Trialkylboranes react with chloroacetonitrile in the presence of potassium 2.6-di-tert-butylphenoxide to give nitriles. This reaction, coupled with hydroboration by 9-bora-bicyclononane, allows selective cyanomethylation of alkenes. The application of this reaction to trialkylborane structures derived from alkene structures of polyisoprenes and polybutadienes has been considered. In order to determine the polydienic structures best suited for this reaction, we have studied model molecules of 1.4 and 1.2 polybutadienes and of 1.4 and 3.4 polyisoprenes and then applied the process to the corresponding polymers. 相似文献
Isoprene units in natural rubber (NR) and its synthetic analogues were quantified by 1H-NMR spectroscopy using polyethylene glycol (PEG) as an internal standard. The effect of PEG and rubber concentrations, molar ratio of rubber/PEG, measuring temperature and scan number on the quantification was investigated to establish the respective working range. Analysis of commercial grades of NR revealed that the differences in 1,4 isoprene content is caused by the production process and feedstock, in which proteins and lipids were found to be the major impurity in NR. Gel fraction of NR has insignificant effect on the measurement of 1,4 isoprene content. Furthermore, the new method was found to produce good results for the quantification of 1,4 and 3,4 units of synthetic polyisoprenes. 相似文献
The rheological properties of narrow-molecular-weight-distribution linear and star-branched polyisoprenes have been determined using both shearing and stretching deformations. At all strain rates studied the tensile stress measured under transient and steady-state conditions did not increase above the linear viscoelastic value. The absence of an enhanced tensile stress for the branched polymer is in contrast to what is observed for branched low-density polyethylene. An explanation for the difference is proposed. Additional remarks are made about the broad distribution of relaxation times observed for star-branched polyisoprenes and about the approach to steady state in constant-strain-rate and constant-stress tests. 相似文献
The 13C-NMR spectra of hydrogenated polyisoprenes were investigated. Polyisoprenes containing various ratios of 1,4 and 3,4 units were prepared with n-BuLi/Et2O and were hydrogenated by using p-toluenesulfonylhydrazide. 13C-NMR signals of the hydrogenated polymers were assigned for the triad sequences arising respectively from the 1, 4 and 3, 4 units of polyisoprenes. On the basis of these assignments it was confirmed that 1,4 and 3,4 units were randomly distributed in the polyisoprenes prepared with n-BuLi and that these polymers did not contain appreciable amounts of head-to-head or tail-to-tail 1,4 linkages. 相似文献
Through neodymium-mediated coordinative chain transfer copolymerizaiton(CCTcoP), polyisoprenes bearing dual hydroxylated mini-blocky chain-ends were prepared via a three-step strategy. Kinetic studies revealed that, the polymerization demonstrated typical features of CCTcoP across the whole polymerization process, i.e., quasi-living polymerization characteristic, tunable molecular weights, narrow molecular weight distributions, and atom economies. Comparing to previously reported CCTP homopolyme... 相似文献
A number of linear, four- and six-branched regular star polyisoprenes were synthesized by anionic polymerization techniques in benzene using lithium as the counterion and polyfunctional silicon chloride compounds as the coupling agents. Light-scattering measurements in dioxane were performed in order to establish Θ solvent conditions. Determinations of the radius of gyration of the polymers of different structure indicate that g = 〈S2〉0,br/〈S2〉0,lin agree closely with random flight calculations for the ratios. Intrinsic viscosities determined in a Θ solvent establish g′ = [η]br/[η]lin to be 0.773 and 0.625 for the four- and six-branched polyisoprenes, respectively. In a good solvent g′ values are slightly lower. These values are compared with theoretical estimates. Viscosities of 19.29% (w/w) solutions of the polyisoprenes in n-decane at 25°C are correlated with the intrinsic viscosities of the polymers under Θ conditions. 相似文献
The present work reports some new results on the dependence between the stereospecificity of the anionic propagation of isoprene and the nature of the active centers. The stereospecificity of the propagation of macroheterobicylce separated ion pairs does not depend on the nature of the alkali counterions, and the microstructure of the polyisoprenes obtained does not differ from that of those produced by free ions. Variations in the microstructure were observed in the propagation of contact ion pairs, depending on the nature of the cation and the solvent used. These variations are likely to be related to the degree of intimacy of the contact ion pairs. There are two main factors which affect the stereospecificity of the propagations in different ways: the size of the cation and the donating power of the solvent. Finally, the external solvation of the propagation ions pairs was confirmed by the microstructure of polyisoprenes synthesized in mixed (inert + donor) solvents. 相似文献
The dynamic shear modulus and the flow rate through capillaries under constant pressure and under constant velocity of the piston, have been measured for polybutadienes and polyisoprenes of narrow molecular weight distribution with molecular weights ranging, respectively, from 3.8 × 104 to 5.8 × 105 and from 1.06 × 105 to 6.02 × 105. The phenomena of the discontinuous increase of volume flow rate and self-oscillatory flow regime at critical rates of deformation have been considered in detail. It is proposed that these phenomena are due to the induced transition of the polymer from the fluid to the high-elastic state at higher deformation rates. As a result, an inference has been made that polybutadienes and polyisoprenes with a narrow molecular weight distribution in the high-elastic state, behave in certain respects as crosslinked polymers incapable of displaying fluidity. The quantitative relationships among the viscoelastic characteristics measured under dynamic regimes, the parameters determining the critical flow regimes, and the molecular weights of polybutadienes and polyisoprenes have been worked out. 相似文献
An efficient and simple method for the synthesis of symmetric macromolecules by photoinduced switching from radical polymerization to a radical coupling reaction is reported. Structurally well-defined telechelic polyisoprenes and ABA-triblock copolymers were prepared by successive organotellurium-mediated living radical polymerization (TERP) under thermal conditions, followed by a polymer-end radical coupling reaction under photoirradiation. 相似文献
Two methods of differentiating between natural rubber and synthetic cis-1,4-polyisoprenes have been examined. Both techniques depend on the presence of Ziegler-Natta catalyst residues in the synthetic polymers. The major pyrolysis product of cis-1,4-polyisoprenes at 350°C is 1-methyl-4-(1-methylethenyl)cyclohexene. This can undergo disproportionation to yield 1-methyl-4-(1-methylethyl)benzene and methyl-(1-methylethyl)cyclohexenes. It is this disproportionation reaction, catalyzed by Ziegler-Natta catalyst residues or by carbon black, that is responsible for the different product ratios obtained on pyrolysis of natural rubber and Ziegler-Natta catalyzed cis-1,4-polyisoprenes. Lithium alkyl-polymerized polyisoprenes undergo this secondary disproportionation reaction only in the presence of carbon black. Derivative thermogravimetric traces of black-filled sulfur vulcanizates of natural rubber and synthetic polyisoprenes are significantly different because polymerization catalyst residues promote cyclization of the polymer. 相似文献