Anhydrosugars polymerization

Both unsubstituted bicyclic monomers and related anhydrosugars polymerize by cationic mechanism [148]. In sugar derivatives, the remaining free hydroxyl groups have to be blocked before polymerization by typical methods known in carbohydrate chemistry. 

In general, it is possible to obtain higher degrees of polymerization by ring-opening polymerization than with polycondensation. Two types are used in oligo and polysaccharide syntheses orthoester synthesis and cationic anhydrosugar polymerization. 

Cationic polymerization of anhydrosugars give (after deblocking) polysaccharides with reasonably high molecular weights (DP in the range of 102-103) and in some systems, e.g., in the polymerization of 2,3,4-tris-... 

In the polymerization of anhydrosugars, however, it is desirable to prepare the polymers having the structure identical with that occurring in natural polysaccharidess. This can not always be achieved. 

Polymerization of bicyclic acetal (1,4-anhydrosugar) composed of fused 5- and 6-membered rings may involve opening of both rings, because a 6-membered ring exists in enforced unfavorable boat conformation [150-152] ... 

The lack of specificity is thus related to similar ring strain of both fused rings. 1,6-Anhydrosugars, in contrast, polymerize exclusively with opening of 5-membered ring because the 6-membered ring, being in more favorable conformation, is essentially strainless [153]. 

In the cationic polymerization of 1,2-anhydrosugars, however, racemization is clearly detected indicating unimolecular opening of 3-mem-bered ring [154] ... 

In the polymerization of 1,6-anhydrosugars due to the lack of strained ring, propagation proceeds as SN2 reaction with inversion of configuration, thus typically, sterically uniform polysaccharides are obtained [153]. 

Polysaccharides having a regular structure were first obtained by polymerization of anhydrosugars or step-by-step elongation. For synthesis of irregular structures, a step-by-step elongation by polycondensation is the usual method at least to produce the repeating unit which is then polymerized. These routes have been discussed previously in the review of Kochetkov (313). 

U-Anhydrosugar derivatives which have a bicyclic skeleton corresponding to have been shown to undergo polymerization. 1,U-Anhydro-2,3-di-O-methyl-L-arabinose (l6) and 1,U-anhydro-2,3,6-tri-O-methyl-D-galactose (ij) yielded under the influence of Lewis acid initiators amorphous polymers containing both furanose and pyranose rings. 

Carvalho, J.D.S., Prins, W., and Schuerch, C. (1959) Addition polymerization of anhydrosugar derivatives. 1. A polyanhydroglucose. Journal of the American Chemical Society, 81,4054—4058. 

The bicyclic acetal, 6,8-DBO, possesses the structural skeleton of the 1,6-anhydrosugars and, because it is readily prepared from acrolein dimer, its cationic polymerization has been investigated. Mostly Lewis acids (BF3, PF5) were used as initiators, and at low temperature (-78 °C) polymers with Mn up to 10 could be obtained although dispersity was broad. ... 

Cationic polymerization of anhydrosugars has been reviewed and since then there has been little activity in this area. More recent studies revealed, however, that cationic polymerization of anhydrosugars displayed many of the elements of controlled polymerization a linear increase in molecular weights with conversion and block copolymer formation. 

In the polymerization of 1,4-anhydrosugar with blocked hydroxy groups initiated with BF3 Et20, a linear increase in molecular weight with conversion was observed although dis-persity was not narrow 1.6-1.7). Block copolymers... 

Cationic polymerization of 1,6-anhydrosugar with blocked hydroxy groups initiated by the cumyl chloride (2-chloro-2-phenylpropane)/Znl2 initiating system gave polysaccharides with relatively low dispersity 1.25). The kinetic... 

There are also reports on the cationic polymerization of anhydrosugars having free hydroxy groups. Anhydrosugars shown in Scheme 27 were polymerized at high temperatures (>130 °C) with sulfonium salts as initiators to highly branched polysaccharides with in the range between 3 x lO and... 

The steric course of polymerization of anhydrosugars depends on the size of the fused rings. Thus, for polymerization of 1,2- and 1,4-anhydrosugars, racemization has been observed and explained by participation of alkoxycarbenium ions in the propagation step (e.g., for 1,2-anhydrosugars, see Scheme 29). 

For the configuration-conformational properties of simple carbohydrates, as well as their reactivities, comprehensive treatises should be consulted (Pigman and Horton, 1972). It has to be mentioned on closing, that recent achievements in macromolecular science have established the ways to the synthesis of linear homopolysaccharides and copolysaccharides (normally by cationic polymerization) of different suitably protected anhydrosugars (Schuerch, 1981). These syntheses open the route to the availability for our studies of a variety of tailor made carbohydrate polymers. A few synthetic polysaccharides have in fact already been used to investigate immunological and allergic reactions in experimental animals and the interaction of plant lectins with carbohydrates (Schuerch, 1981). 

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