In quasiliving polymerizations with reversible chain transfer (QL0R systems), polymers with narrow molecular weight distribution can be obtained, It has been shown that while in true living systems (L00) R = 1, and in quasiliving systems with irreversible chain transfer (QL01) R = 2 is the limiting value of polydispersity, in QL0R systems r = 4/3 is the polydispersity to which the distribution of the polymer tends with increasing polymerization time. This limit is independent of the rate of reinitiation; the course of the R vs t curves is, however, determined by the various rate constants. 相似文献
All attempts at synthesizing side chain liquid crystalline polymers (SCLCPs) with chiral smectic C (sc*) mesophases simply functionalize one terminal group of the mesogen with a chiral substituent and attach the other terminus to the polymer backbone through a spacer. If a sc* mesophase is observed, it is usually in the less desirable sc*-sA phase sequence. We propose that SCLCPs with laterally attached (vs terminally attached) mesogens offer an ideal architecture for obtaining sc* meso- phases. This is because extended mesogens symmetrically disubstituted with longn-alkoxy groups can be attached to the polymer backbone through a chiral spacer. Thus, mesogens which typically form the desirable sc*-n phase sequence can be used, and the chiral group can be introduced at the center of the mesogen which should result in high values of spontaneous polarization. We are not only using mesogens which exhibit sc*-n phase sequences, but are also attempting to induce smectic layering into laterally attached systems which typically form nematic mesophases by electron-donor-acceptor interactions and immiscible hydrocarbon/fluorocarbon components. Smectic layering was successfully induced in 2,5-bis[(4'-n-alkoxybenzoyl)oxy]toluenes when the n-alkoxy substituents were terminated with perfluorinated segments. 相似文献
Herein, we report a cooperatively palladium/norbornene‐catalyzed polymerization, which simplifies the synthesis of functional aromatic polymers, including conjugated polymers. Specifically, an A2B2C‐type multicomponent polymerization that is based on an ortho amination/ipso alkynylation reaction was developed for the preparation of various amine‐functionalized arylacetylene‐containing polymers. Within a single catalytic cycle, the amine side chains are site‐selectively installed in situ by C?H activation during the polymerization process, which represents a major difference from conventional cross‐coupling polymerizations. This “in situ functionalization” strategy enables the modular incorporation of functional side chains starting from simple monomers, thereby conveniently affording a diverse range of functional polymers. 相似文献
Cationic Pd(II) -complexes with weakly coordinating ligands were used for the olefin addition polymerization of strained polycyclic olefins. The cyclic structure of the monomers remained intact during the reaction which contrasts with products obtained from the olefin metathesis polymerization. The Pd(II) -catalyzed polymerizations showed the features of a “living” polymerization, when norbornene and selected exo-substituted norbornene derivatives were used as the monomers. Endo- and exo-dicyclopentadiene, exo-1,2-dihydrodicyclopentadiene, endo, exo-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene and endo, exo-1,4,5,8-dimethano-1,4,4,a,5,8,8a-hexahydronaphthalene were converted into the corresponding rigid polymers. The exo-substituted monomers were found to polymerize at a higher rate than the corresponding or similar endo-substituted monomers. The polymerization of norbornadiene and the subsequent thermal elimination of cyclopentadiene resulted in the formation of polyacetylene. 相似文献
Polymerization of norbornene using PdCl2 as catalyst is generally carried out at high temperatures in the absence of solvent. Low yields of polymer with relatively undefined molecular weights are obtained under these conditions. We describe the first example of polymerization of norbornene in water dispersion or aqueous emulsions catalyzed by PdCl2. Good yields of polymers and oligomers were obtained. Furthermore, in aqueous emulsions, novel microlatex which cannot be obtained by radical or ionic routes were synthesized, with particles sizes (10 nm.) generally only observed in microemulsion polymerizations.The stereochemistry of the polymers of low molecular weight was partially elucidated by13C NMR. 相似文献
Free radical polymerization of acrylonitrile (AN) in ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 2,2;m1-azobisisobutyronitrile (AIBN) as initiator was investigated. Early investigations on polymerizations using ionic liquids indicate that they serve as especially good solvents to achieve high molecular weight polymers. Free radical polymerizations result in higher molecular weight polymers, for ionic liquids have low chain transfer constants and act to stabilize the active radical during the process of polymerization. The thermal stability of polymers synthesized in ionic liquids have be improved obviously than that in traditional solvents. 相似文献
The synthetic details of solution polymerization in benzene and bulk polymerization of vinylferrocene are reported. In benzene solutions, with azobisisobutyronitrile (AIBN) as the initiator, small yields of low-polydispersity low molecular weight (M?n ? 5000) polyvinylferrocene is obtained. However, high yields can be obtained by continuous or multiple AIBN addition. Higher molecular weight polymers and binodal polymers can be obtained as the monomer concentration is increased. In bulk polymerizations, yields of 80% can be obtained. The molecular weight increases as temperature decreases from 80 to 60°C in bulk polymerizations, and an increasing amount of insoluble polymer results. The soluble portion is often binodal, the higher molecular weight node consisting of an increasingly branched structure. Lower molecular weight polymer was readily fractionated into narrow fractions from benzene–methanol systems, but higher molecular weight polymer proved impossible to fractionate into narrow fractions due to branching. 相似文献
Summary: Homopolymers and diblock copolymers that contain maltose or glucose residues have been prepared by ring‐opening metathesis polymerization of norbornene derivatives using a molybdenum–alkylidene initiator, Mo(CHCMe2Ph)(N‐2,6‐iPr2C6H3)(OtBu)2 ( A ). These polymerizations took place not only in a living fashion ( = < 1.2) but also with almost quantitative initiation. Two types of ruthenium initiators, (Cy3P)2RuCl2(CHPh) ( B ) and (IMesH2)(Cy3P)RuCl2(CHPh) ( C ), have also been used to compare initiator performance under the same conditions.
