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Unexpected Polypseudorotaxanes Formed from the Self-assembly of β-Cyclodextrins with Poly(N-isopropylacrylamide) Homo- and Copolymers

Duan, Nannan, Lu, Hang, Ye, Lin, Zhang, Ai-Ying, Feng, Zeng-Guo
TheJournal of physical chemistry 2019 v.123 no.23 pp. 5004-5013
Homo, X-ray diffraction, ambient temperature, aqueous solutions, beta-cyclodextrin, composite polymers, crystal structure, propylene glycol
Compared with polypseudorotaxanes (PPRs) formed from the self-assembly of β-cyclodextrins (β-CDs) with poly(propylene glycol) (PPG) and γ-CDs with poly(N-isopropylacrylamide) (PNIPAAm), the ratio of the inner cavity size of β-CD to the cross-sectional area of PNIPAAm appears not appropriate for their self-assembly. For a better understanding of the possibility of β-CDs including PNIPAAm and the crystal structure of PPRs formed therefrom, the PNIPAAm homo- and copolymers were subjected to self-assembly with β-CDs in an aqueous solution at room temperature. The results revealed that when β-CDs meet thicker PNIPAAms, the self-assembly takes place, not only giving rise to PPRs by a manner of main-chain inclusion complexation but also presenting the PPRs a matched over-fit crystal structure different from those of either a matched tight-fit β-CD–PPG PPR or a mismatched over-fit γ-CD–PNIPAAm PPR. This is most likely due to the thicker PNIPAAm adapting its unfavorable main-chain cross-sectional area to fit into the cavity of β-CDs by changing the side-chain conformations. Based on the X-ray diffraction patterns, a monoclinic crystal system was created from these PPRs and the unit cell parameters calculated were as follows: a = 15.3 Å, b = 10.3 Å, and c = 21.2 Å; β = 110.3°; and space group P2. It suggested that this matched over-fit crystal structure would possess a Mosaic crystal structure rather than a typical channel-like one.