Dialkylzinc Compounds as Chain Transfer Agents in Ethylene and Propylene Polymerizations Catalyzed by Metallocene Catalysts

Dialkylzinc compounds (ZnR₂) with the alkyl groups of different steric hindrance were used as chain transfer agents in ethylene and propylene polymerizations catalyzed by two conventional metallocene catalysts including rac-Et(Ind)₂ZrCl₂ and rac-Me₂Si[2-Me-4-Ph-Ind]₂ZrCl₂. In general, catalyst activities for ethylene polymerizations are barely affected by chain transfer agents, regardless of the R type; however, there are significant activity reductions in propylene polymerizations when the R in ZnR₂ is less hindered, and as R becomes bulkier, catalyst activities are gradually restored. ZnR₂ and metallocene catalyst active sites tend to form a reversible and catalytically inactive complex, thus, the geometry congested ZnR₂ would reduce complex formation tendency and hence, decrease its negative effect on catalyst activities.     

Dialkylzinc Compounds as Chain Transfer Agents in Ethylene and Propylene Polymerizations Catalyzed by Metallocene Catalysts

Dialkylzinc compounds (ZnR₂) with the alkyl groups of different steric hindrance were used as chain transfer agents in ethylene and propylene polymerizations catalyzed by two conventional metallocene catalysts including rac-Et(Ind)₂ZrCl₂ and rac-Me₂Si[2-Me-4-Ph-Ind]₂ZrCl₂. In general, catalyst activities for ethylene polymerizations are barely affected by chain transfer agents, regardless of the R type; however, there are significant activity reductions in propylene polymerizations when the R in ZnR₂ is less hindered, and as R becomes bulkier, catalyst activities are gradually restored. ZnR₂ and metallocene catalyst active site tend to form a reversible and catalytically inactive complex, thus the geometry congested ZnR₂ would reduce complex formation tendency and hence decreased its negative effect on catalyst activities.     

A Strategy for Controlling the Polymerizations of Thiyl Radical Propagation by RAFT Agents

The ability to extend the polymerizations of thiyl radical propagation to be regulated by existing controlled methods would be highly desirable, yet remained very challenging to achieve because the thiyl radicals still cannot be reversibly controlled by these methods. In this article, we reported a novel strategy that could enable the radical ring-opening polymerization of macrocyclic allylic sulfides, wherein propagating specie is thiyl radical, to be controlled by reversible addition–fragmentation chain transfer (RAFT) agents. The key to the success of this strategy is the propagating thiyl radical can undergo desulfurization with isocyanide and generate a stabilized alkyl radical for reversible control. Systematic optimization of the reaction conditions allowed good control over the polymerization, leading to the formation of polymers with well-defined architectures, exemplified by the radical block copolymerization of macrocyclic allylic sulfides and vinyl monomers and the incorporation of sequence-defined segments into the polymer backbone. This work represents a significant step toward directly enabling the polymerizations of heteroatom-centered radical propagation to be regulated by existing reversible-deactivation radical polymerization techniques.     

Recent Advances in the Study of Multifunctional Initiators in Free Radical Polymerizations

An overview of a systematic investigation of a tetrafunctional peroxide initiator's behaviour is presented. The study focuses on three main areas of research: kinetic experiments, polymer characterization and modelling efforts. The kinetic investigation compared the behaviour of the tetrafunctional initiator (JWEB50) to that of a monofunctional counterpart (TBEC) for a variety of monomers. Although higher rates of polymerization were generated with JWEB50 for all monomers investigated, switching from a mono‐ to a tetrafunctional initiator actually decreased the polymer molecular weight for methyl methacrylate. While chromatographic characterization methods were able to detect branching in polystyrene samples produced with JWEB50, this was not the case for poly(methyl methacrylate). However, evidence of branching was clearly observed for both polystyrene and PMMA when rheological methods were employed. In order to explain the experimental results, a mathematical model was developed. Through its use, the concentration and chain length of various polymer structures (i.e., linear, star or coupled stars) was found to depend upon monomer type and reaction conditions.

     

可控自由基聚合与其它活性聚合方法结合制备嵌段共聚物

可控自由基聚合与其它聚合方法结合,可以制备多种类型的嵌段共聚物,因此得到了广泛关注。本文着重介绍可控自由基聚合与离子开环聚合、阴离子聚合、烯类单体的阳离子聚合及其它活性聚合方法结合制备嵌段共聚物的研究现状和进展。     

无外给电子体的Ziegler-Natta丙烯聚合复相催化剂研究进展

从新型给电子体、催化剂性能、活性中心模型、催化和聚合机理研究及理论模拟这了无外给电子体的Ｚｉｅｇｌｅｒ－Ｎａｔｔａ丙烯聚合复相催化剂的研究进展。     

Solvent Influence on Propagation Kinetics in Radical Polymerizations Studied by Pulsed Laser Initiated Polymerizations

The influence of the reaction medium (organic solvents, water, ionic liquids, supercritical CO₂) on the propagation rate in radical polymerizations has very different causes, e.g., hindered rotational modes, hydrogen bonding or electron pair donor/acceptor interactions. Depending on the origin of the solvent influence propagation rate coefficients, k_p, may be enhanced by up to an order of magnitude associated with changes in the pre‐exponential or the activation energy of k_p. In contrast, non‐specific interactions, size and steric effects lead to rather small changes in the vicinity of the radical chain end and are reflected by modest variations in k_p.

