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

Abstract

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 (Me21-β-CD, 1) and heptakis[2,3,6-tri-O-methyl- d3]-β-cyclodextrin ((Me-d3)21- β-CD, 2) to cationic ring-opening polymerisation (CROP). Reactions were pe rformed with BF3 · OEt2, methyl triflate (MeOTf), and Et3OSbCl6. Products were compared with respect to their degree of polymerisation (DP) and the average block length (BL). Highest DP was observed with BF3 · OEt2, while Et3OSbCl6 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. 1H NMR spectroscopy, GLC, GLC-MS, ESI-MS and MALDI-TOF-MS were applied for detailed investigations of side reactions. During incubation with BF3 · OEt2, a stereroisomeric β-CD with one β-glucosidic linkage (Me21-β-CD6α1β, 3a (Me-d3)3)21-β- CD6α1β, 3b) is formed as an intermediate, while linear Me21- and (Me-d3)21-maltoheptaose (4a/b) was detected in the early stage of the reaction promoted by MeOTf. In the case of Et3OSbCl6, 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 Et3OSbCl6, and that polymerisation is also initiated by protons. Copolymerisation of heptakis[2,3,6-tri-O-benzyl]-β-cyclodextrin (Bn21-β- CD, 5) and Me21-β-CD (1) and subsequent debenzylation yielded a polymer of only 1,4-glcp-Me3- and 1,4-glcp-residues. Reactivity of Bn21-β-CD was significantly lower than of Me21-β-CD, resulting in higher average block length of 1,4-glcp-Me3-units. © Springer Science+Business Media, Inc. 2006.