Mechanistic studies on cationic ring-opening polymerisation of cyclodextrin derivatives using various Lewis acids

In order to investigate the potential of cyclodextrins for the preparation of block-like substituted polysaccharides, we submitted mixtures of heptakis[2,3,6-tri-O-methyl]-β-cyclodextrin (Me-β-CD, 1) and heptakis[2,3,6-tri-O-methyl- d]-β-cyclodextrin ((Me-d)- β-CD, 2) to cationic ring-opening polymerisation (CROP). Reactions were pe rformed with BF · OEt, methyl triflate (MeOTf), and EtOSbCl. Products were compared with respect to their degree of polymerisation (DP) and the average block length (BL). Highest DP was observed with BF · OEt, while EtOSbCl was the most active initiator. Average block length decreased from 14 in the early stage of product formation to about 2 due to competing chain transfer reaction. H NMR spectroscopy, GLC, GLC-MS, ESI-MS and MALDI-TOF-MS were applied for detailed investigations of side reactions. During incubation with BF · OEt, a stereroisomeric β-CD with one β-glucosidic linkage (Me-β-CD, 3a (Me-d))-β- CD, 3b) is formed as an intermediate, while linear Me- and (Me-d)-maltoheptaose (4a/b) was detected in the early stage of the reaction promoted by MeOTf. In the case of EtOSbCl, both intermediates (3a/b, 4a) can be observed during the lag phase of polymerisation, but to a very low degree. End group analysis by GLC reveals that some alkyl exchange occurs at position 3 and 6 in the presence of EtOSbCl, and that polymerisation is also initiated by protons. Copolymerisation of heptakis[2,3,6-tri-O-benzyl]-β-cyclodextrin (Bn-β- CD, 5) and Me-β-CD (1) and subsequent debenzylation yielded a polymer of only 1,4-glcp-Me- and 1,4-glcp-residues. Reactivity of Bn-β-CD was significantly lower than of Me-β-CD, resulting in higher average block length of 1,4-glcp-Me-units. © Springer Science+Business Media, Inc. 2006.