首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 46 毫秒
1.
Computer simulations show that ion pair aggregation can be responsible for the unusual dependence of the initial rate of polymerization on the concentration of added salt in the cationic polymerization of styrene initiated by RCl/SnCl4/NRCl. Addition of small amounts of tetraalkylammonium chloride to the system reduces the rate of polymerization due to the decrease of the concentration of propagating free cations. Subsequent salt addition leads to a small rate increase, and then the rate decreases at higher [salt]0/[SnCl4]0 ratio. The simulations show that the rate increase can be ascribed to the formation of aggregates of ion pairs and thus to a higher overall proportion of carbocations resulting in faster polymerization. The decrease of the polymerization rate at higher concentrations of added salt can be explained by the conversion of free SnCl4 to SnCl anions which are weaker Lewis acids. The effect of various equilibrium constants on the total concentration of carbocations as a function of added salt is simulated.  相似文献   

2.
To elucidate the reaction mechanism of radiation-induced polymerization of the styrene—silica gel system, the influence of H2O as adsorbed water and inhibitor of cationic polymerization was investigated by two methods. Monomer conversion decreased as H2O increased. In general, percent grafting decreased as H2O increased, but the presence of a small amount of H2O increased the percent grafting. Grafting at 16 Mrad has a maximum value at a water content of about 0.2%. This seems to be due to two effects of H2O: percent grafting increases due to restraint of cationic polymerization by H2O, but the percent grafting decreases due to adsorption water which interrupts the contact of styrene with silica gel. In GPC spectra, the low molecular weight peaks of both graft polymers and homopolymers decreased when H2O was added. The GPC results suggest that the number of positive holes which initiate cationic polymerization is very large.  相似文献   

3.
The polymerization of styrene was carried out in a cyclohexane solution of natural rubber with stannic chloride. It was found that the grafting copolymerizations of styrene took place as well as the cyclization of rubber. The rate of polymerization of styrene was proportional to the second power of the concentration of styrene and to the concentrations of stannic chloride and natural rubber, respectively. The overall activation energy was about 6 kcal./mole. The percentage grafting increased with increasing concentration of rubber. On the other hand, the grafting efficiency showed the reverse tendency. The percentage grafting could be increased to 150% by the addition of nitrobenzene, a polar solvent.  相似文献   

4.
To investigate the mechanism of radiation-induced grafting in this system, the increase of monomer concentration in the polyethylene film in styrene vapor was evaluated by measuring the weight increase and formulated to be V([M] ? [M]). The decay of radical concentration was also measured by ESR and the rate constant of the decay was determined. The alkyl type radical was affected only a little by styrene, while the allyl type radical was much affected by styrene. A new computer investigation method was proposed to clarify the reaction mechanism. The data obtained were substituted into differential equations and used to calculate the pattern of increase of the degree of grafting for the preirradiation method with reaction in the vapor phase. Results of these calculations suggest that only allyl type radicals induce grafting reactions and that the grafting reaction seldom occurs in the region of grafted polystyrene.  相似文献   

5.
The graft polymerization of styrene onto preirradiated poly(isobutylene oxide) (PIBO) with methanol and benzene was studied. The order of grafting yield and of the number-average molecular weight of graft chains decrease in the order; undiluted styrene > styrene–methanol (1:1) solution > styrene–benzene (1:1) solution. A kinetic treatment to calculate rate constants from the rate of grafting and the molecular weight of the graft chain was proposed. The propagation rate constant kp was 0.2–0.3 l./mole-sec and the termination rate constant kt was 1.0–16.0 l./mole-sec. The ratio kp/kt in this heterogeneous system was larger than that in homogeneous system by a factor of about 104–105.  相似文献   

6.
Chain-transfer reactions to alkylbenzenes were investigated in the polymerizations of phenylacetylene and styrene by WCl6 in benzene at 30°C. In the polymerization of phenylacetylene, alkylbenzenes did not work as chain-transfer agents, and further ethyl iodide was not a terminating agent. These findings suggest that the polymerization of phenylacetylene by WCl6 differs from the conventional cationic or anionic mechanisms. On the other hand, the ability of alkylbenzenes as chain-transfer agents in the polymerization of styrene by WCl6 increased in the following order: toluene < p-xylene < m-xylene < o-xylene. This order is similar to that in the polymerization by SnCl4. These results indicate that the polymerization of styrene by WCl6 proceeds by a conventional cationic mechanism.  相似文献   

