lactide in a controlled fashion, yielding polymers with very low polydispersity indexes. Kinetic studies show a second-order dependency on [LA] and a first-order on [2] with magnesium complex 2 as an initiator. While zinc complex 4 is used as an initiator, the polymerization rate has a first order dependency on both [LA] and [4].     

有机锡化合物引发D,L-丙交酯的开环聚合

D,L-丙交酯在有机锡化合物四苯基锡(Ph4Sn)、三正丁基醋酸锡(Bu3SnOAc)、辛酸亚锡(SnOct2)等催化剂引发下开环聚合,得到高分子量的聚D,L-乳酸,讨论了聚合单体D,L-丙交酯纯度、聚合温度、聚合时间、聚合体系真空度以及引发剂有机锡催化剂与聚合单体D,L-丙交酯的比例对聚D,L-乳酸合成的影响,通过红外光谱和核磁共振光谱对聚D,L-乳酸的结构进行了表征。实验结果表明,D,L-丙交酯开环聚合反应的较佳工艺路线是:D,L-丙交酯重结晶5次,聚合温度140℃,聚合时间24 h,聚合体系真空度13.3~133.3 Pa,引发剂有机锡催化剂与单体D,L-丙交酯比例0.2‰~0.5‰,此时聚D,L-乳酸的粘均分子量最高,分别达到32.46×104,33.43×104和41.36×104。     

Winsor I-like (micro)emulsion polymerization of styrene initiated by oil-soluble initiator

Batch emulsion polymerization of styrene initiated by an oil-soluble initiator and stabilized by non-ionic emulsifier (Tween 20) has been investigated. The rate of polymerization vs. conversion curve shows the two non-stationary rate intervals typical for the non-stationary-state polymerization. This behavior is a result of the continuous particle nucleation and the decrease of monomer concentration at the reaction loci with increasing conversion. The initial increase of the polymerization rate is attributed to the increase of particle number and the polymerization proceeding under the monomer-saturated condition—the Winsor I-like (micro)emulsion polymerization. The decrease of the polymerization rate is the result of the depressed transfer of monomer from the monomer reservoir to the reaction loci. Above 50 °C the monomer emulsion separates into two phases: the upper transparent monomer phase and the lower blue colored (microemulsion) phase. The polymerization mixture consists of the microdroplets (act as the reaction loci) and large degradable monomer droplets (act as the reservoir monomer and emulsifier). The continuous release of emulsifier from the monomer phase and the microdroplets induce the continuous particle nucleation up to high conversion. The initial formation of large particles results from the agglomeration of unstable growing particles and monomer droplets. The size of large (highly monomer-swollen) particles decreases with conversion and they merge with the growing particles at ca. 40-50% conversion. The coarse initial emulsion transformed during polymerization to the fine (semitransparent) polymer emulsion as a result of the continuous particle nucleation, the shrinking of highly monomer-swollen polymer particles and the depletion of monomer droplets. The low overall activation energy of polymerization is mainly ascribed to the decreased barrier for entering radicals into the latex particles with increasing temperature.     

Ring-opening polymerization of ε-caprolactone initiated by cyclopentadienyl sodium

Polymerization of ε-caprolactone had been investigated with cyclopentadienyl sodium as an initiator. The effects of reaction time, temperature, and concentration of the initiator on the yield and molecular weight of the polymer were discussed. It was shown that the high molecular weight of poly(ε-caprolactone) (13 × 10⁴) was obtained with cyclopentadienyl sodium initiator, and the mechanism of polymerization was also discussed. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1273–1276, 1998     