     

Mechanistic Transformations Involving Radical and Cationic Polymerizations

Mechanistic transformation approach has been widely applied in polymer synthesis due to its unique feature combining structurally different polymers prepared by different polymerization mechanisms.Reported methods for the formation of block and graft copolymers through mechanistic transformation involve almost all polymerizations modes.However,certain polymerization processes require extensive purification processes,which can be time-consuming and problematic.Recent developments on controlled/living polymerizations involving radical and cationic mechanisms with the ability to control molecular weight and functionality led to new pathways for mechanistic transformations.In this mini-review,we systematically discussed relevant advances in the field through three main titles namely(i)from radical to cationic mechanism,(ii)from cationic to radical mechanism,and(iii)application of specific catalyst systems for both radical and cationic polymerizations.     

Templating Organosilicate Vitrification Using Unimolecular Self-Organizing Polymers Prepared from Tandem Ring Opening and Atom Transfer Radical Polymerizations

A unimolecular templating star-shaped polymer with a compatibilizing outer corona, prepared by tandem ROP/ATRP procedures, was dispersed into a thermosetting organosilicate. The organic polymer was thermalized to leave behind its latent image in the matrix with a pore size that reflected the size of the polymer molecule, and provided the expected reduction in dielectric constant.     

Living Polymers in Ionic and Radical Polymerizations: Recent Developments

The discovery of living polymers, that is, assemblies of polymer molecules formed by anionic polymerization which may grow without chain-breaking reaction and may react subsequently with other monomers and various reagents through their end-groups, has led to great progress in the knowledge of the mechanism of anionic polymerization and to the synthesis of a large variety of well-defined block copolymers, graft co-polymers, and polymers with functionalized end-groups. Since only a limited number of the current monomers are polymerizable by an anionic mechanism, many attempts have been made to obtain similar results by polymerizing other monomers by cationic, radical, and Ziegler polymerization. Systems making it possible to work at temperatures higher than those used for many anionic and most cationic polymerizations would be particularly interesting.     

Germanium- and Tin-Catalyzed Living Radical Polymerizations of Styrene and Methacrylates

Summary : Ge and Sn (non-transition-metal) catalyzed living radical polymerizations were developed. Low-polydispersity (M_w/M_n ∼ 1.1–1.3) polystyrenes, poly(methyl methacrylate)s, poly(glycidyl methacrylate)s, and poly(2-hydroxyethyl methacrylate) with predicted molecular weights were obtained with a fairly high conversion in a fairly short time. The pseudo-first-order activation rate constant k_act for the styrene/GeI₄ (catalyst) system was large enough, even with a small amount of GeI₄, to explain why the system provides low-polydispersity polymers from an early stage of polymerization. The retardation in the polymerization rate observed for the styrene/GeI₄ system was kinetically proved to be mainly due to the cross-termination between the propagating radical with GeI. Attractive features of the Ge and Sn catalysts include their high reactivity hence small amounts (1–5 mM) being required under a mild condition (at 60–80 °C), high solubility in organic media without ligands, insensitivity to air hence sample preparation being allowed in the air, and minor color and smell. The Ge catalysts may also be attractive for their low toxicity.     

介孔材料孔道内进行的非自由基聚合反应研究进展

介孔材料在催化、吸附、分离等领域有潜在的应用价值.近来,研究人员将介孔材料的有序孔道作为聚合微反应器,在其内部进行了许多类型的聚合反应,如自由基聚合反应和非自由基聚合反应.本文综述了近二十年来在介孔材料孔道内各类单体进行的非自由基聚合反应,包括氧化聚合反应、配位聚合反应、缩合聚合反应、开环聚合反应和阳离子聚合反应,阐述...     

Transient Behavior and Gelation of Free Radical Polymerizations in Continuous Stirred Tank Reactors

Summary: Using the authors' previously developed method for the general kinetic analysis of non‐linear irreversible polymerizations, the simulation of free radical homogeneous polymerization systems with terminal branching and chain transfer to polymer has been carried out for continuous stirred tank reactors. Its improved accuracy on the numerical evaluation of generating functions has been exploited in order to perform their numerical inversion and chain length distributions could also be estimated with or without the presence of gel. A comparison with alternative techniques emphasizing the effect of their simplifying assumptions on the accuracy of calculations is also presented.

