Regiospecific Poly(ethylene-co-vinyl alcohol) by ROMP of 3-Acetoxycyclooctene and Postpolymerization Modification for Barrier Material Applications

Abstract

Ethylene vinyl alcohol (EVOH) is an oxygen barrier polymer used to prevent premature degradation of food, pharmaceuticals, and other products due to its semicrystallinity, strong intermolecular interactions, and consequently low free volume. EVOH is made using traditional free-radical copolymerization, which leads to little structural regularity. We utilized ringopening metathesis polymerization (ROMP) to determine how regioregularity impacts the barrier properties of EVOH-related materials. A regioregular (head-to-tail) polymer was synthesized from ROMP of 3-acetoxycyclooctene, followed by hydrogenation and deacylation to give a linear, highly regioregular EVOH (PH3OHCOE) containing the equivalent of 75 mol % ethylene units. The same process was carried out with 5-acetoxycyclooctene, but the resulting polymer (PH5OHCOE) is regiorandom. Both polymers were compared to an industry benchmark, EVOH-44, containing 44 mol % ethylene units. After processing, differential scanning calorimetry showed that the semicrystalline PH3OHCOE had a higher melting temperature and enthalpy of melting compared to semicrystalline PH5OHCOE, indicating that PH3OHCOE is more crystalline. This was confirmed by wide-angle X-ray scattering (WAXS). WAXS, rheological studies, and polarized optical microscopy showed that PH3OHCOE has a more well-defined crystal structure, a higher degree of hydrogen-bonding between −OH groups, and a higher glass transition temperature compared to PH5OHCOE. These differences were also highlighted in their tensile behavior, where PH3OHCOE and EVOH-44 exhibited brittle failure compared to the ductile behavior observed for PH5OHCOE. Oxygen barrier testing demonstrated that regioregular PH3OHCOE had an oxygen permeability more than a factor of 3 lower than regiorandom PH5OHCOE but still higher than EVOH-44, while water barrier testing showed that PH3OHCOE had the lowest water permeability, more than 6 times lower than EVOH-44. These results highlight the importance of regioregularity on the barrier properties of EVOH-like materials and show that structural regularity can lower oxygen permeability while maintaining low water permeability at the low vinyl alcohol content.

Crystal Structures of Isotactic and Atactic Poly(1-pentamethylene alcohol)

“…The melting point, heat of fusion, and oxygen barrier of the regiospecific polymer were higher than for the regiorandom analog. The regularity of monomer addition in the former also led to relatively sharp WAXD peaks while one broad pattern was observed for the regiorandom …”

“…For example, ROMP of cyclooctene derivatives permits investigation of the importance of precision placement of alcohol pendants by producing one carbinol per every eight backbone carbons in a precise (regiospecific) or purposefully imprecise (regiorandom) fashion. 9 Although both systems are presumably atactic, there were major differences in their properties. The melting point, heat of fusion, and oxygen barrier of the regiospecific polymer were higher than for the regiorandom analog.…”

“…Therefore a balance of oxygen and water barrier properties can be targeted from EVOH systems through judicious selection of comonomer composition coupled with X c . A recent work has demonstrated that such balance could be achieved by precision systems with ethylene backbones and a relatively high content (∼25%) of OH pendants …”

“…On this path, remarkable progress in ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis polymerization has made it possible to synthesize linear EVOH analogs with OH pendants placed at precise distances along a polyethylene backbone. For example, ROMP of cyclooctene derivatives permits investigation of the importance of precision placement of alcohol pendants by producing one carbinol per every eight backbone carbons in a precise (regiospecific) or purposefully imprecise (regiorandom) fashion . Although both systems are presumably atactic, there were major differences in their properties.…”

“…A recent work has demonstrated that such balance could be achieved by precision systems with ethylene backbones and a relatively high content (∼25%) of OH pendants. 9 Due to the strong correlation between barrier properties and X c , there has been interest in contrasting the crystalline properties of atactic versus more stereoregular PVA. 10,11 However, despite efforts to increase PVA stereoregularity, the isotacticity remained relatively low, with 78% being the highest reported from NMR calculations based on mm triads.…”