Efficient Low-Temperature Atom Transfer Radical Coupling and Its Application to Synthesis of Well-Defined Symmetrical Polybenzamides

Radical coupling reactions have been widely used for preparation of symmetrical macromolecules. In previous studies of atom transfer radical coupling (ATRC), coupling efficiency above 90% has been generally attained at high temperature (T = 70-110 degrees C) by using M-n < 5000 precursors. From the mechanistic viewpoint, we designed a strategy for styrene-assisted ATRC from methacrylic macroradicals at low temperature (St/PMMA-Br/(CuBr)-Br-I/Cu-0/Me6TREN). Analysis of the products (H-1 NMR, H-H COSY, MALDI-TOF mass spectra and gel permeation chromatography) indicated that this provides an efficient "convergent" or coupling approach, if a high degree of chain-end functionality is preserved. We then applied this concept in combination with chain-growth condensation polymerization (CGCP), which usually affords quantitative chain-end functionality. Couplings of high-molecular-weight AB-type diblock polybenzamide, 2-bromoisobutyryl-terminated poly(N-octyloxybenzyl-m-benzamide)-block-poly(N-octyl-m-benzamide) (M-n = 9300, M-w/M-n = 1.09), were conducted to yield ABA-type triblock polybenzamides with high coupling efficiency (>94%). The molecular weight (M-n similar to 18000) was doubled and a narrow molecular weight distribution (M-w/M-n < 1.18) was maintained. Selective removal of the octyloxybenzyl groups was achieved, resulting in a poly(N-H-m-benzamide) segment (i.e., A block). Thermal transitions of the diblock and triblock polybenzamides were examined by DSC. In the case of triblock polybenzamides, the T-g value shifted from 45 to 62 degrees C after removal of the octyloxybenzyl groups; this might be ascribed to a confinement effect of the segments at the extremities via strong intermolecular hydrogen-bonding interaction.