Synthesis of Helical Poly(phenylacetylene)s Carrying Dendritic Pendants with Varied Branching Densities Through Polymerization in Different Solvents

Helical poly(phenylacetylene)s (PPAs) have received extensive attention because of their features in dynamic chirality and promising applications. Therefore, understanding relationship among the polymer molecular structures, polymerization conditions and tunability of their chirality is of key scientific value. Recently, we developed a novel class of dendronized PPAs carrying 3-fold dendritic oligo(ethylene glycols) (OEGs) via alanine linkage, and found that these bulky polymers exhibited tunable helical conformations through thermally-mediated dehydration and aggregation. Herein, we report on synthesis of a homologous series of dendronized PPAs that carry 2-fold, 3-fold or 6-fold dendritic OEG pendants, and focus on effects of molecular topological structures, peripheral units and polymerization solvents on the thermoresponsiveness and their conformation switching behaviors. Effects of branching density and peripheral units (ethoxyl or methoxyl) of the dendritic OEG pendants were examined, and found to play a decisive role on the helical conformation and thermoresponsiveness of these dendronized PPAs due to their different bulkiness and overall hydrophilicity. In addition, different polymerization solvents were checked for their possible influence on the polymerization, thermoresponsive behavior and the chirality of the resulting polymers. For polymerization in selective solvents like water or methanol, the obtained dendronized PPAs exhibited weak thermal transitions, while polymerization in non-selective solvent like THF furnished PPAs with characteristic thermoresponsive behavior, indicating that solvents were involved in the process of polymerization of the dendronized macromonomers. More interestingly, different chiralities of the PPAs through polymerization in various solvents were retained, irrelevant to the purification process and solvents treatments. This work suggests that the topological structures together with polymerization solvents can modulate the thermoresponsive behavior and helical conformation of the dendronized PPAs.