1.3- Dioxolane polymerization

Immediately after the start of dioxolane polymerization, initiated by the triethyloxonium salt Et30 + B, the carboxonium centres present are of the form [127]... 

The kinetic evidences, however, were obtained that concentration of active species in 1,3-dioxolane polymerization decreases with time in the... 

It has been shown, however, that in the copolymerization much more reactive 1,3-dioxolane polymerizes first and is practically completely consumed at still low conversion of 1,3,5-trioxane [139]. Thus, by polymerization alone, it would not be possible to achieve the random distribution of 1,3-dioxolane units along the chain. Both analysis of the microstructure of the chain [140], as well as thermal behavior of the copolymer [141], indicate, however, that nearly random distribution of comonomer units is indeed attained by scrambling process similar to that described earlier for sequential polymerization of 1,3-dioxolane and 1,3-dioxepane. 

In 1968, Yamashita et al. [127] reported a study of the kinetics of 1,3-dioxolane polymerization initiated by Et30 BF4 and carried out in CH2 CI2 solution. They concluded that the mechanism, deduced from the structure of the initiation product and the molecular weight of the polymer, is simply... 

In 1973 Penczek and Kubisa [137] published a detailed study of the interaction of triphenylmethyl hexachloroantimonate with 1,3-dioxolane. Polymerization occurs as a result of the interaction but the triphenylmethyl salt itself does not initiate the polymerization. Instead the sequence of reactions... 

In conclusion, in the kinetics of dioxolane polymerizations with many catalysts, the initiation mechanism is complex and inefficient. The degree of efficiency seems to be related both to the cation and to the anion. Again as in the case of cyclic ethers and cyclic sulphides, an independent measurement of the number of active sites seems essential for precise kinetics. The most probable fep for the polymerization seems to be of the order of 10—501 mole sec . With careful choice of polymerization conditions a kinetically reversible polymerization occurs, but the molecular weight of the polymer produced is not related to the initiator concentration, probably as a result of a transfer reaction. 

Alkoxycarbenium Ions. Methoxycarbenium hexachloroantlmonate prepared by a reaction developed by Olah (31) was exploited by Penczek et al. (32) to solve one of the most disputed controversies concerning the mechanism of 1,3-dioxolane polymerization. 

Recently we stressed that the proportion of the cyclic fraction is given by the ratio [M]cril./([M]o — [M]e — [M]crll ), thus it is different for different starting monomer concentrations, as shown in Fig. 3.2 for 1,3-dioxolane polymerization 18). Figure 3.2 is based on the data of Ref.14), (note that only the proportions change, the absolute value of rM]cri, does not depend on [M]o). 

Acetate groups are present at both ends of the polymer molecules as shown above7 This was confirmed by analytical evidence. The initiation of dioxolane polymerization by boron trifluoride-etherate is pictured differently ... 

Scheme 5 A simplified reaction scheme for 1,3-dioxolane polymerization initiated with protonic acid (Z=H) or an alkylating agent (Z=R). See text for differences in effectiveness of end- and backbiting in these systems.

It has also been shown that living polydioxolane can be reacted directly onto polystyrene, whereby grafting takes place (Scheme 26). This is a kind of Friedel-Crafts reaction, resulting from the attack of the active site onto the benzene nucleus of a monomer unit. In this case, however, polydioxolane (PDXL) may remain ungrafted. The absence of any proton-donating impurity in the medium is required. The dioxolane polymerization has to be initiated by oxocarbenium salts and not by systems containing protons. 

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