Redox control of a polymerization catalyst by changing the oxidation state of the metal center

The activity of cerium alkoxide complexes supported by a Schiff base ligand was controlled using redox reagents during the ring-opening polymerization of L-lactide. The rate of L-lactide polymerization was modified by switching in situ between the cerium(III) and cerium(IV) species.     

Progress of Polymerization of D,L-Lactide Through Differential Scanning Calorimetry and Gel Permeation Chromatography

Melt polymerization conditions for D,L-lactide initiated with tetraphenyltin were studied with regard to polymer molecular weight. The present study was undertaken to investigate the progress of polymerization of D,L-lactide through differential scanning calorimetry (DSC), and also to explore the correlation between melt polymerization conditions and molecular weight. The physical characteristics, such as glass transition temperature (Tg) of the polymer and melting transition (Tm) of D,L-lactide are correlated with GPC data. DSC data shows that the Tm of D,L-lactide is 122.8 at 150°C polymerization time. ΔHf is 83.2 J g-1, and Tg of polymer is untraceable. At 180°C the Tm is 101.4°C, ΔHf is 34 J g-1, and Tg is around 29.5°C. The drop in Tm and ΔHf clearly shows the conversion of D,L-lactide to polymer. The maximum increment to molecular weight of polymer is achieved at 160°C and 8 h. After a short induction period, the slow polymerization of D,L-lactide resulted in maximal molecular weight followed by an almost constant value of molecular weight. This revised version was published online in July 2006 with corrections to the Cover Date.     

Production of High Purity Multi-Walled Carbon Nanotubes from Catalytic Decomposition of Methane

Acid-based purification process of multi-walled carbon nanotubes (MWNTs) produced via catalytic decomposition of methane with NiO/TiO2 as a catalyst is described. By combining the oxidation in air and the acid refluxes, the impurities, such as amorphous carbon, carbon nanoparticles, and the NiO/TiO2 catalyst, are eliminated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images confirm the removal of the impurities. The percentage of the carbon nanotubes purity was analyzed using thermal gravimetric analysis (TGA). Using this process, 99.9 wt% purity of MWNTs was obtained.     

乙酰丙酮铁催化丙交酯开环聚合的研究

以乙酰丙酮铁 [Fe(acac) 3]为催化剂进行D ,L 丙交酯的开环聚合及在聚乙二醇 (PEG)存在下的开环共聚 ,研究了催化剂用量、反应温度和反应时间对聚合反应的影响以及PEG用量对共聚反应的影响 ,并探讨了丙交酯开环聚合机理 .结果表明 ,Fe(acac) 3是按配位 插入机理催化丙交酯开环聚合的 ;在本文的聚合条件下 ,大部分聚合的单体转化率都达 90 %以上 ,聚合产物的粘均分子量最高可达 6 6 0 0 0 ,均显示出较好的催化性能 .在PEG存在下 ,PEG作为引发剂参入了丙交酯的开环聚合 ,D ,L 丙交酯是沿着PEG分子两端开环聚合的 ,分子链的链端结构是以羟基为端基的乳酰基结构单元 ,Fe(acac) 3有促进PEG参与聚合成酯的作用 .     

Kinetics of D,L–Lactide Polymerization Initiated with Zirconium Acetylacetonate

The kinetic of D,L-lactide polymerization in presence of biocompatible zirconium acetylacetonate initiator was studied by differential scanning calorimetry in isothermal mode at various temperatures and initiator concentrations. The enthalpy of D,L-lactide polymerization measured directly in DSC cell was found to be ΔH=−17.8±1.4 kJ mol⁻¹. Kinetic curves of D,L-lactide polymerization and propagation rate constants were determined for polymerization with zirconium acetylacetonate at concentrations of 250–1000 ppm and temperature of 160–220 °C. Using model or reversible polymerization the following kinetic and thermodynamic parameters were calculated: activation energy E_a=44.51±5.35 kJ mol⁻¹, preexponential constant lnA=15.47±1.38, entropy of polymerization ΔS=−25.14 J mol⁻¹ K⁻¹. The effect of reaction conditions on the molecular weight of poly(D,L-lactide) was shown.     

