Synthesis and characterization of star-shaped poly(ethylene-co-propylene) polymers bearing terminal self-complementary multiple hydrogen-bonding sites

The polymerization of isoprene was initiated with 3-(tert-butyldimethylsilyloxy)-1-propyllithium (TBDMSPrLi), which contained a silyl-protected hydroxyl functionality. Living poly (isoprenyllithium) with controlled molecular weight and narrow molecular weight distribution coupled efficiently with divinylbenzene to form well-defined star-shaped polymers. Both linear and star-shaped polymers were subsequently hydrogenated to poly (ethylene-co-propylene) and deprotected quantitatively to yield terminal primary hydroxyl functionality. High conversions of hydroxyl functionality to the 2-ureido-4[H-1]-pyrimidinone (UPy) quadruple hydrogen-bonding group were achieved using isocyanate coupling and subsequent reaction with 6-methylisocytosine. Non-functionalized and UPy-functionalized linear and star-shaped poly(ethylene-co-propylene)s were characterized using H-1 NMR spectroscopy, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), tensile testing, and melt rheology. The observed glass transition temperatures were independent of molecular architecture for the UPy-functionalized polymers and nonfunctionalized analogues using both DSC and DMA. Tensile testing revealed the UPy-functionalized star polymers (UPy-Star) exhibited a higher Young's modulus and lower percent elongation at failure compared to the UPy-telechelic polymers with M-n of 24 000 g/mol (UPy-24K-T) analogues. UPy-Star polymer exhibited a rubbery plateau region over a well-defined frequency range, and in contrast, the UPy-functionalized linear polymers were in the terminal flow regime, which suggested greater association for the star-shaped polymers. In addition, complex viscosity data revealed a non-Newtonian behavior for the star-shaped polymers in contrast to linear analogues, which is also consistent with a highly associated structure.