ISSN:1022-1360    

DOI:10.1002/masy.19940880103

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractA living functionalization method has been investigated for group transfer polymerization (GTP) of poly(alkyl methacrylates) using ethyl 2‐phenyl‐2‐butenoate (EPB). The end‐capping reactions of EPB to living trimethylsilyl ketene acetal‐ended poly(methyl methacrylate) (PMMA) chain ends have been systematically studied and characterized by SEC, VPO, UV‐visible spectrosocopy,1H and13c NMR spectroscopy. The results of sequential monomer addition, varying stoichiometry and copolymerization indicate that EPB effects efficient chain end functionalization only at stoichiometric concentrations; chain transfer reactions (chain transfer constant = 0.4) occur with excess EPB and during copolymerization with MMA. Chain transfer reactions (chain transfer constant = 0.1) also occur when copolymerizing ethyl 2‐methyl‐2‐b

ISSN:1022-1360    

DOI:10.1002/masy.19940880104

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe methylation (CH3I) of 1,3‐bis(2‐pyridyl)butyllithium in THF at −78°C is highly meso‐selective (>98%) but the selectivity decreases with increasing cation‐size or ‐coordination. The reaction of 1,3‐bis(2‐pyridyl)butyllithium with other electrophiles such as i‐C3H7Br, PhCH2Cl, Me3SiCl (CD3)2CO and 4‐vinylpyridine is also stereoselective under these conditions giving meso‐like products. On the other hand, addition of 2‐vinylpyridine is only slightly selective (64%) and this is consistent with the 65% meso content of P2VP formed by polymerization in the presence of Li ion. The chemistry of the above reactions is rationalized by intramolecular coordination of Li or other cations by the penultimate 2‐pyridyl group and this is supported by equilibria involving proton abstraction of 1,3‐bis(2‐pyridyl)butane and similar compounds by bases having Li, Na or K counterions. It is shown that the tendency for intramolecular chelation is highest for Li and lowest for K ion. Temperature dependence of the above equilibrium shows that intramolecular chelation of Li and Na ions is exothermic (−1.4 and −1.3 kcal respectively) whereas the AH for K ion is very small (−0.5 kcal). Entropies of chelation are slightly positive for Li (0.8 e.u.) and negative for Na and K ions (‐2.60 and −0.40 e.u. respectively). The lack of stereoregular polymerization of 2‐VP in the presence of Li ion is most likely due to the requirement that the Li ion of the newly formed 2‐pyridyl anion is coordinated with the 2‐pyridyl group of the previous asymmetric center. Thus it would appear that intramolecular coordination of metal ion by penultimate 2‐pyridine does

ISSN:1022-1360    

DOI:10.1002/masy.19940880105

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractAbstract: Catalysis of GTP of MMA with nucleophilic anions on cross‐linked polystyrene supports was studied. With anion‐bound supported catalyst, evidence is presented for formation in solution of ester enolates as reaction intermediates. Study of the cyanide‐catalyzed initiation of GTP of MMA by TMSCN, has provided quantitative data for the association constant Kafor the complexation of cyanide by TMSCN and, by inference, an upper limit for the Kafor the association of this nucleophilic anion with silyl ketene acetals. The effects ofi‐propyl‐ andt‐butoxy‐silyl analogs of TMSCN on anion‐complexation and on initiation and propagation of GTP are discussed. Coordination by hydrogen‐bonding of nucleophilic anions to acetonitrile is shown to be the mechanism for “livingness‐enhancement” of anion‐catalyzed GTP at low concentrations of acetonitrile. GTP was used to prepare an ABC triblock dispersant, poly(methacrylic acid)‐block‐poly(2‐phenylethyl methacrylate)‐block‐poly(ethoxytriethylene glycol methacrylate), and the surface activity of an aqueous solution of the potassum salt was compared with

