摘要:

AbstractSome of the most important unsolved problems of cationic polymerizations are reviewed together with the most significant progress made recently.In the field of initiation reactions, there are more and more data showing that initiation of ethylenic monomers by Friedel Crafts catalysts does not always necessitate a coinitiator. The different hypotheses already put forward to explain these “direct initiations” are discussed and a new mechanism is proposed. The mechanisms of termination and transfer reactions are also examined.Concerning the mechanisms of propagation reactions, the difficulties in determining the concentration of active centers are outlined. Some progress was made by using stable carbocations as initiators, but some discrepancies between the results are discussed (existence of apparently negative activation energies). The different types of active species in equilibrium are examined, together with the nature of the non dissociated species. The differences between the propagation rate constants over free ions, measured in systems initiated by high energy radiations or more classical cationic initiators, are also discus

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750401

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY

摘要:

AbstractThe rôle of carbocations in cationic polymerizations is discussed, differentiating σ‐ (i.e.alkene and arene), π‐ (i.e.alkane) andn‐donor (i.e.ether, sulfide, nitrogen compound) monomers as the reactive nucleophiles. The mechanism of the initiation step in alkene polymerization is evaluated considering both protic (i.e.coinitiator containing) and aprotic systems. The aluminum bromide catalyzed polymerization of isobutylene, as an example, can involve besides the conjugate protic acid, bromoalanation intermediates in aprotic media, which, however, themselves rapidly can undergo elimination, forming HBr in the originally acid free system. The involvement of dialkylhalonium ions in alkyl halide solvent systems is also of substantial interest and may explain increasing deactivation in the order RI>RBr>RCl. Oxidative polycondensation of π‐donor arenes (such as benzene) and σ‐donor alkanes (such as methane) is discussed in the context of the general carbocation concept, involving trivalent carbenium ion intermediates in the former and pentacoordinated carbonium ions in the latter case, as the reactive alkylating agents responsible for the ionic propagation reactions.n‐Donor monomers, such as ethers, sulfides or miscellaneous nitrogen compounds, undergo polymerizationvianucleophilic displacement reactions on the polarized carbocationic sites of intermediate alkylated or proto

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750402

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY

摘要:

AbstractSome of the factors which influence, or must be contained in, a Comprehensive Theory of the cationic polymerisation of olefins are discussed from new points of view, starting from the author's new theory concerning initiation by metal halides. Alleged determinations of the rate constantk p+are criticised because they ignored two equilibria which make the concentration of growing polymer chains [P n+] in general less than the nominal concentration,c0, of initiator. One of them is a binary ionogenic equilibrium involving metal halide MtXnand polymer halide PnX. The other is between metal halide and olefin. An equation relating [P n+] toc0is deduced which takes account of these equilibria. The importance of the second equilibrium is illustrated by the way in which it provides simple explanations of hitherto obscure phenomena. Some of these arise during the interaction of aluminium halides with olefins in different circumstances, others concern the different interactions which can take place between titanium tetrachloride and 1,1‐diphenylethylene. The fact that titanium tetrachloride appears to be able to initiate without co‐initiator in some systems, but requires one in others is explained by the self‐ionisation of the metal halide and the effects of

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750403

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY

摘要:

AbstractProblems of interpretation of the mechanisms in the polymerisation of styrene by HClO4in CH2Cl2, especially of chain propagation are discussed and experimental results from recent studies of the bimodal molecular weight distributions found in polymers prepared at 0 C and below, and of the kinetics of the rapid, non‐stationary, precursor polymerisation at −80 C and other low temperatures are presented. The former indicate simultaneous, but independent, propagation at 0 C by pseudo‐ionic and ionic species. The latter reveal the transient existence at low temperatures of an ionic species (absorbing at 340 nm) identified as the (poly)styryl carbenium ion. The measurements permit estimates of the free‐ion propagation rate constant at −80 C to −45 C, and suggest the unexpected conclusion that propagation by ion pairs is several fold faster than by free ions. Both species however appear to be not simple, but strongly complexed by monomer. It is speculated also that monomer complexation of the perchloric acid initiator plays an important rôle in determining whether pseudo‐ or true ionic mechanis

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750404

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY

摘要:

AbstractThe cationic copolymerization of isobutylene and β‐pinene (2(10)‐pinene) yields random copolymers which exhibit interesting physical properties.E.g., high molecular weight copolymers containing isobutylene/β‐pinene in the ratio of 90/10 and 97/3 are sulfur‐vulcanizable ozone‐resistant elastomers. Copolymer synthesis was accomplished with Friedel‐Crafts halidese.g., C2H5AlCl2, BF3,etc.using ethyl chloride diluent at low temperatures. The structure of the copolymers was determined by NMR spectroscopy. The reactivity ratios of isobutylene and β‐pinene are stongly affected by temperature, however, are much less sensitive to the particular Lewis acid used. Interestingly, with C2H5AlCl2, the individual reactivity ratios become equal to unity belowca.−100°C,i.e., the copolymerization becomes azeotropic.The mechanism of cationic olefin polymerizations induced by various alkylaluminum compounds has been investigated by model reactions. The rate of the reactions. The rate of the reactionis strongly affected by the nature of the (CH3)3CX and the CH3X solvent (X=Cl, Br. I). Unexpectedly, the sequence of rates of this reaction was found to be (CH3)3CCl>(CH3)3CBr>(CH3)3Cl. Further, an intensive study of the 2,4,4‐trimethyl‐1‐pentene/(CH3)3Al/(CH3)3CCl model system gave a series of important mechanistic insights.E.g., if an alkylaluminum compound having a β‐hydrogen in respect to aluminum is used as the coinitiatore.g., (C2H5)3Al, [(CH3)2CHCH2]3Al,etc., rapid hydridation terminates the reaction:\documentclass{article}\pagestyle{empty}\begin{document}$$ \ldots - {\rm R}^ \oplus + \left( {{\rm C}_2 {\rm H}_5 } \right)_3 {\rm AlCl}^ \odot \to \ldots - {\rm RH} + \left( {{\rm C}_2 {\rm H}_5 } \right)_2 {\rm AlCl} + {\rm C}

