No abstract

A semi-crystalline precision polymer with a high molar mass (>100 kg mol–1), a moderate dispersity (∼1.6), and an isotactic alcohol pendant on each and every fifth carbon of a linear polyethylene backbone is synthesized through highly regioregular ring-opening metathesis polymerization of (S) or (R)-3-(tert-butyldimethylsiloxy)­cyclopentene followed by olefin hydrogenation and alcohol deprotection. The thermal and semi-crystalline properties of these materials are compared to analogues with atactic alcohol pendants and varying degrees of head-to-tail (HT) regioregularity. For atactic polymers with HT from 77 to 99%, the change in glass transition temperature (T g) is minimally affected (47 ± 2 °C), but the crystalline melting temperature (T m) increases substantially from 96 to 137 °C. When highly isotactic alcohol pendant groups are present (90% enantiopure monomer and HT = 96%), the T g increases slightly to ∼53 °C, but the T m increases dramatically to 190 °C. Although the wide-angle X-ray diffraction patterns of atactic and isotactic materials are similar and infer equivalent crystal unit cell packing, the isotactic sample develops ∼40% crystallinity, which is double that observed for an atactic sample (20%) with high HT = 99%. The thermal stability of all samples was >360 °C. Such investigations present unexplored insights on how isotactic and precision microstructure affect material properties of polymeric systems outside the two-carbon branch periodicity of isotactic polymers from vinyl monomers. Synthesis, structural characterizations, thermal properties in addition to intermolecular hydrogen bonding, and other crystalline structural data are discussed comparatively based on tacticity.

Photoresponsive polymers, typically activated with direct excitation of an azobenzene moiety, are an intriguing class of materials for application as adaptive structures. Here, we introduce triplet excited state sensitization as a mechanism to harness light in a stilbene-based photopolymer (i.e., the carbon analogue of azobenzene). While the undoped film shows no response, the sensitized film exhibits polarization dependent bending under 445 nm irradiation via (1) sensitizer excitation, (2) triplet energy transfer, (3) stilbene isomerization, and (4) subunit reorientation. This work is the first to demonstrate stilbene-based photopolymers and triplet sensitization as a low energy light harvesting mechanism in photomechanics.

We have determined the crystal structures of highly regioregular (>97% HT additions) isotactic-and atactic-poly(1pentamethylene alcohol)s (PPMA, −[(CH 2 ) 4 CH(OH)] n −) by a quantitative analysis of 2-dimensional wide-angle X-ray diffraction data. The unit cell houses two chains in a planar-zigzag conformation, with orthorhombic symmetry and parameters a = 8.637 Å, b = 4.987 Å and c (chain axis) = 12.840 Å for the it-PPMA and a = 8.76 Å, b = 5.06 Å, and c (chain axis) = 12.840 Å for the at-PPMA. The space group is P2 1 2 1 2 1 for both polymers. The zigzag planes are nearly parallel to the rolled plane of the doubly oriented sample. These regularly packed chains are aggregated in twinned domains having two opposite orientations of the unit cells. In the case of at-PPMA, the random stereochemical positioning of the alcohol pendants results in a more expanded unit cell than the corresponding cell of it-PPMA. Intermolecular hydrogen bonds form along the b axis for both the it-and at-species, the strength of which is slightly different depending on the OH configuration, as judged from infrared spectral information as well as the O•••O intermolecular distances.