Dioxolans, polymerizability

Methylene- 1,3-dioxolane (15) is polymerized by peroxide initiator to produce a polyester 16 which is regarded as the product of the hypothetical ring-opening polymerization of the non-polymerizable heterocycle of y-butyrolactone (Eq. (14)).13). The opening of the cyclic species bearing a free radical 17 is the key step. 

These changes observed in H-NMR are illustrated in Fig. 1. The same figure describes tte chemical shift of the acidic proton as a function of the ratk) of concentrations [(C2Hs)2O]/[H0SO2CF3] for a non-polymerizable ether. This system qualitatively behaves in the same way as a syrtem with 1,3-dioxolane however, obviously, no upfield shift (due to polymerization and formation of HO—CH2-. .. groups) is observed for diethyl ether. 

Polymerizability of 5-membered cyclic acetals (1,3-dioxolanes) have been studied in much greater detail. 1,3-Dioxolanes adopt the stable envelope conformation. One of the carbon atoms in the ethylene group is located at the tip of the flap, giving a dihedral angle between the cis- neighbouring hydrogens of 35°47). 

Substitution in dioxolanes decreases their polymerizability. Although the strain in the monomer may increase with substitution, the conformational strain of the macromolecule apparently prevails. The increase of the strain that destabilizes the monomer comes from the replacement of the cis-geminal C4—H and C5— H bond oppositions with greater C4—H and C5-substitutent interactions (for mono-substitu-... 

The bulk polymerization of 4-methyl-1,3-dioxepane at 0 °C yields 60% polymer. Further substitution decreases polymerizability as with 1,3-dioxolanes. 2,4-Dimethyl-1,3-dioxepane gives oligomers in limited yield, the 2,2-dimethyl-derivative dimerizes and the 4,4-dimethyl derivative does not polymerize at all53). 

Like THF, cyclic acetals (e.g., 1,3-dioxolane and 1,3,5-trioxane) are polymerizable only with cationic initiators. The ring-opening polymerization of 1,3,5-trioxane (cyclic trimer of formaldehyde) leads to polyoxymethylenes (see Example 3.24), which have the same chain structure as polyformaldehyde (see Example 3.22). They are thermally unstable unless the semiacetal hydroxy end groups have been protected in a suitable way (see Example 5.7). Like the cyclic ethers, the polymerization of 1,3,5-trioxane proceeds via the addition of an initiator cation to a ring oxygen atom, with the formation of an oxonium ion which is transformed to... 

Of the cationically-polymerizable cyclic acetals, 1-3-dioxolan has received the most attention. By means of an ion-trapping technique, quantitative measurements of the concentration of active centres have been made and correlated with initiator incorporation to confirm the living characteristics of the linear polymers under certain conditions. The oxycarbenium ion nature of these propagating centres has been verified by C-n.m.r. spectroscopy. Methyl substitution alters the polymerizability of the homologous 1,3-dioxepanes significantly and steric hindrance in the initiator can influence both the homopolymerization and copolymerization of cyclic acetals. ... 

Cyclization during the polymerization of 1,3-dioxolane can generate cationically-polymerizable oligomers. Of these oligomers, 1,3,6,9-tetraoxacycloundccane has been subjected to kinetic investigations. - ... 

Jedliriski analysed in a series of papers the H-NMR spectra of various substituted 1,3-dioxolans in order to understand the stereochemistry of these monomers. Then, following earlier work, described previously for the unsubstituted dioxolenium salts, studied the kinetics of H transfer from these monomers to the triphe-nylmethylium cation 23, as the first reaction, preceding the true initiation. This approach, is complementary to that of Okada,which gives a thermodynamic in -formation about the polymerizability, while Jedlirtski tends to characterize the influence of structure (stereochemistry) on the rate of reactions pertinent to elementary reactions. There are till now, however,no... 

According to Astle (4o) et al. trioxocane can be obtained by condensation of diethylene glycol with paraformaldehyde it is easily polymerizable by cationic catalysts or by electrochemical initiation (41). Copolymerization with e.g. trioxane or dioxolane are possible, too (41). Kinetics, thermodynamics and mechanism of homopolymerization have been studied in detail by several authors. According to the analytic results of Weichert (42) the structure of the polymers of /2/... 

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