双官能团引发剂引发甲基丙烯酸甲酯的聚合速率

研究了双官能团引发剂2,5-双(2-乙基己酰过氧化)-2,5-二甲基己烷（TX-141）引发甲基丙烯酸甲酯（MMA）的自由基聚合动力学,考察了引发剂浓度、聚合温度对聚合动力学的影响.结果表明：该体系的聚合活化能为91 kJ•mol^-1左右,计算得到TX-141引发剂的分解活化能为140 kJ•mol^-1左右,与实验值接近;TX-141引发剂的反应级数约为0.72,说明MMA聚合终止基元反应中单基和双基终止并存.同时与相似半衰期的过氧化二苯甲酰（BPO）比较,发现当TX-141引发剂浓度为BPO的1/2时,两者在各个聚合温度下的聚合动力学曲线几乎相同;但相对分子质量有明显增加,随聚合转化率增加,TX-141与BPO引发的聚合物数均分子量之比从1.2变化到1.33;由TX-141引发的聚合物低转化率时DSC曲线出现放热峰,而高转化率以及BPO引发的聚合物则没有.说明双官能团引发剂TX-141引发聚合时,在低转化率下TX-141引发剂的2个过氧键没有全部断裂,随聚合进行,断裂程度加深.     

Ring-opening polymerization of 2-oxazolines initiated by chloromethyl groups introduced onto carbon black surface

Summary The ring-opening polymerization of 2-oxazolines (OXZs) was found to be initiated by chloromethyl groups introduced onto carbon black surface. The introduction of chloromethyl groups onto the surface was achieved by the reaction of carbon black with 3,3-bischloromethylbenzoyl peroxide in carbon tetrachloride. During the polymerization, poly-OXZs were grafted from carbon black based on the propagation of the polymers from the surface: percentage of grafting increased with an increase of conversion and reached 40–60%. The polymerization was accelerated by the addition of potassium iodide. Poly-OXZ-grafted carbon black produced stable colloidal dispersions in both hydrophobic and hydrophilic solvents.     

Ring-opening polymerization of ϵ-caprolactone initiated by rare earth complex catalysts

The polymerization of caprolactone (ϵ-CL) was initiated by yttrium triisopropoxide {Y(OPⁱ_r)₃}, bimetallic isopropoxide of yttrium and aluminum {Y[Al(OPⁱ_s)₄]₃}, yttrium and tin(II) {Y[Sn(OPⁱ_r)₃]₃}, and tin(II) and yttrium {Sn[Y(OPⁱ_r)₄]₂}, respectively. The polymerization was carried out through coordinative insertion of a monomer into the free metal-oxygen bond. The molecular weight and yield of poly(ϵ-caprolactone) (PCL) were affected drastically by the mol % of the initiator to the monomer (C_o/M_o). The results showed that Y(OPⁱ_r)₃ and Sn[Y(OPⁱ_r)₄]₂ were more effective than were {Y[Al(OPⁱ_r)₄]₃} and {Y[Sn(OPⁱ_r)₃]₃} for the polymerization of ϵ-CL. When polymerization was conducted at 5°C using Y(OPⁱ_r)₃ as the initiator or at 10°C using Sn[Y(OPⁱ_r)₄]₄ as the initiator, polymers with a molecular weight of 45.9 × 10³ or 54.0 × 10³ and high yield resulted in 30 or 5 min, respectively. The polymer was characterized by FTIR, H-NMR, and GPC. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1295–1299, 1997     

High pressure polymerization of glycidol. Kinetics studies

High pressure polymerization of glycidol was carried out at four different pressures ranging from p = 0.1 MPa up to p = 800 MPa at room temperature (T = 298 K). To monitor polymerization kinetics at high pressure dielectric spectroscopy was employed. Constant rates of polymerization at given thermodynamic conditions were determined form the analysis of dc conductivity as well as variation of the static dielectric permittivity. To be sure that both quantities can be used to monitor progress of the polymerization reaction, complementary FTIR studies were carried out at ambient pressure. The vibration of the C–O–C epoxy rings assigned to the band in the 760–880 cm⁻¹ range was chosen to follow progress of the monomer conversion. The rate constant “k” determined from the Fourier Transform Infrared data analysis, was found to be in good correlation with dielectric data. Furthermore, having rate constant obtained for polymerization carried out at different pressures and constant temperature the activation volume was determined ΔV = −3 cm³/mol. Finally, analysis with the use of MALDI-TOF² MS² equipment showed that high pressure favors formation of polymers of greater molecular weight. Moreover, it was found that mechanism of polymerization at high pressure is quite different with respect to the reaction carried out at ambient pressure. It turned out that side reaction occurring upon polymerization of glycidol are completely eliminated at high compression.     

