Induction period dioxolane

Where 1,3-dioxolane is the nucleophile then the kinetic product equivalent to 15 is sterieally unfavoured, and so a slow reaction to thermodynamic products 17 equivalent to 16, seems most likely to account for the observed induction period... 

On the other hand copolymer with a trioxane unit at the cationic chain end (Pi+) may be converted intp P2+ by cleavage of several formaldehyde units. These side reactions change the nature of the active chain ends without participation of the actual monomers trioxane and dioxo-lane. Such reactions are not provided for in the kinetic scheme of Mayo and Lewis. In their conventional scheme, conversion of Pi+ to P2+ is assumed to take place exclusively by addition of monomer M2. Polymerization of trioxane with dioxolane actually is a ternary copolymerization after the induction period one of the three monomers—formaldehyde— is present in its equilibrium concentration. Being the most reactive monomer it still exerts a strong influence on the course of copolymerization (9). This makes it impossible to apply the conventional copolymerization equation and complicates the process considerably. 

To investigate the copolymerization of trioxane with dioxolane and to determine r1 by the excess method, a molar ratio of trioxane to dioxolane of 100 1.8 was used. All polymerizations were run in methylene dichloride at 30°C. with SnCl as initiator. To reduce the influence of formaldehyde production at the beginning of copolymerization, dioxolane was added to the solution of trioxane and initiator only at the end of the induction period—i.e., at the appearance of the first insoluble polyoxy-methylene. After various reaction times polymerizations were terminated by adding tributylamine. Monomer conversions were determined by gas chromatography, the liquid phase being injected directly. When conversions were small, isolation and analysis of the copolymer yielded more accurate results. 

The induction periods observed in the polyrnwization of 1,3-dioxolane initiated by acids (e. g. perchloric acid trifluoromethaneailfonfc acid is apparently due to the equilibrium shifted toward much less reactive secondary oxonium ions at the early stages (short chains) of polymerization. 

In 1971 Lyudvig et al. [97] reported an investigation of the polymerization of 1,3-dioxolane using precision purification of reactants and Et3 0 SbCl6 as initiator. Under these conditions polymerization with different initial monomer concentrations are reported to take place without any induction period. Lyudvig et al. report that the polymerization is first order with respect to both monomer and initiator concentration. Polymerization of 1,3-dioxolane is a reversible process. The final kinetic equation takes a form similar to eqn. (6). A UV spectroscopic method was used to investigate the nature of the active centre in the polymerization. Very briefly, what these workers found was that the maximum they observed for the polymerizing mixture was different from that which could be attributed to a simple cyclic oxonium ion. Hence they propose that the active centre has the polymeric tertiary oxonium ion structure... 

Yamashita [128] reports that the kinetics of polymerization of 1,3-dioxolane initiated by BF3. Et2 O are quite complex and that initiation is not a simple reaction. In 1973 Rozenberg et al. [130] reported a kinetic study of such a polymerization in CHjClj solution. They e ain found an induction period, but this time stated that the acceleration is the result of the autocatalytic action of the macromolecules formed. Their conclusions are based (i) on the relationship of the rate coefficients of BF3. Et2 0 reactions with cyclic monomer or with polymer chain and (ii) on the decrease of the induction period on addition of polymer or methylal to a polymerizing mixture. The mechanism of initiation suggested is... 

For TOX copolymerization with ethylene oxide (EO), EO was first converted to low molecular weight copolymer and cyclic oligomers such as dioxolane (DOL) and 1,3,5-trioxepane (TOP) (4). For either EO and DOL as comonomer with TOX, they were found to be preferentially incorporated during the induction period (5). 

It was concluded that the ring expansion resulting from successive insertions of EO units (-CH2CH2O-) into cyclic formals issued from TOX is the main initial process of the reaaion during the induction period. The appearance of TXP and then DXL is delayed until most of EO is consumed. This conflicts with the previously proposed mechanism involving the prior formation of dioxolane by reaction of formaldehyde with the EO oxonium followed by the reversible insertion of formaldehyde unit (OCH2) into the DXL oxonium leading to TXP. 

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