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-d 3]-β-cyclodextrin ((Me-d 3)21-β-CD, 2) to cationic ring-opening polymerisation (CROP). Reactions were performed 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-d 3)21-β-CD6α 1β, 3b) is formed as an intermediate, while linear Me21- and (Me-d 3)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/b) 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.