The polymerization mechanism of lactide initiated with zinc (II) acetylacetonate monohydrate

The paper presents the mechanism of lactide polymerization initiated with zinc (II) acetylacetonate monohydrate. However, the actual initiator of this reaction is the complex containing a metal-oxygen bond, formed by the exchange reaction of acetylacetonate ligand with deprotonated lactide derivative. The described reaction results in the release of free acetylacetonate and formation of transitional zinc complex with metal-oxygen bond connecting the zinc atom with derivative of lactide, incorporated - as a new, active in polymerization ligand. Polylactide chain propagation process, which constitutes the following stage of the reaction, is caused by a typical, well known, coordination-insertion ring opening polymerization. The proceeding polymerization maximum yield at the applied conditions does not exceed about 70% in benzene solution and 90% at bulk.     

Thermal analysis of the styrene bulk polymerization and characterization of polystyrene initiated by two methods

Among the accidents caused by highly exothermic reactions, the proportion of polymerization thermal runaway accident accounts for about 48%. The mechanism of styrene polymerization in normal reaction process had been studied thoroughly in the literature. However, the interpretation of mechanism of runaway reactions was lacking. Based on the complicated mechanism of styrene polymerization by thermoinitiated and benzoyl peroxide (BPO) initiated. Differential scanning calorimeter and adiabatic rate calorimeter were devoted to analyze the thermal runaway processes of styrene polymerization. The initial exothermic temperature, adiabatic temperature rise, and maximum reaction temperature of BPO-initiated synthesis reaction were lower than those of thermoinitiated polymerization. Moreover, nuclear magnetic resonance imaging, gel permeation chromatography, and Fourier transform infrared spectrometry were used to characterize the polymerization products. The polystyrene initiated by the two methods had the same hydrogen structure, and BPO-initiated process included the effects of the thermoinitiated process. However, their molecular weight and distribution uniformity differed considerably. The free radicals produced by BPO decomposition participated in the chain reaction of styrene polymerization, accelerated instantaneous grain growth, and promoted the formation of short-chain polystyrene. In brief, the BPO-initiated polymerization process exhibited the desired thermal safety characteristics and gained product under thermal runaway was still polystyrene.     

Modeling Rheological Properties of HDPE Based on Polymerization Kinetics

A three-step modeling approach to obtain rheological properties from the kinetics of the metallocene-catalyzed homopolymerization of ethene is developed. With that, linear as well as slightly branched high-density polyethylene is investigated. Therefore, a deterministic kinetic model of the metallocene-catalyzed polymerization of ethene is set up and coupled with a Monte Carlo algorithm to obtain molecular weight distributions and polymeric microstructures. The topological information is used as input for the rheology model to compute linear and non-linear rheological properties.     

