摘要:

AbstractSome fundamental aspects of carbocation chemistry are discussed in their relationship to our better understanding of carbocationic olefin polymerization and alkane condensation reactions including those of methane. Whereas trivalent alkyl cations, i.e. R3C+play the key role in the former, the latter is made possible through the intermediacy of five coordinate carbocations of the CH5+type. In conventional acid catalyzed reactions carbocations are always in equilibrium with their corresponding olefins, whereas under superacidic, stable ion conditions carbocations can be generated and reacted free of such equilibria. The chemical consequences of these differences are discussed in polymerization, as well as in alkylative or oxidative alkane condensation reactions.

ISSN:0258-0322    

DOI:10.1002/masy.19880130103

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractCarbocations, carbanions and free radicals are the most important reactive intermediates in organic chemistry and the majority of bond‐forming and bond‐breaking reactions involve their participation at some stage. We will review briefly our studies, over the past five years, of the direct coordination reaction between carbocations and carbanions to form covalent bonds in solution. We have gathered considerable thermodynamic and kinetic information for this reaction.Particular emphasis will be placed on recent work on the measurement and analysis of heterolysis energies. Heterolysis is the reverse of the coordination reaction and we have obtained heterolysis energies in solution for over 160 bonds by measuring the heats of reaction between several resonance‐stabilized carbocations and a variety of carbanions, oxyanions, nitranions a thiophenoxide ions in solution. The results are correlated by very simple equations so that heats of heterolysis may be predicted with considerable precision from readily available data.Carbocations and carbanions can also react by electron transfer to form free radicals. We will describe recent experiments which demonstrate that under appropriate conditions all three types of reactive intermediates can coexist at equilibrium in solution with the electrons shuttling between the three different sp

ISSN:0258-0322    

DOI:10.1002/masy.19880130104

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

ISSN:0258-0322    

DOI:10.1002/masy.19880130105

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe ionisation (Ar2CHCl + BCl3⇌ Ar2CH+BCl−4) and dissociation (Ar2CH+BCl−4⇌ Ar2CH++ BCl−4) equilibria of diarylmethyl chlorides in boron trichloride/dichloromethane solution have been studied by conductimetry, photometry and1H NMR spectroscopy. Small differences in the UV‐vis spectra of diaryl‐carbenium tetrachloroborates, which have been observed in solutions of low and high tetrachloroborate concentration, can be attributed to the formation of 1:1 ion‐pairs in the more concentrated solutions. Low temperature calorimetry was used to determine the heats of addition of diarylcarbenium tetrachloroborates to 2‐methyl‐1‐pentene (Ar2CH+BCl−4+ H2C‐CRR' + Ar2CH‐CH2‐CRR'Cl + BCl3), and it is estimated that the standard free enthalpy of this reaction is greater than 0 for systems with pKR+>−2.6. Kinetic studies have shown that paired and unpaired diarylcarbenium tetrachloroborates exhibit identical reactivity towards alkenes. A rationalisation for the different situation in carbocationic and carbanionic polymerisation is presented. The rate constants for the initiation of isobutene, styrene and isoprene polymerisation by diarylcarbenium ions have been determined, and it is shown, how propagation rate constants can be estimated from l

ISSN:0258-0322    

DOI:10.1002/masy.19880130106

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe nature and the stability of the various types of propagating species involved in cationic polymerization of ethylenic monomers are examined. After a short discussion of the evidence for and against the presence of covalent species in styrene polymerization, recent data about the identification of carbocationic active centres involved in the polymerization of para‐substituted α‐methyl‐styrenes are given. Carbenium ions were generated in methylene chloride either by reaction of the p‐substituted cumyl chloride with SbF5or by reaction of the monomer with trifluoromethanesulfonic acid (HA). They are stable around −70°C, but some of them give secondary reactions at higher temperature. The various types of species were identified by1H NMR correlated with UV spectroscopy.For the p‐tert‐butyl derivative, monomeric cations remain stable at −70°C (and even at higher temperature) while oligomeric ones are less stable, giving indanic derivatives. With p‐methoxy‐α‐methylstyrene, the monomeric, dimeric and trimeric cations are stable at −63°C but give indanic derivatives at −40°C and above. For both monomers, monomeric cations show a single maximum at 350 or 367 nm with a high absorption coefficient (≁ 30000). In the presence of an excess of monomer, a bathochromic shift of this maximum (≁ 15 nm) is observed, together with the appearance of a new (smaller) maximum around 480 nm. The presence of these two maxima could be correlated with that of “isomerized” species, some of which could be identified by NMR. The possible occurrence of a similar situation with other monomers and particul