7.
The cationic polymerization of styrene with the 2-phenyl-2-propanol (CumOH)/AlCl3 · OBu2 initiating system at various dibutyl ether concentrations in a mixture of 1,2-dichloroethane and n-hexane (55:45 v/v) at −15 °C was investigated. The experimental results showed that an increase in dibutyl ether concentration leads to a noticeable decrease in the polymerization rate as well as to the more controlled polymerization in terms of molecular weight (Mn) and molecular weight distribution (MWD) evolutions. The kinetic investigation revealed that the polymerization proceeds in two stages. The first stage is characterized by high polymerization rate and slow initiation relative to propagation. During this stage molecular weight decreases or does not change and MWD increases with conversion. In the second stage considerably slower quasiliving polymerization of styrene occurs. The quasiliving nature of the styrene polymerization by the CumOH/AlCl3 · OBu2 system is proved and mechanistic scheme of the polymerization is proposed.  相似文献   

8.
Effects of a common-ion salt, n-Bu4NClO4, on the cationic polymerization of styrene and p-chlorostyrene by acetyl perchlorate were studied in a variety of solvents at 0°C. In polymerization (in CH2Cl2) which yielded polymers with a bimodal molecular weight distribution (MWD), addition of the salt suppressed the formation of higher polymers, but affected neither the molecular weight nor the steric structure of the lower polymers. The polymerization rate decreased with increasing salt concentration and became constant at or above a certain concentration. In nitrobenzene, on the other hand, the MWD of the polymers was unimodal and steric structure was unchanged even in the presence of salt at a concentration 50 times that of the catalyst. However, the polymerization rate and the polymer molecular weight decreased monotonically as salt concentration increased. On the basis of these results, it was concluded that the ion pair in methylene chloride differs from that in nitrobenzene, and that the species in the latter solvent is similar in nature to free ions. The fractional contribution of the dissociated and nondissociated propagating species to polymer formation was determined from the rate depression caused by addition of the salt.  相似文献   

9.
Cationic polymerization of 2-phenylbutadiene (2-PBD) has been investigated. Polymerization were performed by SnCl4·TCA, WCl6, and BF3·OEt2 as catalysts in methylene chloride. 2-PBD polymerized easily and gave low molecular weight polymers. The polymerization proceeded to give a polymer having 1,4-structure without 1,2- or 3,4-structure. The double bonds of the polymer were partially consumed, probably owing to cyclization and chain-transfer reactions. 2-PBD was 0.66 times as reactive as styrene and 1.2 times as reactive as isoprene in the copolymerization at ?78°C by SnCl4·TCA in methylene chloride. Reactivities of ring-substituted 2-PBD obeyed the Hammett relation with ρ+ = ?2.04. The 13C chemical shift of ring-substituted 2-PBD was measured. Chemical shift values for C1 and C3 were correlated with Hammett σ, but those for C2 and C4 were almost unaffected by the substituents. On the basis of experimental results, the transition state of the cationic polymerization of 2-PBD was depicted as a benzylic cation rather than a phenylallylic one.  相似文献   

10.
A quite small dose of a poisonous species was found to induce living cationic polymerization of isobutyl vinyl ether (IBVE) in toluene at 0 °C. In the presence of a small amount of N,N‐dimethylacetamide, living cationic polymerization of IBVE was achieved using SnCl4, producing a low polydispersity polymer (weight–average molecular weight/number–average molecular weight (Mw/Mn) ≤ 1.1), whereas the polymerization was terminated at its higher concentration. In addition, amine derivatives (common terminators) as stronger bases allow living polymerization when a catalytic quantity was used. On the other hand, EtAlCl2 produced polymers with comparatively broad MWDs (Mw/Mn ~ 2), although the polymerization was slightly retarded. The systems with a strong base required much less quantity of bases than weak base systems such as ethers or esters for living polymerization. The strong base system exhibited Lewis acid preference: living polymerization proceeded only with SnCl4, TiCl4, or ZnCl2, whereas a range of Lewis acids are effective for achieving living polymerization in the conventional weak base system such as an ester and an ether. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6746–6753, 2008  相似文献   

11.
In this paper, a new water-soluble initiator system, 2-bromopropane/CuSO4/sodium ascorbate, was used as the initiator for emulsion polymerization. Radical emulsion polymerization of styrene was successfully carried out at 80 °C by using sodium dodecylbenzenesulfonate as the emulsifier. The 2-bromopropane/CuSO4/sodium ascorbate-initiated emulsion polymerization shows the controlled free-radical polymerization features with linear growth of molecular weight. Polystyrene with a relatively high molecular weight and a narrow molecular weight distribution can be synthesized by this method. On the other hand, stable polystyrene latex can be obtained, and the size of the polystyrene latex increased with the increase in monomer conversion.  相似文献   