Predicted CLD before gelation (t = 1 h), after gelation (t = 15 h, steady state), and close to gel point for a free radical polymerization with transfer to polymer in a CSTR with τ = 60 min.     

Use of Allylbenzene and Allyl Phenyl Ether as Chain-Transfer Agents in Radical Polymerization

The performance of allylbenzene and allyl phenyl ether as chain-transfer agents in radical polymerization of styrene, methyl acrylate, butyl methacrylate, vinylpyrrolidone, and vinylcaprolactam was evaluated.     

Aluminum Alkyls as Highly Active Catalytic Chain Transfer Agents

During the production of free radical initiated low‐density polyethylene (LDPE), it was discovered that the addition of low levels of alkyl aluminum compounds caused the molecular weight of the LDPE to drop precipitously. Further investigation demonstrated that aluminum‐alkyl compounds are among the most effective chain transfer agents ever utilized. It was also shown that polymer chains, which transfer to Al alkyl species, contain almost exclusively vinyl terminated end groups. A catalytic chain transfer mechanism is proposed in which chain transfer occurs from a growing polymer chain to an aluminum center followed by beta hydride elimination to produce a vinyl terminated polymer chain and a new aluminum hydride bond. This new aluminum hydride bond can then undergo further chain transfer reactions. This is the first time such a catalytic chain transfer mechanism has been reported. As little as 10–20 mol ppm aluminum alkyl species decreased the degree of polymerization by a factor of 2 resulting in chain transfer constant (C_s) values as high as 1000–2000. Density functional theory (DFT) study elucidated the catalytic cycle of triethylaluminum (TEA). It is discovered that, depending on the reaction conditions, TEA can serve as a conventional as well as catalytic chain transfer agent.     

α－甲基丙烯酸烯丙酯的合成及其自由基、阴离子聚合的研究

合成了α－甲基丙烯酸烯丙酯（ＡＭＡ），并对其自由基、阴离子聚合进行了探讨。结果发现，该单体难以进行选择性自由基聚合，但可用作多种单体自由基聚合的交联剂。用１，１′－二苯基己基锂在ＴＨＦ中引发ＡＭＡ，可顺利地进行α位双键的选择性阴离子聚合，分子量实测值与计算值基本一致。在较低温度下（≤－６０℃），可得窄分布ＰＡＭＡ（Ｍｗ／Ｍｎ＝１．１２～１．１５）。随聚合温度升高，间同和无规聚合物含量分别呈下降和上升趋势。ＧＰＣ、１ＨＮＭＲ及ＦＴＩＲ鉴定表明，用阴离子聚合法可得到溶于多种溶剂、每个重复单元上均定量带有烯丙基双键的窄分布官能性ＰＡＭＡ。     

Critical Evaluation of the Microwave Effect on Radical (Co)Polymerizations

Critical evaluations of the microwave effect on initiation, propagation, and termination during conventional radical polymerizations (RPs) of methyl methacrylate (MMA) and random copolymerization of styrene (St) with (meth)acrylates are examined by comparing microwave heating (MWH) and conventional heating (CH). Poly(methyl methacrylate) with similar , / , and conversion are obtained under precisely controlled temperature, indicating very small changes of propagation rate constant. Rate enhancement in the absence of precise temperature control is mostly due to the higher reaction temperature of the reaction mixture than the apparent value indicated on display. Rates of initiator decomposition under well‐controlled temperature are essentially the same for MWH and CH.     

Propagation Kinetics of Isoprene Radical Homopolymerization Derived from Pulsed Laser Initiated Polymerizations

The propagation kinetics of isoprene radical polymerizations in bulk and in solution are investigated via pulsed laser initiated polymerizations and subsequent polymer analyses via size‐exclusion chromatography, the PLP‐SEC method. Because of low polymerization rate and high volatility of isoprene, the polymerizations are carried out at elevated pressure ranging from 134 to 1320 bar. The temperatures are varied between 55 and 105 °C. PLP‐SEC yields activation parameters of k_p (Arrhenius parameters and activation volume) over a wide temperature and pressure range that allow for the calculation of k_p at technically relevant ambient pressure conditions. The k_p values determined are very low, e.g., 99 L mol^?1 s^?1 at 50 °C, which is even lower than the corresponding value for styrene polymerizations. The presence of a polar solvent results in a slight increase of k_p compared to the bulk system. The k_p values reported are important for determining rate coefficients of other elemental reactions from coupled parameters as well as for modeling isoprene free‐radical polymerizations and reversible deactivation radical polymerization with respect to tailored polymer properties and optimizing the polymerization processes.