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81.
The amounts of tetraoxane produced in the polymerization of trioxane catalyzed by BF3·O(C2H5)2 were measured in various solvents. The maximum amount of tetraoxane produced depends on the nature of solvent used. This amount was independent of the initial concentration of the catalyst in ethylene dichloride and in nitrobenzene. On the other hand, in benzene, the amount of tetraoxane produced decreased slightly with increasing initial catalyst concentration. This result was explained by the reaction of tetraoxane produced with the residual catalyst as well as with the active center. The maximum amount of tetraoxane produced decreased, other conditions being similar, in the order, nitrobenzene > ethylene dichloride > benzene solvent. This order may be explained in terms of a longer lifetime of the active center in the more polar solvent, leading to the formation of tetraoxane.  相似文献   
82.
The reactivity of trans-1-alkoxybutadienes in cationic homopolymerization and copolymerizations and structure of the polymers produced were investigated. 1-Ethoxybutadiene is polymerized easily at ?78°C by various acidic catalysis. The reactivity of 1-ethoxybutadiene was similar to that of ethyl vinyl ether. The polymers produced possessed molecular weights of several thousands, and were composed of 70–95% 1,4 structure and 5–30% 3,4 structure. In the copolymerization of ethyl vinyl ether (M1) with 1-ethoxybutadiene at ?78°C in toluene by boron trifluoride diethyl etherate, r1 = 1.15, r2 = 2.62. From the Hammett plot of the relative reactivities of alkoxybutadienes (alkoxy: CH3O, C2H5O, i-C3H7O), the reaction constant p* was determined to be ?2.9. Results of the present study were compared with those of various butadiene derivatives.  相似文献   
83.
New sequence-regulated macromonomers ( 3 ) with a vinyl ether terminal were prepared by the HI/ZnI2-mediated living cationic polymerization of vinyl ethers: CH3? CH(OR1)? CH2CH(OR2)? C(COOEt)2CH2CH2OCH?CH2 ( 3a : R1 = nBu, R2 = CH2CH2OCOPh; 3b : R1 = iOct, R2 = CH2CH2Cl). The synthesis consisted of three consecutive steps: (i) quantitative addition of hydrogen iodide to the first vinyl ether into an adduct [CH3? CH(OR1)? l]; (ii) propagation of a second vinyl ether from the adduct in the presence of zinc iodide; and (iii) quenching the resulting AB-type heterodimeric living intermediate with a carbanion [θC(COOEt)2CH2CH2OCH?CH2] carrying a vinyl ether group. The HI/ZnI2-initiated living cationic polymerization of 3a and 3b yielded narrowly distributed polymers $\left( {\overline {DP}} _{_n } \sim 10 \right)$ consisting of a poly(vinyl ether) backbone and sequence-regulated oligomer branches. The terminal vinyl ether function of 3 was also utilized to prepare pentamers and hexamers with controlled sequence of functional vinyl ethers by selective dimerization and chain extension reactions with HI/ZnI2. © 1993 John Wiley & Sons, Inc.  相似文献   
84.
Cationic polymerization of cyclopentadiene induced by titanium tetrachloride–trichloroacetic acid was investigated in a toluene solution at ?69 to ?77°C. All manipulations were handled under vacuum conditions. Time–conversion curves were determined accurately by following the exothermicity of the fast reaction in an adiabatic system. The polymerization kinetics were developed on the basis of a fast initiation reaction and a nonstationary-state concentration (diminishing concentration) of active species, and the propagation rate constant k2 was determined by substituting either an initial rate of polymerization or a final conversion for the kinetic equations. k2 for the present system was determined to be 350 l./mole-sec, which is larger than those so far reported for some vinyl monomers in cationic polymerization. The present method can be commonly applied to reactive monomers for the determination of k2. The nature of termination reaction is discussed in connection with the determination of k2.  相似文献   
85.