12.
研究了在少量吡啶(Py)存在下由水(H2O)四氯化钛(TiCl4)体系引发苯乙烯于二氯甲烷正己烷中进行碳正离子聚合,分别考察[Py]、[H2O]和[TiCl4]对聚合速率、产物分子量与分子量分布的影响.实验结果表明,少量亲核试剂吡啶(Py)对聚合反应起着重要作用,可有效地降低聚合速率和使分子量分布变窄;随着[H2O]和[Py]降低或[TiCl4]增加,聚合产物的分子量增加,而分子量分布指数(Mw Mn)基本维持在1.8左右;随着[Py]增加,聚合速率降低;随着[H2O]和[TiCl4]增加,聚合速率提高.聚合速率对单体浓度呈一级动力学关系,对Py、H2O和TiCl4的反应级数分别为-0.72、0.72和1.86.聚合速率对TiCl4浓度呈接近二级动力学关系,这可能与体系中TiCl4主要以二聚体形式存在有关.聚合转化率和产物分子量均随着反应时间延长而逐渐增大,PS的数均分子量与转化率呈线性增加关系.  相似文献   

13.
Ultra-high molecular weight polyethylene (UHMWPE) powder was irradiated by gamma rays using a 60Co source. Simultaneous and pre-irradiation grafting was performed in air and in inert atmosphere at room temperature. The monomer selected for grafting was styrene, since the styrene-grafted UHMWPE could be readily post-sulfonated to afford proton exchange membranes (PEMs). The effect of absorbed radiation dose and monomer concentration in methanol on the degree of grafting (DG) is discussed. It was found that the DG increases linearly with increase in the absorbed dose, grafting time and monomer concentration, reaching a maximum at a certain level. The order of rate dependence of grafting on monomer concentration was found to be 2.32. Furthermore, the apparent activation energy, calculated by plotting the Arrhenius curve, was 11.5 kJ/mole. Lower activation energy and high rate dependence on monomer concentration shows the facilitation of grafting onto powder substrate compared with film. The particle size of UHMWPE powder was measured before and after grafting and found to increase linearly with increase in level of grafting. FTIR-ATR analysis confirmed the styrene grafting. The grafted UHMWPE powder was then fabricated into film and post-sulfonated using chlorosulfonic acid for the purposes of evaluating the products as inexpensive PEM materials for fuel cells. The relationship of DG with degree of substitution (DS) of styrene per UHMWPE repeat unit and ion exchange capacity (IEC) is also presented.  相似文献   

14.
Complexes of methyl methacrylate and methacrylonitrile with Lewis acids (SnCl4, AlCl3, and BF3) were copolymerized with styrene at ?75°C under irradiation with a high-pressure mercury lamp in toluene solution. The resulting copolymers consisted of equimolar amount of methyl methacrylate or methacrylonitrile and styrene, regardless of the molar ratio of monomers in the feed. NMR spectroscopy showed the copolymers to have an alternate sequence. The tacticities of the copolymers varied with the complex to have an alternate sequence. The tacticities of the copolymers varied with the complex species: the copolymer from the SnCl4 complex system had a higher cosyndiotactieity, while those from the AlCl3 and the BF3 complex systems showed coisotacticity to predominate over cosyndiotacticity. NMR spectroscopic investigation of the copolymerization system indicated the presence of a charge-transfer complex between the styrene and the methyl methacrylate coordinated to SnCl4. The concentration of the charge-transfer complex was estimated to be about 30% of monomer pairs at ?78°C at a 1:1 molar ratio of feed. The growing end radicals were identified as a methyl methacrylate radical for the AlCl3 complex–styrene system and a styrene radical for the SnCl4 complex–styrene system by the measurement of the ESR spectra of the copolymerization systems under or after irradation with a high-pressure mercury lamp. The tacticity of the resulting polymer appears to be controlled by the structure of the charge transfer complex. In the case of the SnCl4 complex a certain interaction of SnCl4 with the growing end radical seems to be a factor controlling the polymer structure. These copolymerizations can be explained by an alternating charge-transfer complex copolymerization scheme.  相似文献   

15.
The bulk polymerization of styrene was investigated with tetramethylthiuram disulfide (TMTD) as an initiator in the presence of 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) at 123 °C. The polymerization proceeded in a controlled/living way; that is, the polymerization rate was first‐order with respect to the monomer concentration, and the molecular weight increased linearly with conversion. The molecular weights of the polymers obtained were close to the theoretical values, and the molecular weight distributions were relatively low (weight‐average molecular weight/number‐average molecular weight = 1.1–1.3). The rate of polymerization with TMTD as an initiator was faster than that with benzoyl peroxide, and the rate was independent of the initial concentration of TMTD in the presence of TEMPO. The obtained polystyrene was functionalized with ultraviolet‐light‐sensitive ? SC(S)N(CH3)2 groups, which was characterized with 1H NMR spectroscopy. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 543–551, 2005  相似文献   

