Spiro ortho carbonate

Finally, the unsaturated spiro ortho carbonates were been shown to undergo double ring opening in a free radical polymerization to introduce a carbonate group in the backbone of an addition polymer (19). 

The chemistry of a wide variety of unsaturateo spiro ortho carbonates and unsaturated spiro ortho esters have been discussed in detail previously (14,19,24,27-X). They not only are useful for monomers that expaiu on polymerization Qa,32-21) but are useful as dental filling materials (38), for the syntHesTs of oligomers capped with hydroxyl groups, pro( ction of biodegradable polymers, and enhancement of thermal stability of copolymers (39-45) ... 

An inspection of Fig. 1 indicates that the densities of the monomeric spiro ortho carbonate and the polymer appear to cross above 200 . At that point, one would expect no change in volume during polymerization since the two materials have the same density. Above this critical temperature, one would expect to get shrinkage during the polymerization. Unfortunately, the polymerization cannot be carried out conveniently in this temperature range with the catalysts now available since carbon dioxide is liberated and the polycarbonate is not obtained in a pure form. At the lower end of the temperature scale the two lines appear to Intersect again, but one would expect below the glass transition of the polymer that the density line would become more nearly horizontal and become essentially parallel to the line of the density of the monomer. 

Figure 2. Densities of the monomeric unsaturated spiro ortho carbonate and related polyoxycarbonate vs. temperature...

By a very similar synthetic scheme, other unsaturated spiro ortho carbonates were prepared. 

A potential use of this monomer is in the area of dental fillings in which a slurry containing 20% of very fine crystals of the unsaturated spiro ortho carbonate I in 60% of the adduct of methacrylic acid to bisphenol-A diglycidyl ether (Bis-GMA) plus 20% trimethylolpropane trimethacrylate produces on polymerization a material with essentially no change in volume. An investigation of a bubble test on tooth enamel showed that this copolymer had nearly double the adhesion to the tooth structure that the base resin had without the addition of the unsaturated spiro ortho carbonate. The copolymer also had improved impact strength but yet essentially the same modulus, and filled composites appeared to have somewhat improved abrasion resistance. 

Since the synthesis of the spiro ortho carbonates through the tin compounds could be modified to produce unsymmetrical materials, we undertook the synthesis of the unsymmetrical 2-methylene-l,5,7,ll-tetraoxaspiro[5.5]undecane by the accompanying set of reactions. 

By a very similar synthetic scheme, other unsaturated spiro ortho carbonates were prepared. Bulk polymerization or solution polymerization in chlorobenzene gave soluble polymer if the reaction was stopped at low conversion. 

Figure 5B. Radical ring opening pol3nnerization of spiro ortho carbonate vinyl monomers with expansion in volume (W. J. Bailey et al.).

These reactions do not involve a straightforward cyclization by electrophilic attack of the nitrene on an ortho carbon atom 69 - 70, but they occur by initial ipso attack to form a five-membered ring spiro intermediate (71), which subsequently undergoes rearrangements. Alternatively, it has been suggested that the aziridine 72 is formed directly from 69. The pathway taken is probably substituent dependent. 

The mechanism for the conversion of the A -oxide (94) to the o-methylaminophenylquinoxaline (96) involves an initial protonation of the A -oxide function. This enhances the electrophilic reactivity of the a-carbon atom which then effects an intramolecular electrophilic substitution at an ortho position of the anilide ring to give the spiro-lactam (98). Hydrolytic ring cleavage of (98) gives the acid (99), which undergoes ready dehydration and decarboxylation to (96), the availability of the cyclic transition state facilitating these processes. ... 

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