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750405

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY

摘要:

AbstractSalts of stable carbocations such as triphenylmethyl and tropylium ions were used conveniently to estimate free ion propagation rate coefficients (k p+) for cationic homopolymerization of a series of alkyl vinyl ethers andN‐vinylcarbazole. Methyl vinyl ether and 2‐chloroethyl vinyl ether gave values ofk p+approximately one order of magnitude less than those of branched alkyl derivatives, and an explanation in terms of conformational effects is presented. Results are compared with those from radiation induced polymerizations, and with data for related hydride ion abstraction processes, in an attempt to evaluate solvent effects on free ion reactivity.The scope and applicability of stable carbocations to cationic polymerizations are surveyed and complicating side reactions exemplified by the systems: xanthylium ion—1,1‐diphenylethylene, tropylium ion—indene, xanthylium ion—THF, C6H5CO+SbCl 6−—THF, and by the effect of perchlorate anions on chain transfer in polymerizati

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750406

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY

摘要:

AbstractTo elucidate the feature of the propagation reaction in cationic polymerization, the studies on the effect of a β‐methyl group on the reactivity of vinyl ethers and styrenes are summarized. Although the electron density on a β‐carbon is decreased by β‐methyl substitution, the β‐methyl group increases the reactivity of vinyl ethers. Therefore, in the transition state of the propagation reaction of vinyl ethers, it is estimated that a propagating chain interacts with π‐electrons of the α‐carbon and ethereal oxygen in addition to the β‐carbon.Cis‐propenyl ethers have been reported to be several times as reactive astrans‐propenyl ethers in a non‐polar solvent. However, in a polar solvent,cis‐ andtrans‐propenyl ethers show nearly the same reactivity, and with isopropyl andtert‐butyl propenyl ethers having a bulky alkoxyl group, thetrans‐isomer is found to be more reactive than thecis‐isomer. These phenomena are interpreted by the t

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750407

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY

摘要:

AbstractThe results obtained by various authors in the study of cationic polymerizations carried out in the presence of an electric field of high strength, that we propose to call “electro‐influenced polymerizations”, are shortly reviewed.The most relevant results obtained in the author's laboratories, for the “electroinfluenced” cationic polymerization of anethole, isobutyl vinyl ether and cyclohexyl vinyl ether with constant or periodically reversed field polarity are illustrated.The analogies between these results and those obtained from studies on electroinitiated cationic polymerizations of various vinyl monomers with tetraalkylammonium perchlorates, tetraalkylammonium iodides or tetraalkylammonium triiodides are emphasized.On the basis of these analogies, we interpret the increase of the polymerization rate which is observed in some cases when a d.c. field is applied to a cationically polymerizing system as being due to electrodic processes, namely the anodic generation of new active centres through electrolysis of the counterions. This new interpretation can be formalized in a mathematical model which fits most of the experimental results so far obtained for electro‐influenced cationic polymerizations initiate

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750408

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY

摘要:

AbstractIonizing radiations generate both free radicals and ions upon interaction with matter and under appropriate conditions either free radical or ionic polymerizations are observed. Cationic polymerizations are favored by the presence of halogenated solvents and by low reaction temperatures. Selective inhibitors and copolymerization studies make it possible to determine the relative proportions of the cationic and the free radical contributions to the overall process under a variety of experimental conditions (temperature, reaction medium, radiation dose‐rate). Pure, carefully dehydrated hydrocarbon monomers undergo cationic polymerization when irradiated at room temperature. This reaction proceeds at a very high rate and is believed to involve free propagating cations. Propagation rate constants are available for the following monomers: styrene (kp= 3,5·106at 15°C), α‐methylstyrene (kp= 4.106at 0°C), cyclopentadiene (kp= 6.108at −78°C), isobutene (kp= 1,5·108at 0°C), and isobutyl vinyl ether (kp= 3.105at 30°C). The present views on the mechanism of radiation induced cationic chain initiation and chain propagation are brie

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750409

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY

摘要:

AbstractCationic polymerizations of some cyclic imino ethers using methyl iodide as initiator were found to propagateviacovalent growing species of alkyl iodide nature. The propagation mechanism of the polymerization was examined by direct observation of the reaction system by means of NMR spectroscopy and by indirect kinetic analysis. Among six monomers, 2‐oxazoline (1), 2‐phenyl‐2‐oxazoline (2), and 2‐phenyl‐5,6‐dihydro‐4H‐1,3‐oxazine (3) were found to be polymerized with CH3Iviacovalent growing species. On the other hand, 2‐methyl‐2‐oxazoline (4) and 5,6‐dihydro‐4H‐1,3‐oxazine (5) were polymerized with CH3Iviaionic growing species. For the polymerization of 5‐methyl‐2‐oxazoline (6) initiated by CH3I, a mechanism of propagationviaan equilibrium mixture of ionic and covalent propagating species (22and21, respectively) was presented. In all cases of the polymerizations of these six monomers by methylp‐toluenesulfonate as initiator, the growing species were ionic ones having structures of the corr

ISSN:0025-116X    

DOI:10.1002/macp.1974.021750410

出版商:Hüthig&Wepf Verlag    

年代:1974

数据来源: WILEY