16.
Living cationic copolymerization of amide‐functional vinyl ethers with isobutyl vinyl ether (IBVE) was achieved using SnCl4 in the presence of ethyl acetate at 0 °C: the number–average molecular weight of the obtained polymers increased in direct proportion to the monomer conversion with relatively low polydispersity, and the amide‐functional monomer units were introduced almost quantitatively. To optimize the reaction conditions, cationic polymerization of IBVE in the presence of amide compounds, as a model reaction, was also examined using various Lewis acids in dichloromethane. The combination of SnCl4 and ethyl acetate induced living cationic polymerization of IBVE at 0 °C when an amide compound, whose nitrogen is adjacent to a phenyl group, was used. The versatile performance of SnCl4 especially for achieving living cationic polymerization of various polar functional monomers was demonstrated in this study as well as in our previous studies. Thus, the specific properties of the SnCl4 initiating system are discussed by comparing with the EtxAlCl3?x systems from viewpoints of hard and soft acids and bases principle and computational chemistry. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6129–6141, 2008  相似文献   

17.
The γ‐initiated reversible addition–fragmentation chain‐transfer (RAFT)‐agent‐mediated free‐radical graft polymerization of styrene onto a polypropylene solid phase has been performed with cumyl phenyldithioacetate (CPDA). The initial CPDA concentrations range between 1 × 10?2 and 2 × 10?3 mol L?1 with dose rates of 0.18, 0.08, 0.07, 0.05, and 0.03 kGy h?1. The RAFT graft polymerization is compared with the conventional free‐radical graft polymerization of styrene onto polypropylene. Both processes show two distinct regimes of grafting: (1) the grafting layer regime, in which the surface is not yet totally covered with polymer chains, and (2) a regime in which a second polymer layer is formed. Here, we hypothesize that the surface is totally covered with polymer chains and that new polymer chains are started by polystyrene radicals from already grafted chains. The grafting ratio of the RAFT‐agent‐mediated process is controlled via the initial CPDA concentration. The molecular weight of the polystyrene from the solution (PSfree) shows a linear behavior with conversion and has a low polydispersity index. Furthermore, the loading of the grafted solid phase shows a linear relationship with the molecular weight of PSfree for both regimes. Regime 2 has a higher loading capacity per molecular weight than regime 1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4180–4192, 2002  相似文献   

18.
The polymerization of trioxane catalyzed by stannic chloride (SnCl4) in ethylene dichloride was studied and compared with the results obtained with boron trifluoride etherate, BF3·O(C2H5)2, as catalyst. Under the same conditions, the polymerization rate was larger with SnCl4 than with BF3·O(C2H5)2, while at a fixed polymer yield the molecular weight of the polymer obtained by SnCl4 was lower than with the BF3·O(C2H5)2 catalyzed reaction. The overall activation energy of trioxane polymerization with SnCl4 was 11.0 ± 0.8 kcal/mole. The kinetic orders of catalyst and monomer were determined to be close to 2 and 4, respectively. A certain amount of tetraoxane was also produced in an early stage of the polymerization with SnCl4 similar to BF3·O(C2H5)2-catalyzed reaction. However, the maximum amount of tetraoxane produced at 30°C was larger with SnCl4 than with BF3·O(C2H5)2. In addition, a ten-membered ring compound (pentoxane) was isolated in the solution polymerization of trioxane catalyzed by both SnCl4 and BF3·O(C2H5)2. The confirmation of pentoxane formation is strong evidence for the back-biting reaction mechanism.  相似文献   

19.
The influence of parameters such as styrene dilution and active site concentration on the polymerization of styrene in the presence of a di-tert-butyl nitroxide adduct (A-T) was examined. It is confirmed that the rate of styrene polymerization is independent of A-T concentration, with no monomer dilution effect. An increase in radical concentration generated in the medium leads to faster propagation, but the molecular weight of the polymers formed is alway controlled by the A-T concentration.  相似文献   

20.
Graft polymerization of styrene onto preirradiated poly(isobutylene oxide) was carried out at 25°C. The concentration of active sites for grafting was estimated by means of ESR and by the activation analysis of bromine atoms bound to the chain ends. Kinetic data such as differential amount of active sites (DACT), growth rate (GR), and average lifetime (τ) were obtained to calculate the molecular weight distribution of graft chains by the Monte Carlo simulation method. The number-average molecular weight of the graft chain was calculated from the equation, M n = 2GRτ (mw), which agreed well with the observed value.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号