β-环糊精/聚(DL-丙交酯)接枝共聚物的合成与表征

以β-环糊精(β-CD)为接枝骨架、DL-丙交酯(DLLA)为接枝单体,三乙胺为催化剂,合成了β-环糊精/聚(DL-丙交酯)接枝共聚物(PCDLA).利用IR、1H-NMR、DSC、WXRD和GPC等方法对接枝共聚物的结构进行了表征,测定了共聚物的分子量,并研究了反应投料比对单体转化率(C%)、接枝率(G%)和接枝效率(GE%)的影响.结果表明,在三乙胺催化下,DL-丙交酯与β-环糊精能够发生聚合反应得到接枝共聚物,当DL-丙交酯与β-环糊精结构单元的摩尔比为30∶1,反应时间为10h时,接枝反应的接枝率(G%)和接枝效率(GE%)可分别达到182·9%和21·4%.随着接枝共聚物中β-环糊精含量的增加,共聚物的亲水性得到了改善.     

醇铁化合物引发丙交酯开环聚合的研究

分别以乙醇铁、正丙醇铁、异丙醇铁、正丁醇铁为引发剂进行D,L-丙交酯和L-丙交酯的本体开环聚合,研究了在130℃的聚合温度下引发剂用量和聚合时间对聚合反应的影响.结果表明这些醇铁化合物对丙交酯开环聚合都有较好的引发作用;聚合36h,单体转化率可达90%以上.单体转化率在引发剂/单体摩尔比为1/1000时最高,然后随引发剂用量增加和聚合时间延长而降低.乙醇铁表现出最高的引发活性,聚合产物的相对粘均分子量最高可达7·28×104[聚(D,L-丙交酯)]和19·00×104[聚(L-丙交酯)].醇盐配体对聚合产物的分子量和分子量分布影响显著,随醇铁配体体积增大,聚合产物的分子量逐渐降低,分子量分布也逐渐加宽.1H和13C-NMR分析表明醇铁对L-丙交酯的开环聚合没有发生消旋化,对D,L-丙交酯的开环聚合有一定的等规加成选择性.MALDI-TOF MS分析指出D,L-丙交酯在开环聚合过程中发生了分子间的酯交换反应,用13C-NMR评价了各醇铁引发体系在聚合过程中的酯交换程度.但基于谱峰分辨原因,醇铁配体对立构加成选择性和酯交换的影响的规律性不明显.     

Redox control of a ring-opening polymerization catalyst

The activity of an yttrium alkoxide complex supported by a ferrocene-based ligand was controlled using redox reagents during the ring-opening polymerization of L-lactide. The oxidized complex was characterized by X-ray crystallography and (1)H NMR, XANES, and M?ssbauer spectroscopy. Switching in situ between the oxidized and reduced yttrium complexes resulted in a change in the rate of polymerization of L-lactide. Synthesized polymers were analyzed by gel permeation chromatography. Polymerization of trimethylene carbonate was also performed with the reduced and oxidized forms of an indium alkoxide complex. The indium system showed the opposite behavior to that of yttrium, revealing a metal-based dependency on the rate of polymerization.     

四臂星形嵌段共聚物s-PDLLA-b-PEG的合成

季戊四醇与D,L-丙交酯开环聚合制得末端为羟基的四臂星形聚乳酸(s-PDLLA);s-PDLLA与羧基封端的聚乙二醇单甲醚(CT-mPEG)完成酯化反应,合成了以季戊四醇为核,以聚乳酸为内部嵌段、聚乙二醇为外部嵌段的四臂星形聚(D,L-乳酸)-聚乙二醇嵌段共聚物(s-PDLLA-b-PEG),其结构经1H NMR,IR和GPC表征。     

Modeling of L-lactide Polymerization by Reactive Extrusion

The kinetics of L-lactide ring-opening polymerization initiated by stannous octoate and triphenylphosphine was investigated in a batch apparatus (Haake Rheocord Mixer). Based on the experimental data, a kinetic model is developed, considering a coordination-insertion mechanism. Reactive extrusion experiments were further conducted for the same polymerization process, on a co-rotating twin screw extruder. The melted material flow and mixing was described by using the Ludovic® commercial simulator. Based on the developed kinetic model and simulated flow of L-lactide polymerization mixture, a mathematical model of reactive extrusion process is formulated, describing the evolutions of monomer conversion and average molecular weight along the extruder. The model is predicting with a reasonable good accuracy the experimental data.     

