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 heptakis2,3,6-tri-O-methyl]-β-cyclodextrin (Me₂₁-β-CD, 1) and heptakis2,3,6-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-CD_6α1β, 3a (Me-d ₃)₂₁-β-CD_{6α 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 heptakis2,3,6-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.