Solid-State 13C-NMR Investigation of Di- and Tri-block Biodegradable Copolymers Isolated In Their Inclusion Compounds with Cyclodextrins

Abstract

We have synthesized and characterized two biodegradable copolymers: poly(e-caprolactone)-poly(L-lactide), (PCL-b-PLLA) diblock copolymer, and poly(e-caprolactone)-poly(propylene glycol), (PCL-PPG-PCL) triblock copolymer. The lengths of each block in the diblock were determined to be: PCL = 92 and PLLA = 84. While in the tri-block, each PCL block length was found to be 45, and the PPG was 60. Inclusion compounds (ICs) of these copolymers with alpha- and gamma- cyclodextrins (CDs) were formed and characterized. The solid crystals of the ICs were found to be in the channel packing mode, allowing the copolymers to be isolated as straight chains within the cylindrical channels of the CDs. We have investigated the conformations and dynamics of the isolated di- and triblock copolymer chains entrapped within the channels of alpha- and gamma-cyclodextrins (CD) using solid-state 13C-NMR techniques. The PCL block chains isolated within the cavities of alpha-CD, adopt a conformation similar to that of the bulk semi-crystalline di- and tri-block copolymers. While in gamma-CD/IC, an upfield shift by approximately 1 ppm was observed for the PCL block chains in the triblock. The spin-lattice relaxation time T1 (C) measurements confirmed the semi-crystalline morphology of both of these copolymers. A dramatic difference was observed in the mobility of the polymer chains in the bulk compared to the ICs. In the absence of intermolecular interactions, the isolated chains experienced much rapid mobility relative to their behavior in the bulk. This result reflects on the role of cooperative interactions between the polymer chains in both the bulk di- and tri-block copolymers. The length scale of proton spin diffusion was probed using T1(1H) and T1rho (1H). The T1(1H) in the bulk copolymers averaged to a single value, while the proton T1 indicated that the copolymers were phase-separated. In the ICs, exchange of proton magnetization through spin diffusion was observed between the polymer chains and the CDs, but it was not totally efficient. 2D heteronuclear correlation method was also employed to monitor the nature of proton communication in these samples. Intra-block exchange of proton magnetization was observed in the bulk copolymers at short mixing times. In the ICs, intra-block 1H-1H spin communication was observed for the isolated chains, in spite of the physical closeness between the isolated chains and CD molecules efficient proton spin diffusion was not observed.