| The desire to tune macroscopic properties by controlling the underlying microscopic structure is a driving force in many different areas of scientific research, including polymer science. In living ring-opening metathesis polymerization (ROMP), the subject of this dissertation, there are a variety of different ways to alter the microscopic structure through synthesis. This is in part due to the presence of double bonds in the polymeric backbone, which can influence properties both through their isomeric structures (cis vs. trans) and through their removal by catalytic hydrogenation. Here, we demonstrate the ability to tune a variety of microstructural parameters of our ROMP polymers through synthesis, and investigate the resulting effects on macroscopic properties.;ROMP and subsequent hydrogenation provide access to crystalline, glassy, and rubbery polymers, representing essentially the entire spectrum of polymer properties. These include hydrogenated polynorbornene (hPN), a highly crystalline polymer with Tm° = 156°C; hydrogenated poly(5-hexylnorbornene) (hPHN), a rubbery amorphous polymer with Tg = -22°C; and hydrogenated polymethyltetracyclododecene (hPMTD), a glassy polymer with Tg = 163°C. The microstructure of block copolymers of hPN, hPHN, and hPMTD can be controlled by varying block sequence, block lengths, and number of blocks. We used this control to design and synthesize thermoplastic elastomers (TPEs) containing both crystalline and glassy hard segments, with the aim of capturing the mechanical properties of conventional all-amorphous triblock TPEs, while forming the solid-state structure by crystallization from a single-phase melt. To accomplish this, we synthesized symmetric pentablock copolymers with the architecture crystalline-glassy-rubbery-glassy-crystalline. With this pentablock architecture and appropriate selection of block lengths, crystallization from a single-phase melt causes a layer rich in the glassy block to form around the crystallites, limiting their lateral growth and generating composite hard domains with both crystalline and glassy components. The pentablocks show the low initial modulus, strain-hardening behavior, and small permanent set desired for TPEs, while retaining an easily-processed single-phase melt.;We found that the chain microstructure (cis/trans ratio) in the ROMP of norbornene and methyltetracyclododecene (MTD) using a so-called Schrock-type initiator to be a strong function of monomer concentration, providing a convenient means for tuning the average trans content in the resulting polymers. The results are explained based on a literature kinetic description for the behavior of this initiator, which allowed for the development of a quantitative model to describe the observed experimental data. In contrast, ROMP of MTD using the first-generation Grubbs initiator showed no dependence of the trans content on monomer concentration.;hPN is unusual in that it is highly crystalline despite having an atactic placement of its backbone cyclopentylene rings. Furthermore, it has a crystal-crystal transition at elevated temperatures (∼110°C) where the crystal structure transforms from a conventional, three-dimensionally-ordered monoclinic lattice at lower temperatures to a high-temperature crystal polymorph that is rotationally disordered and shows a pseudo-hexagonal packing transverse to the chain axes. The tacticity of hPN, measured to first order as the ratio of meso to racemo dyads (m:r), was varied by altering the synthesis conditions, and was shown to have a direct correlation with the temperature at which the crystal-crystal transition occurs (T cc). Small changes in m:r (from 0.8 to 1.1) are sufficient to raise Tcc by nearly 20°C. When heated above Tcc, hPN crystals thicken at a rate much greater than conventional three-dimensionally-ordered crystals, but below the rates shown by the two-dimensional hexagonal (columnar) phase formed by some polymers.;Finally, we report the synthesis of novel ROMP poly(5-phenylethylnorbornene), a polymer with pendant phenyl groups. Saturation of the backbone double bonds by catalytic hydrogenation to impart thermal stability can also result in two different hydrogenated derivatives depending on the type of catalyst used for hydrogenation: a derivative with the phenyl groups intact and a derivative with the phenyl groups converted to cyclohexyl groups. In comparing the two different hydrogenated ROMP derivatives, we found that Tg decreased by only 2°C (from 28°C to 26°C) upon replacement of the phenyl groups. This is in contrast to what we observed upon hydrogenation of the phenyl groups in the addition polymer of 5-phenylethylnorbornene, where the Tg increased by 34°C from 215°C to 249°C upon hydrogenation, a remarkable difference in magnitude and direction compared with the ROMP polymers. |
