Mechanistic Aspects of Anhydro Sugar Polymerization Related to Polymer Structure and Synthetic Applications

5 Mechanistic Aspects of Anhydro Sugar Polymerization Related to Polymer Structure and Synthetic Applications 

The recovery of the deblocked polysaccharide may be most conveniently achieved by sodium in liquid ammonia. Usually, deblocking is accompanied by chain scission, but this is negligible under proper conditions. For example, debenzylation of a polymer with DPn close to 900, which requires the removing of 2700 benzyl ether functions, causes a three- to four-fold decrease in DPn 20,21 Sterically pure linear polysaccharides with a number average degree of polymerization of 150-300 can be prepared by cationic polymerization of ether derivatives of 1,6-anhydro sugars followed by deblocking. 

Research in this field was directed mainly towards synthetic aspects and information concerning the mechanism and kinetics of the polymerization are scarce. 

Lewis acids were usually used as initiators, PFS being the most common choice. Recently, 31P- and 19F-NMR was used to study kinetic features of anhydro sugar polymerization initiated with PFS 29,30). PFS generated by the thermal decomposition of p-ClC6H4N2+ PF was introduced into a solution of anhydro sugar (8-30) in CH2C12, 

The intensity of PFf signals was constant troughout the polymerization, whereas the intensity of the signals of PF40- decreased with time. On the basis of these observations the following reaction scheme was proposed. 

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