Dioxolan-2-ylium ions

On the basis of kinetic studies on the polymerization of 80 with 1,3-dioxolan-2-ylium ion, an Ac2 mechanism (bimolecular addition on a carbenium ion) has been proposed for this polymerization. This mechanism assumes the bimolecular reaction of the cyclic dioxacarbenium ion with the monomer as the rate-determining step and predicts the preparation step to be nonstereospecific, that is, the formation of a polymer consisting of cis- and frans-2,4-linked 1,3-dioxolane rings. 

When a strong electrophile, such as a triphenyl methylium ion, abstracts a hydride ion from the 2-position of 1,3-dioxolan, the product is the 1,3-dioxolan-2-ylium or (for short) dioxolenium ion as shown in Equation (5.3) ... 

CH2CI2, 25 °C), while an attack of another monomer molecule on an oxonium ion is apparently slow. Thus, instead of simple initiation by addition, as shown in Scheme 13, hydride transfer takes place and l,3-dioxolan-2-ylium salt and triphenylmethane are formed quantitatively as shown in Scheme 8. 1,3-Dioxolan-2-ylium salts, which are relatively stable and can be isolated as pure crystalline compounds, are thus the tme initiators, formed in situ in the DXL-PhsC A system. 

The initiation with some carbocations, especially trityl, does not involve direct addition to monomer. The carbocation abstracts a hydride ion from the a-carbon of monomer and the newly formed carbocation initiates polymerization [Afsar-Taromi et al., 1978 Kuntz, 1967]. This hydride ion abstraction is so facile with 1,3-dioxolane that it is used to preform stable l,3-dioxolan-2-ylium salts (XII) that can be used subsequently as initiators [Jedlinski et al., 1985],... 

More recently Penczek and his collaborators (134) have used l,3-dioxolan-2-ylium salts to initiate the polymerisation of THF in a variety of solvents. In these systems initiation is clean and efficient, and in the case of carbon tetrachloride as solvent, the only significant ionic species present are ion pairs. Rate constants /c for propagation in this solvent were obtained directly from conversion/time data and are a measure of the reactivity of ion pairs. At 25° C (tTHF]0 = 8.0 M) the value of /c (4.0 x 10 2 M-1 sec-1) was independent of the counter-ion employed, in the series AsFg, PFg, and SbFg. 

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