ISSN:1022-1360    

DOI:10.1002/masy.19940880106

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThis paper is a brief survey on the preparation of latexes bearing covalently bonded functional groups using various routes. Recent methods are presented using water soluble functional (or surface active) initiators, monomers or macromonomers. For tailor synthesis, the copolymerisation approach of a basic monomer with a functional monomer under emulsifier‐free conditions is generally the more suitable and versatile one. More progress should be pursued in this field in order to design better adapted chemical structures which will improve their use in radical‐initiated heterogeneous conditions and favour surface incorporat

ISSN:1022-1360    

DOI:10.1002/masy.19940880107

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractConventional free radical polymerization processes are limited by their inability to be used for the synthesis of resins of controlled molecular weights and complex architecture. New processes are being developed in which the free radical propagating chain is reversibly terminated, allowing the free radical polymerization to behave in a living fashion. A review of these processes is presented, along with preliminary results of chain extension studies of nitroxide‐terminated polystyrene as a model for block copolymer synthesi

ISSN:1022-1360    

DOI:10.1002/masy.19940880108

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe nature of the growing species in living cationic polymerizations of vinyl ethers was discussed on the basis of in‐situ direct analysis of model growing ends [CH3Cδ+H(OiBu)… Bδ…MXn] by multinuclear NMR spectroscopy. The model reactions were to mimic the polymerizations initiated with protonic acid/metal halide systems [HB/MXn; HB = HCl, CF3CO2H, (C6H5O)2PO2H; MXn= ZnCl2, SnCl4], whereallof the three components of the growing end [carbocation (∼∼C+), counter‐anion (B−), and activator (MXn)] were analyzed by monitoring such nuclei as1H,13C,19F,31P, and119Sn incorporated therein. The results demonstrated: (i) the MXn‐assisted formation ofsp2‐type carbocations from the precursor CH3CH(OiBu)‐B; (ii) the interaction of MXnwith the counteranion B−; and (iii) the suppression of dissociated carbocationic intermediates under the living po

ISSN:1022-1360    

DOI:10.1002/masy.19940880109

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractAn organoaluminum compound with bulky phenoxide groups as a Lewis acid can accelerate much the living polymerization of alkylene oxide initiated with aluminum porphyrin by the coordinative activation of the monomer. This concept can be extented to the polymerization initiated with aluminum Schiff base and tetraazaannulene complexes.

ISSN:1022-1360    

DOI:10.1002/masy.19940880110

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe nanoscopic domain of structural complexity, which ranges from 1 to 100 run on a particle size scale includes a relatively unexplored area of science which resides between classical chemistry and molecular biology. This rapidly growing area of science is referred to as nanoscopic chemistry and architecture. Concepts evolving in this area lead to a rich variety of precise structures, architecture and properties. These concepts are based on dendritic macromolecules in general and on Starburst® dendrimers in particular. They envision dendrimers as fundamental building blocks which may be used to synthesize nanoscopic compounds, clusters, polymers, etc. Accordingly, dendrimers are regarded architecturally as functional analogues of atoms; therefore, their potential role in nanoscopic chemistry may be compared to that of the atoms in classical chemistry

ISSN:1022-1360    

DOI:10.1002/masy.19940880111

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractA linear neutral repetitive peptide, (Leu‐Gln‐Pro)n(n = 1‐100), an analog of the Ca2+ion‐binding domain of the matrix protein, amelogenin, has been synthesized. The polypeptide is mostly unordered in trifluoroethanol (TFE) at 25°C and its circular dichroism (CD) spectrum resembles that of the protein itself. In TFE the CD spectrum of (Leu‐Gln‐Pro)nreveals that the polypeptide interacts strongly with divalent cations Mg2+, Ca2+, Sr2+, and Ba2+accompanied by conformational rearrangement from a random to a more‐ordered one. In the presence of Li+, Na+and K+ions no such conformational change has been observed. The CD curves of the complexes suggest the presence of type I β‐turns and reinforce the hypothesis that the region Gln112‐Leu138in amelogenin is the Ca2+‐binding domain. In the solid state, powder X‐ray diffraction suggests that the polymeric (LQP)nmay exist a

ISSN:1022-1360    

DOI:10.1002/masy.19940880112

出版商:Hüthig&Wepf Verlag    

年代:1994

数据来源: WILEY