Lyotropic Phases Formed by 'Molecular Botllebrushes'

摘要

The polymerization of macromonomers (that is. end-func-tionaiized oligomers. Scheme 1) has been a matter of extensive research in order to prepare well-defined comblike polymers. However, the homopolymerization of macromonomers did not yield degrees of polymerization significantly exceeding the length of a macromonomer itself, unless copolymerized with conventional small monomers.A major breakthrough was achieved by Tsukahara et al., who in a radical mechanism ho-mopolyrrierized anionically preparedoligostyrenes with molar masses between 10~3 and 10~4 gmol~(-1) that were end-functional-ized with methacryloyl groups and obtained degrees of polymerization (weight average) P_w of up to 1000. Recent structure characterization of such long-chain polymacromonomers revealed that the main chain exhibits an almost rodlike conformation as measured by the Kuhn statistical segment length of l_k>1000 A. Apparently, the extended structure of the poly-methacrylate (PMA) main chain is caused by the strong overcrowding of the oligostyrene side chains, which are only separated by a contour distance l = 2.5 A at the polymer backbone. Also, because of the length of the side chains, the diameter or cross-section of this curved cylindrical macromolecule or "bot-tlebrush" is quite large (d≈70-150 A for a side chain of molar mass M_n — 3950 gmol~(-1), dependent on whether hydrodynamic data or X-ray investigations of the cross-sectional radius of gyration were used for the determination of diameter). Still, the aspectratio L_2/d(L_w is the weight-averaged contour length, L = p -l. and d the diameter of the chain) is of the order of 10-50 for most of the samples, and accordingly, such polymers should form liquid-crystalline phases.