Ring-opening polymerization of cyclic esters by cyclodextrins

Synthetic polymers, typically prepared by addition polymerization or stepwise polymerization, are used constantly in our daily lives. In recent years, polymer scientists have focused on more environmentally friendly synthetic methods such as mild reaction conditions and biodegradable condensation polymers, including polyesters and polyamides. However, challenges remain in finding greener methods for the synthesis of polymers. Although reactions carried out in water are more environmentally friendly than those in organic solvents, aqueous media can lead to the hydrolysis of condensation polymers. Furthermore, bulk polymerizations are difficult to control. In biological systems, enzymes synthesize most polymers (proteins, DNAs, RNAs, and polysaccharides) in aqueous environments or in condensed phases (membranes). Most enzymes, such as DNA polymerases, RNA polymerases, and ribosomes, form doughnutlike shapes, which encircle the growing polymer chain. As biopolymers form, the active sites and the substrate-combining sites are located at the end of the growing polymer chain and carefully control the polymerization. Therefore, a synthetic catalyst that could insert the monomers between the active site and binding site would create an ideal biomimetic polymerization system. In this Account, we describe cyclodextrins (CDs) as catalysts that can polymerize cyclic esters (lactones and lactides). CDs can initiate polymerizations of cyclic esters in bulk without solvents (even water) to give products in high yields. During our studies on the polymerization of lactones by CDs in bulk, we found that CDs function not only as initiators (catalysts) but also as supporting architectures similar to chaperone proteins. CDs encircle a linear polymer chain so that the chain assumes the proper conformation and avoids coagulation. The CDs can mimic the strategy that living systems use to prepare polymers. Thus, we can obtain polyesters tethered to CDs without employing additional solvents or cocatalysts. Although CD has many hydroxyl groups, only one secondary hydroxyl group attaches to the polyester chain. In addition, the polymerization is highly specific for monomer substrates. We believe that this is the first system in which the catalyst includes monomers initially and subsequently activates the included monomers. The catalyst then inserts the monomers between the binding site and the growing chain. Therefore, this system should provide a new environmentally friendly route to produce biodegradable functional polymers.     

Non-equilibrium molecular weight studies on the metathesis polymerization of cyclopentene initiated by WCl6AliBu3: Effect of catalyst ageing time

A study has been made of the molecular weight and distribution changes that take place during the polymerization of cyclopentene initiated by the non-living metathesis catalyst $\text{WCl}{}_6\text{AliBu}{}_3$. The system appears to be characterized by two distinct phases. In the first phase, the molecular weight distribution, which is bimodal, would appear to be kinetically controlled and the bimodal distribution to result from the presence of two kinetically independent metathesis species. The initial molecular weight distribution would, therefore, appear to be dependent upon the relative concentrations of these active species. The second phase of the polymerization is a slow conversion of the molecular weight distribution to that of an equilibrium distribution after a long reaction time.     

Ring-opening polymerization of activated cyclopropanes: polymerization of diethyl 1,1-cyclopropanedicarboxylate by metallic sodium

Summary Diethyl 1,1-cyclopropanedicarboxylate undergoes ring-opening polymerization under anionic conditions. If the cyclopropane is contaminated by diethyl malonate CH₂(COOEt)₂, polymerization can be initiated by metallic sodium. The mechanism of this initiation was found to involve an initial reduction of the malonate by sodium to give the salt (EtOOC)₂CH(^-) Na(⁺) which acts as the real initiator. Poly(diethyl 1,1-cyclopropane dicarboxylate) is soluble in halogenated solvents and was characterized by GPC, IR,¹H- and¹³C-NMR. Molecular weights are low to moderate (typically 10³–10⁴) and are compatible with the low monomer/initiator ratios used in these experiments.Part of this work was presented at the 1993 Spring National ACS Meeting: Penelle J, Clarebout G, Balikdjian I (1993) Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 34(1): 473     

