Solid-state carbon-13-NMR investigation of di- and tri-block biodegradable copolymers isolated in their inclusion compounds with cyclodextrin

We have synthesized and characterized two biodegradable copolymers: poly(&egr;-caprolactone)-poly(L-lactide), (PCL-b-PLLA) diblock copolymer, and poly(&egr;-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 α- and γ-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 α- and γ-cyclodextrins (CD) using solid-state 13C-NMR techniques. The PCL block chains isolated within the cavities of α- and γ-CDs, adopt a conformation similar to that of PCL crystalline blocks in the bulk semi-crystalline di- and tri-block copolymers. The spin-lattice relaxation time T_1C 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 increased mobility relative to their behavior in the bulk. This result reflects 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 T_1H and T_1ρH. T _1H in the bulk copolymers averaged to a single value, while the T _1ρH 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 complete. Two-dimensional solid-state heteronuclear correlation method was also employed to monitor 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.