A route of synthesizing triblock terpolymers in a one‐pot, “one‐step” polymerization approach is presented. The combination of two distinct polymerization techniques through orthogonal catalyst/initiator functionalities attached to a polymeric linker furnishes novel pathways to ABC‐terpolymers. Both polymerizations have to be compatible regarding mechanisms, chosen monomers, and solvents. Here, an α,ω‐heterobifunctional poly(ethylene glycol) serves as polymeric catalyst/initiator to obtain triblock terpolymers of poly(norbornene)‐b‐poly(ethylene glycol)‐b‐poly(l ‐lactic acid) PNB‐PEG‐PLLA via simultaneous ring opening metathesis polymerization and ring opening polymerization in a fast one‐pot polymerization. Structural characterization of the polymers is provided via 1H‐, DOSY‐, and 1H,1H‐COSY‐NMR, while solution and thin film self‐assembly are investigated by dynamic light scattering and atomic force microscopy.
Base-catalyzed hydrogen-transfer polymerization and copolymerization of acrylamide and its methyl-substituted derivatives were studied in pyridine at 110°C. n-Butyllithium was used as an initiator. The observed rates of these homopolymerizations were found to decrease in the following order: acrylamide > crotonamide > methacrylamide > N-methylacrylamide > N-methylcrotonamide > tiglinamide > N-methylmethacrylamide ? α-chlorocrotonamide ? α-cyanocrotonamide = 0. Acrylamide gave the polymer with the highest degree of polymerization among the monomers examined. It was found that the number and the position of the methyl substituent in acrylamide affected significantly both the rate of polymerization and the molecular weight of the polymer. Although all polymers obtained, except the N-methyl derivatives, contained both methanol-soluble and methanol-insoluble fractions, a polyamide structure with unsaturated terminal monomer unit was confirmed by both infrared and NMR determinations. From the NMR determination of the saturated and terminal unsaturated units, the degree of polymerization of the resulting polyamides were also obtained. The monomers were also found to copolymerize by a hydrogen-transfer mechanism. However, the main chain of the resulting copolymers was composed of the more reactive monomer unit, and the less reactive monomer was incorporated only as a terminal unit when a less reactive monomer was copolymerized with a more reactive one. From these results, it was concluded that these polymerizations proceeded via an intermolecular hydrogen-transfer mechanism (i.e., stepwise mechanism). 相似文献
The cationic monomers (CNBr), obtained by quarternization of dimethylaminoethyl methacrylate with n-alkyl bromide containing varying carbon number (N = 4, 8, 12, 14, and 16) were polymerized with radical initiators in water and various organic solvents. The degree of polymerization of the resulting polymers was determined by GPC measurements on poly(methyl methacrylate) samples derived from them. The rate of polymerization of the micelle-forming monomers (N = 8, 12, 14, and 16) in water increases with increasing a chain length of alkyl group, whereas it is little dependent on N in isotropic solution in dimethylformamide. The data on the degree of polymerization for the polymers of C4Br, C8Br, and C12Br show that the polymerization of C12Br with azo initiators in water and benzene gives polymers with a very high degree of polymerization. The results obtained here suggest that highly developed or relatively rigid, aggregated structures of monomers in solution are responsible for the formation of the polymers with a very high degree of polymerization, in addition to an enhanced rate of polymerization. Also considered are the relation of the molecular weight of poly(C12Br) to the viscosity data in chloroform and methanol. 相似文献
Deterministic methods for tuning polymer dispersity are rare, especially for nonradical polymerizations. Reported here is the first example of photomodulating dispersity in controlled cationic polymerizations of vinyl ethers using carboxy‐functionalized dithienylethene initiators. Reversible photoisomerization of these initiators induces changes in their acidities by up to an order of magnitude. Using the more acidic, ring‐closed isomers as initiators results in polymers with lower dispersities. The degree of light‐induced pKa change in the initiators correlates with the degree of dispersity change in polymers derived from the isomeric initiators. The polymerizations are controlled, and dynamic photoswitching of dispersity during the polymerization reaction was demonstrated. This work provides a framework for photomodulating dispersity in other controlled polymerizations and developing one‐pot block copolymerization reactions in which the dispersities of component blocks can be controlled using light. 相似文献