光学活性(甲基)丙烯酰胺的立体定向自由基聚合——立体控制机理探讨

利用Lewis酸三氟甲磺酸镱(Yb(OTf)3)调控(S)-N-(2-羟基-1-苯乙基)甲基丙烯酰胺((S)-HPEMA)和(S)-N-(2-羟基-1-苯乙基)丙烯酰胺((S)-HPEA)的自由基聚合,得到相应的光学活性聚合物;研究了反应条件对聚合物立体结构的影响,发现以甲醇和正丁醇为溶剂时,Yb(OTf)3的存在可显著提高聚合物的mm三元组的含量,而以DMSO为溶剂时,Yb(OTf)3对聚合物的立构规整性没有明显改变;通过1H-NMR研究了Yb(OTf)3与单体之间的相互作用,结果表明单体的酰胺基团与Lewis酸的稀土金属离子间的配位以及单体的羟基与Lewis酸的三氟甲磺酸根间的氢键影响了单体的加成方向和聚合物的立体结构.     

The effect of hydrogen bonding on oligoleucine structure in water: A molecular dynamics simulation study

Molecular dynamics simulations were conducted in order to improve our understanding of the forces that determine polyleucine chains conformations and govern polyleucine self-assembly in aqueous solutions. Simulations of 10 repeat unit oligoleucine in aqueous solution were performed using the optimized potential for liquid simulations (OPLS) - all atom force field using the canonical ensemble for a minimum of 1.3 ns. These simulations provided information on conformations, chain collapse and intermolecular aggregation. Simulations indicate that single isotactic oligoleucine chains in dilute solution assume tightly packed, regular hairpin conformations while atactic oligoleucine assumes a much less regular and less compact structure. The regular, compact collapsed isotactic chain exhibited a greater degree of intramolecular hydrogen bonding and an increased level of hydrophobic t-butyl functional group aggregation compared to the atactic chain. This occurs at the expense of reduced leucine-water hydrogen bonding.     

Stereospecific Polymerization of Propylene: An Outlook 25 Years after Its Discovery

In 1955 Natta and his co-workers from the Istituto di Chimica Industriale del Politecnico di Milano reported the properties of highly crystalline polypropylene and other poly-α-olefins^[1] which posses, at least in long sections of the main chain, asymmetric carbon atoms of the same absolute configuration (isotactic poly-α-olefins). The discovery of the new crystalline polymers was judged at that time “revolutionary in its significance”^[2] and heralded a new era in polymer science and technology. Indeed, crystalline polypropylene, because of its relatively high melting point and its outstanding mechanical properties, has found very extensive application in the field of plastics films and fibers.—25 years after this publication, it is worthwhile examining the further developments following this discovery which has had such a great scientific and industrial impact, and the problems which are still open concerning the structure of the catalytic centers and the mechanisms by which practically completely stereoregular polypropylenes are formed.     

Insertion vs. site epimerization with singly-bridged and doubly-bridged metallocene polymerization catalysts

A statistical model has been employed to determine the unidirectional site epimerization probability, ε, during propylene polymerization with the following C₁-symmetric metallocene precatalysts activated with MAO (MAO = methylaluminoxane): doubly-bridged rac-(1,2-SiMe₂)₂{η⁵-C₅H₂-4-(CHMe(CMe₃))}{η⁵-C₅H-3,5-(CHMe₂)₂}ZrCl₂ (1) and (1,2-SiMe₂)₂{η⁵-C₅H₂-4-(1R,2S,5R-menthyl)}{η⁵-C₅H-3,5-(CHMe₂)₂}ZrCl₂ (2); and singly-bridged Me₂C(3-(2-adamantyl)-C₅H₃)(C₁₃H₈)ZrCl₂ (3) and Me₂Si(3-(2-adamantyl)-C₅H₃)(C₁₃H₈)ZrCl₂ (4). For 1/MAO a steep tacticity dependence on monomer concentration was found, as ε increased from 0.114 to 0.909 as [C₃H₆] decreased from 12.5 M to 0.5 M; similarly, ε increased for 2/MAO from 0.177 to 0.709. For 3/MAO, ε was moderately responsive to an increase in polymerization temperature, as ε increased from 0.000 to 0.485 from T_p = 0-90 °C ([C₃H₆] = 1.1 M). Similarly, ε increased for 4/MAO from 0.709 to 0.913 from T_p = 0-40 °C; at higher temperatures, bidirectional site epimerization was implicated.     

甲基丙烯酸甲酯原子转移自由基聚合等规度研究

在0～100℃温度范围内,由原子转移自由基聚合方法,采用助催化和非助催化体系,引发甲基丙烯酸甲酯聚合,利用¹³CNMR测定聚甲基丙烯酸甲酯的等规度.发现原子转移自由基聚合仍以间同立构为主,随着聚合温度的升高间同立构等规度降低,与通常自由基聚合对有规立构控制特征相似.助催化剂异丙醇铝和活性端羰基配位,对聚合物的立构规整性有一定的影响.     

