Cationic polymerization relationship

According to Table 10, the relationships for cationic polymerization are not as simple in comparison to those above. Although it is clearly possible to assert limits for characterizing the polymerizability, all parameters used characterize the evident polymerizable monomers acroleine (R = —CHO) and methyl vinyl keton (R = —COCH3) or the non-polymerizable monomer vinyl acetate (R = —OCOCH3) contrary to experimental results. 

The acceptor ability of the cation is increased by electron-attracting substituents. The task was to seperate the substituent influences on the reactivity of the monomer from that on the cation and to find a relationship between these influences and the brutto rate constant of the cationic polymerization 76). 

Structure and Reactivity Relationships in the Photoinitiated Cationic Polymerization of 3,4-Epoxy cyclohexylmethyl-3, 4 -epoxy cyclohexane... 

For many cationic polymerizations, the net activation is negative, using the relationships given in Equation 5.11, so that the overall rate of polymerization decreases, for these cases, as the temperature is increased. Further, using Equation 5.16 and since Ep > Ep, the overall DP does decrease as the temperature is increased. This is pictured in Figure 4.4. 

The chemical and kinetic relationships for the anionic polymerization of acrylonitrile follow the same three major steps found for cationic polymerizations (1) initiation, (2) propagation, and (3) termination ... 

Cationic synthesis of block copolymers with non-linear architectures has been reviewed recently [72]. These block copolymers have served as model materials for systematic studies on architecture/property relationships of macromolecules. (AB)n type star-block copolymers, where n represents the number of arms, have been prepared by the living cationic polymerization using three different methods (i) via multifunctional initiators, (ii) via multifunctional coupling agents, and (iii) via linking agents. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

Thus, the three factors solvatmn, large size of anions and the resulting weak interaction with the cation, as well as the stereochemistry of attadc on the onium ion seem to be responsible, either together or separately, for two facts discussed in the Sect. 4.2.4 the identity of for various anions and the close relationship if not identity of and k in the cationic polymerization of heterocyclic monomers. 

Except in very special circumstances (Section 6.4) electroneutrality is maintained in cationic polymerizations by the presence of a negatively charged counter-ion or gegenion, X . This species has no analogue in free radical polymerizations, and indeed much of the early work on cationic (and anionic) reactions was carried out with almost total disregard for the effect of counter-ion. In fact it turns out as we shall see that the counter-ion, and its physical relationship with the growing cation, is of vital importance in the interpretation of kinetic data derived from these systems. The present authors take the view that, while results obtained from polymerizations where such relationships are unknown are qualitatively useful, any quantitative data obtained must be treated with caution. 

J. V. Crivello and U. Varlemann, Structure and Reactivity Relationships in the Photo-initiated Cationic Polymerization of 3,4-Epoxycyclohexylmethyl-3, 4 - Epoxycyclohexane Carboxylate. In Photopolymerization, Vol. 673, A. B. Scranton and C. N. Bowman, Eds., American Chemical Society, Washington D.C., 1997, p. 82. 

Polymer Tacticity. Our initial results on the polymerization of several different p-substituted-a-methylstyrene monomers indicated that there was some relationship between polymer stereoregularity and both the type of initiator and substituent in these monomers ( ). However, our recent investigations with a much wider variety of monomers, catalysts and cocatalysts revealed that the classical approach to analyzing substituent effects in organic reactions, the use of the Hammett pa relationship, gave no simple and self-consistent relationship between tacticity and the a (or a ) constant for the para-substituent. These results are summarized in the data in Table I for the cationic polymerization of a-methylstyrene and a series of five p-substituted-a-methylstyrene monomers initiated with two different Friedel-Crafts catalysts, TiCl and SnCl, either alone or with a cocatalyst benzyl chloride (BC) or t-butyl chloride (TBC), in methylene chloride at -78°C. Where a cocatalyst was used, the initiator was presumably a carbonium ion formed by the following reaction ... 

When studying the cationic polymerization of 1,3,5-trioxane (the short form trioxane )31, 32), investigators ran into peculiarities of formal kinetics, such as apparently high orders with respect to the monomer and their relationship to the nature of the solvent, temperature, etc., induction periods that could not in any way be ex-... 

The wide use of photoinduced radical or cationic polymerization of unsaturated monomers in UV curing technologies [1-3] have stimulated many detailed investigations of the processes involved [4-10]. Time-resolved laser-spectroscopy has been extensively employed [7 and references therein][8][11-20] to study in real time, the dynamics of the excited states of molecules used as photoinitiators. Reaction models accounting for the photoinHiation steps of the polymerization have been proposed which allow relationships between the excited state reactivity and the practical efficiency as photopolymerization initiators to be discussed thoroughiy. 

Bourissou et al. reported recently controlled cationic polymerization of lactones using a combination of triflic acid with a protonic reagent as the initiators [90]. The reaction was carried out in CH2CI2. Results indicated that the process is controlled is a linear relationship between the molecular... 

Over the past ten years the development of onium salt and other cationic photoinitiators has moved from the realm of speculative investigation to the point today at which they are being employed in numerous commercial applications. Much work still needs to be done in this fields particularly to improve our understanding of the relationship between the structure and the photosensitivity of these photoinitiators. As the field advances, one can expect still other new classes of onium salt photoinitiators to be developed as well as continued improvements to be made in the efficiency of the present systems. An understanding of the mechanism of photosensitization should lead to discovery of more efficient photosensitizers and a further broadening of their spectral response in photoinitiated cationic polymerization. 

Photochemical and Radialion Initiatioii.— The topics of photochemical initiation of cationic polymerizations, radiation-induced initiation, and their inter-relationships have been reviewed recently. 

FIGURE 22.13 Mechanism for the cationic polymerization of simple alkenes. With permission of Elsevier from Andjelkovic DD, Valverde M, Henna P, Li F, Larock RC. Novel thermosets prepared by cationic copolymerization of various vegetable oils - synthesis and their stracture-property relationships. Polymer 2005 46 9674-9685 and Kabasaka OS. Styrenation of mahaleb and anchovy oils. Prog Org Coat 2005 53 235-238. 

Since it was considered less difficult to suppress side reaction in the pMOS polymerization due to the stabilization of the growing carbocation, cationic polymerization of pMOS was investigated in detail using iodine. The MWDs of product polymers became unimodal in the polymerization in CCI4 at 0 °C. A nearly linear relationship was observed between the peak molecular weight of the product polymers and monomer conversion, indicative of polymerization mediated by long-lived active species. At -15 °C, the of product polymers increased in almost direct proportion to monomer conversion after the second feed of pMOS. Moreover, block copolymers with IBVE were obtained under similar conditions. 

Fig. 10.6 Relationship between the molecular weight and the monomer/initiator ratio in cationic polymerization in a flow microreactor system...

The ability to ionically polymerize apparently correlates in many cases with the capacity of the substituents to act as electron acceptors (anionic polymerizability) or as electron donors (cationic polymerizability) on the rt-bond of the vinyl group. These relationships should be visible in carefully chosen quantum chemical parameters. 

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