Synthesis of L-Aspartic Acid and Lactone Copolymers

Recently,thesynthesisandapplications0fbiodegradablepolydepsipeptides,alternatingcopolymersofa-aminoacidanda-hydroxyacidwithfunctionalside-chaingroupshavedrawnmoreandmoreattentionl'2.Thisisbecausethefunctionalizedpendantgroups0fthep0lydepsipeptidesmakethemusefulf0rthepreparationofavarietyofpolymer-drugconjugates.However,thepolymerizati0nofdepsipeptideswithfuncti0nalgr0upsisratherdifficultduetotheirbigstructuresandsterichindrance.Furthermore,high-molecular-weightpr0ductsofsuitablephysicalpropert…     

Anionic Polymerization of L-Lactide Effect of Lithium and Potassium as Counterions

Abstract

The polymerization of L-lactide was carried out with t-BuOK and t-BuOLi in the presence and absence of crown ethers. Each polymerization showed living nature only in the limited sense of having a decreasing amount of active species at high conversion. The initiator efficiency was low in each case because of the deprotonation reaction between the tert-butoxide ion and L-lactide. Although termination did not take place with lithium containing initiators to 90% conversion, intra- and intermolecular transesterification was observed to a significant degree. With potassium containing initiators both termination and transesterification occurred.     

A High Efficiency PDMS-Based Stabilizer for Dispersion Polymerization of L-lactide in Supercritical Carbon Dioxide

Dispersion polymerization of lactides and lactones has been studied and has received a great deal of attention used to prepare biodegradable polymers in supercritical carbon dioxide (ScCO₂). The triblock copolymers of poly(?-caprolactone) PCL-polydimethylsiloxane (PDMS)-poly(?-caprolactone) PCL by the feed ratio of 1:2:1 and 1:3:1, respectively were synthesized to be used as the stabilizers in the dispersion polymerization of L-lactide. The fine poly(L-lactide) (PLLA) powders were obtained at the concentrations of 3% to 5%w/w (stabilizer/monomer). The results demonstrated that the triblock copolymers were the effective stabilizers for the dispersion polymerization of L-lactide. It was also found that the cooling process had a significant effect on the particle size of the fine powders. In order to obtain the best PLLA powders, it would be necessary to use the cooling process with stirring.     

交联度对原位聚合法制备聚合物胶束性质的影响及相应的空心球制备

用原位聚合法成功地制备出不同响应温度的温敏性聚乳酸/聚(异丙基丙烯酰胺-co-丙烯酰胺)[P(D,L-LA)/P(NIPAM-co-AM)]核壳胶束. 实验中发现, 壳层的交联剂含量对粒子的尺寸有很大的影响, 当交联剂的摩尔分数从5%提高到15%时, 粒子在25 ℃时的流体力学直径从170.2 nm增加到886.5 nm. 通过对胶束粒子的核进行生物降解, 方便地得到了相应的空心球. 用FTIR监测核的降解过程, 用SEM和AFM检测核降解完全后粒子的外在形貌和内在结构变化. DLS结果表明, 空心球粒子同样具有良好的温度响应性, 其响应温度可通过改变原位聚合时单体AM的含量加以调节.     

Study of radiation-induced polymerization of vinyl monomers adsorbed on inorganic substances. VIII. Polymerization of styrene and methyl methacrylate adsorbed on aerosil

Aerosil is silica having a purity which is very high compared with that of silica gel and having, unlike silica gel, no micropores. To investigate the effects of impurities and micropores on the radiation-induced polymerization of styrene and methyl methacrylate adsorbed on silica gel, the radiation-induced polymerization of styrene and methyl methacrylate adsorbed on Aerosil was carried out. The results of both the styrene–Aerosil 300 system and the methyl methacrylate–Aerosil 300 system were similar to those of the styrene–silica gel and methyl methacrylate–silica gel systems, respectively. This suggests that in the radiation-induced polymerization of both styrene–silica gel and methyl methacrylate–silica gel systems the impurity and the presence of micropores have almost no effects on the reaction mechanism. The effect of aluminum as an impurity was investigated on the styrene–Aerosil MOX 170 system. It was found that aluminum accelerated the cationic polymerization.     