SUPPORTED CATALYST WITHOUT EXTERNAL ELECTRON DONOR FOR PROPYLENE POLYMERIZATION Ⅱ. TACTICITY DISTRIBUTION AND MICROSTRUCTURE OF POLYPROPYLENE*

Propylene was polymerized with a novel supported Ziegler-Natta catalystcontaining 2,2-di-iso-butyl-1,3-dimethoxy-propane (DIBDMP) as internal donor and in theabsence of external donor. The tacticity distribution of polypropylene was obtained by usingtemperature rising elution fractionation (TREF) technique and microstructure of fractionswas studied with ~(13)C-NMR. Compared with the catalyst without electron donor, this cat-alyst gives a considerably narrower tacticity distribution. Fractionation data demonstratethat DIBDMP shows better performance than aromatic diester DNBP (di-n-butyl phtha-late). Chemically inverted propylene units and less stereoblockiness are found in the firstfraction. Possible reasons for these were presented.     

聚甲基丙烯酸甲酯链结构与无扰尺寸关系的研究

本文采用GPC-[η]联用的方法研究了一组链立规结构服从Berlloulli分布的聚甲基丙烯酸甲酯的无扰尺寸。对凝胶色谱柱组的扩展效应对无扰尺寸测定的影响进行了讨论。实验结果与Flory等人的Monte Carlo计算结果一致。本文还修正了独立内旋转条件下计算高分子链无扰尺寸的Ptitsyn公式。实验订定了不同规整度的甲基丙烯酸甲酯在30℃、四氢呋喃中的Mark-Houwink方程[η]=KM~α。     

Homo- and copolymerization of styrene and 1-alkene using Ph2Zn-Et(Ind)2ZrCl2-MAO initiator systems

Homopolymerization of α-olefins (1-C_nH_2n, n = 6, 8, 10, 12, 16 and 18) and their copolymerization with styrene were carried out in toluene at 60 °C using diphenylzinc-ethenylbisindenylzirconium dichloride-methylaluminoxane as initiator system. Atactic polystyrene and almost isotactic poly(α-olefin)s were obtained. Copolymerization of S/α-olefin with this initiator system gave isotactic olefin-enriched copolymers. According to DSC analysis, the homopolymers P(1-C₁₂H₂₄), P(1-C₁₆H₃₂), and P(1-C₁₈H₃₆) as well their styrene copolymers are crystalline.     

Evaluation of molecular mass and tacticity of polyvinyl alcohol by non-equilibrium capillary electrophoresis of equilibrium mixtures of a polymer and a dye

Non-equilibrium capillary electrophoresis of equilibrium mixtures (NECEEM) has been used to characterize polyvinyl alcohol (PVA). Commercial PVA samples with different molecular masses, from M(w)=15 up to 205 kDa, were used. According to the (13)C NMR spectra, the samples also differed in tacticity (stereoregularity). Mixtures of PVA and the anionic azo-dye Congo Red (CR) were injected in the presence of a borate buffer. The electropherograms gave a band and a peak due to the residual PVA-CR complex and the excess dye, respectively, plus a superimposed exponential decay due to the partial dissociation of the complex during migration. The stoichiometry of the PVA-CR complex, q=[monomer]/[dye], reached a maximum, q(sat), which depended on both M(w) and tacticity of PVA. Thus, q(sat) decreased from a molar ratio of ca. 4.9 to 3.6 at increasing M(w) values, this variation also being largely dependent on tacticity. A similar dependence of the electrophoretic mobility of the complex on both M(w) and tacticity was also observed. A possible explanation, based on the formation of a stack of CR ions inside the PVA-CR complex, was proposed and discussed. Finally, at increasing M(w) values, the stability constant of the complex increased slightly, and the pseudo-first order dissociation rate of the complex decreased, this later parameter also showing a dependence on both M(w) and tacticity.     

Stereocontrol Study in Radical Polymerization of 2-Acrylamido-2-methylpropane Sulfonic Acid in the Presence of Lewis Acids

Tacticity of poly-2-acrylamido-2-methylpropane sulfonic acid samples that were prepared via free radical polymerization in four different solvents, ethanol, dimethyl sulphoxide (DMSO), N, N-dimethylformamide (DMF) and water, were studied by ¹³C nuclear magnetic resonance (¹³C-NMR) techniques. In order to change isotacticity, two rare metal triflates, yttrium trifluoromethane sulfonate (Y(OTf)₃) and ytterbium trifluoromethane sulfonate (Yb(OTf)₃), were used. The stereoregularity of the obtained polymers in DMSO, water and DMF in the presence of the mentioned triflates did not show a noticeable alteration. But isotacticity of the samples prepared in ethanol was increased by about 17% in the presence 75 wt% Y(OTf)₃. In the absence of metal triflates, solvents did not have any significant effect on stereocontrol. In addition, the influence of temperature on isotacticity of the prepared samples was studied. By decreasing the reaction temperature, the isotacticity increased slightly.