Cationic polymerization of 2-vinyloxyethyl glycidyl ether (VEGE), a vinyl ether with an epoxy group, was conducted with various initiators in CH2Cl2 in the temperature range from +15 to ?78°C, and the possibility of its selective vinyl polymerization was investigated. BF3OEt2 polymerized both vinyl and epoxy groups of VEGE to yield polymers partially insoluble in organic solvents. HI/I2, iodine, and CF3SO3H gave soluble, low-molecular-weight oligomers with epoxy pendants. 1H-NMR structural analysis of the oligomeric products showed that the epoxy/vinyl ratio of the pendants decreases in the order: 100% epoxy ~ CF3SO3H > HI/I2 ~ I2 ? BF3OEt2. Although HI/I2 or iodine mainly polymerized the vinyl group, the reaction of the vinyl ether-type growing end with an epoxy group of VEGE took place during the polymerization, so that the monomer conversion leveled off at about 40%.  相似文献   
86.
To determine the effect of the dissociation of propagating species on the relative reactivity of monomers, 2-chloroethyl vinyl ether was copolymerized with p-methoxystyrene or with p-methylstyrene by using iodine in various solvents at 0°C. A common-ion salt (tetra-n-butylammonium iodide or tetra-n-butylammonium triiodide) was added to these copolymerization systems in a polar solvent to depress the dissociation of the propagating species. The addition of a common-ion salt increased the vinyl ether content in the copolymer. The more the dissociation of propagating species was depressed, the more the vinyl ether content in the copolymer increased. This effect of common-ion salt was in agreement with that of decreasing solvent polarity which yielded vinyl ether-rich copolymer as well. Therefore, the change of the monomer reactivity ratio by the solvent polarity, which used to be explained in terms of a selective solvation, must be reconsidered from the viewpoint of varying degrees of the dissociation of propagating species.  相似文献   
87.
Propagation mechanism in the cationic polymerization of alkenyl ethers was investigated through the effect of the bulkiness of alkoxy groups on the steric structure of a polymer. In polymerization with BF3O(C2H5)2 in toluene at ?78°C, trans-propenyl ethers having less bulky alkoxy groups–methyl, ethyl, and benzyl propenyl ethers–produced a stereoregular polymer having a threo-meso structure, and the cis isomer a nonstereoregular one having threo-meso and racemic structures. On the other hand, in the polymerization of propenyl ethers having bulky alkoxy groups–isopropyl and 1-methylpropyl propenyl ethers–the trans isomer yielded a nonstereoregular polymer with threo-meso and racemic structures, and the cis isomer a stereoregular one with a erythro-meso structure. This result suggests that a bulky alkoxy group plays an important role in determining the steric structure of the polymer by repulsion between the alkoxy groups of a growing chain end and of a monomer. The effect of solvent polarity on the steric structure of a polymer was also studied.  相似文献   
88.
In the solution polymerization of tetraoxane catalyzed by BF3·O(C2H5)2, trioxane and methanol-insoluble polymer were produced. However, the amounts of these products depend on the nature of solvent used. A critical concentration of tetraoxane is observed for the formation of methanol-insoluble polymer; at less than this critical concentration of tetraoxane no methanol-insoluble polymer is obtained, but trioxane is preferentially produced. This critical concentration of tetraoxane is higher in a more polar solvent, so the amount of methanol-insoluble polymer produced decreases and the amount of trioxane produced increases with increasing the polarity of solvent used. These results may be explained in terms of a stabilization of the active center leading to formation of trioxane by a solvation with solvent.  相似文献   
89.
Lipase-catalyzed kinetic resolutions of 1,1-diphenyl-2-propanol were performed at high temperatures up to 120°C. Burkholderia cepacia lipase immobilized on porous ceramic particles, lipase PS-C II (Amano Enzyme Inc.), gave an enantiopure product at 40–120°C, with the highest conversion (39%) at 80–90°C. The mechanism of high enantioselectivity retained even at 120°C is also described briefly.  相似文献   
90.
Novel oxazaborolidines B-C6F5 were synthesized by modified protocol from C6F5B(OMe)2 (in place of usual C6F5B(OH)2) and the corresponding amino alcohols, aiming to know the π-π stacking and electron-withdrawing effects of C6F5 group in asymmetric reduction of ketones. Although the results were not simply explained by the expected effects, significant difference was observed in the enantioselectivity between the catalysts with B-C6H5 and B-C6F5.  相似文献   
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