Mesogen–Containing Multi–Block Poly(ester–carbonate)s Bearing Ether Side Groups and Their Nanostructures

Aromatic condensation polymers such as poly(ethylene terephthalate) and poly(bisphenol-A carbonate) are used in the form of fibers, films, and bottles due to their mechanical and thermal stabilities. Highly aromatic polyesters are known as thermotropic main-chain liquid-crystalline polymers (LCPs), exhibiting high mechanical and thermal properties, chemical stabilities, and processability. The molecular assembly and alignment of rigid moieties, called mesogen, enable the excellent performance of the LCPs .<br/>Aliphatic condensation polymers represented by poly(L-lactic acid) and poly(trimethylene carbonate) (PTMC) are recognized as biodegradable polymers, which can be degraded in compost and aqueous environments including body and marine with the aid of enzymes and microorganisms. These polymers have been used in medical devices such as sutures and drug delivery carriers. Side-chain functionalized analogs of these polymers have been developed to induce additional complex biological functions for applications in the fields of nanomedicine and regenerative medicine. PTMC analogs have drawn attention as the functionalized biodegradable polymers because of the variety of installable functionalities and facile accessibility in the monomer synthesis. However, the PTMC analogs with bio-related functional side chains are often flexible and have low glass transition. A PTMC analog bearing an ether side chain previously developed exhibited a high level of cell attachment and biocompatibility as well as enzymatic degradability. This polymer was a liquid material with a glass transition temperature (<i>T</i>_g) around –20 °C, and thus limiting its use. In the present study, we have introduced an aromatic ester triad mesogen into the backbone of the ether-functionalized PTMC analog to impart the mechanical stabilities, while maintaining the biological functions and biodegradability.<br/>Our macromolecular design includes multi-block structures where oligocarbonate sequences with ether side chains are connected by the aromatic ester triad mesogens. The mesogen–containing multi–block condensation polymers (<b>P1</b>) were synthesized via two-step polymerization. In the first step, mesogen-containing oligocarbonates (<b>P2</b>) were obtained by ring-opening polymerization of a cyclic carbonate with an ether group, using a bis(hydroxy)-functionalized aromatic mesogen as an initiator. The mesogenic diols showed liquid-crystalline properties. The lengths of the oligocarbonates were controlled by the ratio of the cyclic monomer and the mesogen. The peripheral hydroxy groups of <b>P2</b> were then used for chain extension with adipoyl chloride to give <b>P1</b> with weight-average molecular weights over 1.0 × 10⁵. Unlike <b>P2</b> and the original ether-functionalized PTMC analogs, <b>P1</b> exhibited elastomeric properties. Differential scanning calorimetry of <b>P1</b> only showed glass transition, indicating that they were amorphous. Polarized optical microscopy of <b>P1</b> subjected to shear suggested nanosegregation and alignment of the aromatic mesogens. Furthermore, atomic force microscopy (AFM) of <b>P1</b> revealed that <b>P1</b> formed nanosegregated structures composed of high modulus domains with approximately 10 nm in diameter, surrounded by soft matrices. Nanomechanical evaluations of the polymer surface of <b>P1</b> using the AFM technique indicated that the surface modulus of <b>P1</b> was 4-5 times higher than that of the non-mesogen-containing counterparts.<br/>In summary, we successfully synthesized mesogen–containing aromatic/aliphatic poly(ester–carbonate)s bearing ether groups at the side chains (<b>P1</b>). The two-step polymerization using ring-opening polymerization and subsequent polycondensation gave <b>P1</b> with regulated multi-block structures. Incorporation of the aromatic ester triad mesogens into the main chain of the bio-related side-chain functionalized aliphatic polycarbonates plays an effective role in the enhancement of their mechanical properties.