The influence of morphology on the (hydrolytic degradation of as-polymerized and hot-drawn poly(L-lactide))

 The influence of morpho-logy on the hydrolytic degradation behavior of poly(L-lactide) has been studied. High molecular weight and highly crystalline as-polymerized poly(L-lactide) was obtained in high yields through melt polymerization. Poly(L-lactide) fiber with a draw ratio of 5.6 was obtained by hot-drawing solution-spun fiber. During the bulk degradation of as-polymerized poly(L-lactide), a rapid decrease of molecular weight and tensile properties was observed. This could be explained by the morphology of the material and the presence of thermal stresses and subsequent generation of micro-cracks. The lamellar crystallites in as-polymerized poly(L-lactide) appeared to be very stable towards hydrolysis. The resorption time of high molecular weight as-polymerized poly(L-lactide) in vivo was estimated at 40–50 yr by extrapolation of molecular weight data. Hot-drawn poly(L-lactide) fiber showed exceptional hydrolytic stability under a static load and substantially retained its mechanical properties over a period of more than 5 yr. The high perfection of the crystalline fiber and the elimination of micro-voids obtained by hot-drawing prevented penetration of water and induced surface erosion of the fiber. Received: 10 February 1998 Accepted: 15 May 1998     

Anilido-imine Zinc Complexes: Efficient Catalysts for Ring-opening Polymerization of L-Lactide

A series of anilido-imine zinc complexes ortho-C₆H₄(CH=NAr)(NAr)ZnEt[Ar=2,6-Me₂C₆H₃(2a); Ar=2,6-Et₂C₆H₃(2b)] was synthesized, characterized and used as catalysts for the ring-opening polymerization of L-lactide in the presence of benzyl alcohol. The effects of the structures of catalysts and reaction conditions on the reactivity were investigated. The polymerization of L-lactide initiated by each of these complexes was found to take place with a controlled manner.     

Resorbable initiators for polymerizations of lactones

Numerous salts were prepared from cations and anions belonging to the human metabolism, such as Na, K, Mg, Ca, Zn and Fe in combination with chloride, iodide, hydroxide, carbonate, acetate, stearate, glycolate, L-lactate, D-mandelate and various N-substituted α-amiono acids. All these salts were used as initiators for polymerizations of L-lactide in bulk at 100–180°C. Furthermore, Grignard reagents, hemin and hematin were included in this study. Zn L-lactate was found to be the most useful initiator in terms of reactivity, maximum molecular weight of the isolated poly(L-lactide) and its optical purity. Zn L-lactate initiated copolymerizations of L-lactide and glycolide or ϵ-caprolactone were also studied.     

自由基引发的可注射PLA-b-PEG-b-PLA大分子单体和α-CD超分子结构水凝胶的合成与表征

以数均分子量为6000的聚乙二醇为引发剂,以辛酸亚锡为催化剂引发丙交酯开环,再用甲基丙烯酸酐进行封端生成大分子单体.然后将大分子单体和α-环糊精混合,分别用维生素C和硫酸亚铁与过硫酸铵组成的氧化还原引发剂引发聚合,得到了两种不同结构的超分子结构水凝胶.用1HNMR,FTIR,TGA和XRD等分析测试手段对大分子单体及形成的水凝胶进行了表征.流变仪测试结果表明,该水凝胶固化时间合适,并具有可注射性.     

Synthesis of ultra high molecular weight poly(dimethyldiallyl ammonium chloride)

In the study we reported the synthesis of poly-(dimethyldiallyl ammonium chloride) (PDMDAAC) with ultra high molecular weight. The polymerization method consists in the following: the ultra-high purity monomer synthesized in lab was used as the raw material, and APS as the initiator. APS was added in a lump and the polymerization temperature was increased stepwise to complete the polymerization gradually in 9 h reaction time. The maximum intrinsic viscosity ([η]) of PDMDAAC reached 3.66 dL g^–1 at a monomer conversion rate (Conv) of 96.71%. The structure and properties of PDMDAAC were characterized using FTIR, NMR, GPC-MALLS, and DTA-TG. The effects of polymerization conditions, such as the polymerization temperature, m(DMDAAC), m(APS): m(DMDAAC), and m(Na₄EDTA): m(DMDAAC), on the [η] and Conv of products were investigated. Meanwhile, the effects of the monomer purity on the [η] of products were compared. The results showed that the types and amount of impurities in monomer solution are two key factors, which affect the polymerization and, thus, the molecular weight of products.