Cooperative Folding of Linear Poly(dimethyl siloxane)s via Supramolecular Interactions

The synthesis and characterization of graft copolymers are reported based on linear poly(dimethyl siloxane) (PDMS) and chiral, pendant benzene-1,3,5-tricarboxamides (BTAs). The copolymers differ in degree of polymerization (DP) and BTA graft density. Characterization of the bulk materials at room temperature reveals that the BTAs aggregate in a helical fashion via threefold hydrogen-bond formation within the PDMS matrix. A significant degree of hydrogen bonding persists up to 180 degrees C, regardless of DP and BTA content. Analysis of the solution behavior by H-1 NMR spectroscopy indicates that BTA aggregation occurs in CDCl3, a solvent normally suppressing aggregation. Circular dichroism (CD) spectroscopy in 1,2-dichloroethane shows strong CD effects and reveals that increasing the DP and decreasing the BTA graft density results in an increase in the cooperativity of the BTA aggregation. Dynamic light scattering indicates the formation of particles with sizes of 400 nm. This is the first time that polymers with pendant BTAs show a sharp transition between a nonaggregated and aggregated state, a behavior similar to the one observed for "free" BTAs. The cooperative aggregation is attributed to the strong phase-segregation between the PDMS backbone and the BTAs, in combination with a high propensity of these polymers to form multichain aggregates.