Ring-opening polymerization of lactones catalyzed by decamolybdate anion

An efficient process for the ring-opening polymerization (ROP) of lactones by ammonium decamolybdate (NH₄)₈[Mo₁₀O₃₄] (obtained in situ from thermal decomposition of ammonium heptamolybdate (Hep) (NH₄)₆[Mo₇O₂₄]), is reported for the first time. Polymerization of the ε-caprolactone (CL) and δ-valerolactone (VL) proceeds with quantitative conversions, and high molecular weight polyesters (M_n in the order of 10⁴) with moderate polydispersity (M_w/M_n≤1.81) can be obtained at 150 °C in two hours using a CL/Hep molar ratio of 20,000. Hydroxylic compounds can be efficiently used to control the final molecular weight. Hydroxylic compounds act as both co-initiators and chain transfer agents. Incorporation of the hydroxylic compounds as end groups in the polymeric backbone was corroborated by ¹H and ¹³C-NMR. The molecular weight of poly(ε-caprolactone) (PCL) measured by gel permeation chromatography (GPC) shows a linear dependence on the CL/OctOH ratio. Decamolybdate anion (isolated as a product from the thermal decomposition of heptamolybdate) shows a similar catalytic activity toward polymerization compared to that observed for ammonium decamolybdate obtained in situ. Catalysts recovered after a polymerization batch maintain most of their initial properties, and can be reused to polymerize lactones in an efficient manner.     

Kinetics of UV‐initiated RAFT crosslinking polymerization of dimethacrylates

The UV‐initiated RAFT polymerizations of a series of poly(ethylene glycol) dimethacrylates (PEGDMA) were investigated using differential scanning photocalorimetry (DPC) at room temperature. The rate of the RAFT system was much lower than that of a conventional free radical polymerization. A mild autoacceleration occurred as the addition reaction became diffusion controlled. The influence of the spacer length (CH₂CH₂O)_x between the vinyl moieties of the dimethacrylates on the polymerization kinetics was examined. The polymerization rate of PEGDMA decreased with an increased x value from 4 to 9, but it increased with a further increased x value from 9 to 14. Mechanical properties of the resulting polymers were also examined by dynamic mechanical analysis (DMA). It was concluded that the presence of the RAFT agent during polymerization of multifunctional monomers did not have an effect on the heterogeneity of the polymer network. In comparison with three different PEGDMAs, the PEGDMA with the longest spacer formed the most homogeneous networks with a lower crosslinking density. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Anionic polymerization initiated by lamellar compounds of LiC12: 1. Kinetics of the polymerization of styrene and isoprene — initiator efficiency

The polymerizations of styrene and isoprene initiated by LiC₁₂ in cyclohexane are far slower than those observed in homogeneous media. The initiator's efficiency is estimated from a determination of the polymer molecular weight at differents reaction steps. Keeping in mind the monomer diffusion inside the graphite layers and in allowing some assumptions, some of the propagation rate constants were estimated. These values are much lower than those found in homogeneous polymerization. Temperature speeds up the reaction. The initiator's efficiency depends on the rate at which the graphite separates under the conjugated action of temperature and polymer growth.     

Kinetics of anionic polymerization of lactams. (Solution of non-isothermal kinetic problems by the inverse method)

Wide use has been made of non-isothermic studies for obtaining kinetic data on the anionic polymerization of ?-caprolactam and ω-dodecalactam. Data are obtained by measuring changes in reaction temperature, under conditions of heat exchange with the surroundings and by solving the inverse thermophysical problem. This solution yields kinetic constants, which when substituted into heat conduction and kinetic equations minimize the deviations of calculated temperature dependences on time and the deviations in experimental data. Thus, we have studied qualitatively the polymerization of lactams. Numerical values of the basic kinetic constants in the macrokinetic scheme have been determined. Some new effects, such as self-acceleration in the polymerization of caprolactam and the increase of the apparent activation energy at high degrees of conversion in the polymerization of dodecalactam were detected. The approach may be useful in studying other polymerizations.     

Photochemically masked benzophenone: Photoinitiated free radical polymerization by using benzodioxinone

A new photoinitiating system for free radical polymerization of methyl methacrylate (MMA) is reported. This system consists of benzodioxinone and hydrogen donors such as triethylamine (TEA), N,N-dimethyl ethanol amine (DMEA) and tetrahydrofuran (THF). A feasible mechanism involves photoinduced formation of benzophenone from benzodioxinone and subsequent hydrogen abstraction of photoexcited benzophenone from hydrogen donors to yield radicals capable of initiating polymerization of MMA.