ISSN:0258-0322    

DOI:10.1002/masy.19880130107

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractIt is demonstrated by means of1H NMR spectra in combination with IR spectra that the reaction of δ‐valerolactone with an alkylating reagent results in alkylation of the exocyclic oxygen. The propagation of the resulting cyclic dioxocarbenium ion involves cleavage of the alkyl‐oxygen bond. Initiation and propagation mechanisms induced by Lewis acids such as boron bromide, aluminum bromide, titanium(IV) and zirconium (IV) bromide, tin(IV) bromide and zinc bromide are discussed. It is demonstrated by means of IR and NMR spectroscopy that complexation of the lactones at the exocyclic oxygen is followed by cleavage of the alkyl‐oxygen bond and halogen transfer, so that ω‐bromocarboxylic acids are obtained (after hydrolytic work up). In the case of aluminum, tin and zinc the intermediately formed metal‐oxygen bond initiates rapid polymerization via a nonionic insertion mechanism which is characterized by cleavage of the acyl

ISSN:0258-0322    

DOI:10.1002/masy.19880130108

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractLactams and lactam derivatives may be polymerized by various types of processes, but only the cationic mechanisms provide suitable pathways for the polymerization of all types of lactam monomers. Therefore, the cationic polymerization is predestinated for the synthesis of their copolymers. Random copolymers can be obtained even if the rates of homopolymerization of comonomers differ by several orders of magnitude, either by transacylation of block copolymers, or in a new process designated as polymerization with transient chemical activation. This process is operative e.g. in the copolymerization of an unsubstituted and an N‐acylated lactam. It implies N‐acylation of the unsubstituted lactam by the polymer of the N‐acylated one, followed by the incorporation of the activated monomer and subsequent deacylation of the incorporated monomer

ISSN:0258-0322    

DOI:10.1002/masy.19880130109

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractA new systematization is presented for the polymerization reactions in relation to the so‐called “Ionic Polymerizations'. The basis of the new systematization, is the concept of the reaction between “Electrophile” and “Nucleophile”. The reactants are either ions (cation or anion) or dipoles. Polymerization reactions except free radical polymerization and polycondensation are classified into three categories depending upon the reactivity characteristics (electrophilic or nucleophilic) of the propagating species.a. Polymerization via electrophilic propagating speciesb. Polymerization via nucleophilic propagating speciesc. Polymerization via zwitterionsEmphasis is placed on the polymerization reactions which propagate through covalent species having nucleophilic or electrophilic reactivity. By this systematization, several polymerization systems which have been treated as unusual or exceptional cases are thus reasonably

ISSN:0258-0322    

DOI:10.1002/masy.19880130110

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractWe have studied the reaction of 1,3‐dioxolane with trifluoromethanesulfonic acid in the presence of α,ω‐dihydroxypolytetrahydrofurane and α,ω ‐ dihydroxypolystyrene. Besides polymerization of 1,3‐dioxolane we have observed the occurence of a fast coupling reaction between the initial oligodiols. The mechanism has been studied using a monohydroxylated polystyrene and a model compound, 3‐phenyl‐1‐butanol. We have shown that coupling takes places through transacetalization even under the mild conditions used Ethylene glycol is produced and an acetal bridge is formed between oligodiols. Conditions have been found where high‐molecular‐weight polymer is formed through this polycondensation, in the absence of ho

ISSN:0258-0322    

DOI:10.1002/masy.19880130111

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractSince their discovery in our laboratories as novel photo and thermal initiators for cationic polymerization, diaryliodonium and triarylsulfonium salts have been the object of considerable investigation in our laboratories as well as in a number of other academic and industrial research groups. Recent work in our laboratory has focused on several major themes which will be the topics of this presentation. These include; the synthesis of new photoinitiators with higher photosensitivity, the preparation of polymer bound photoinitiators, the preparation of new cationically polymerizable monomers with high reactivities and lastly, the use of onium salt induced photo and thermally induced cationic polymerization in a number of practical applications.

ISSN:0258-0322    

DOI:10.1002/masy.19880130112

出版商:Hüthig&Wepf Verlag    

年代:1988

数据来源: WILEY