Martinez, H.; Zhang, J.; Kobayashi, S.; Xu, Y.; Pitet, L. M.; Matta, M. E.; Hillmyer, M. A. “Functionalized Regio-regular Linear Polyethylenes from the ROMP of 3-Substituted Cyclooctenes” Appl. Petrochem Res., 2015, 5, 19–25.
It turns out that olefin metathesis is very useful for studying polyethylene (PE); the most common synthetic polymer produced at a staggering >80,000,000,000 kg/yr scale.
Metathesis polymerization of cyclooctene followed by hydrogenation yields a polymer structurally equivalent to linear PE.
The Hillmyer group recently demonstrated that substitution at the 3-position of cyclooctene gives a high degree (in many cases >99%) of head-to-tail (HT) connectivity when polymerized with Grubbs Catalyst® 2nd generation. Hydrogenation yields PE with precise substitution at each 8th carbon. Although PEs with precise substitution have previously been prepared via ADMET, and were studied extensively to better understand PE branching, Hillmyer’s ROMP approach is a great addition in that much higher molecular weights can be achieved.
Polymerizations of the 3-substituted cyclooctenes worked well for ethers, esters, and functional groups in protected form. Further chemical elaboration on the functional groups could lead to interesting and useful graft copolymers and other structures. Maleic anhydride-grafted polyolefins, for example, are important industrial materials. The paper does include examples of substituted cyclooctenes that did not polymerize well – amine, cyano, amide, urea, or silane functionality in the 3-position did not form polymer. 3-hydroxy- and 3-halocyclooctenes gave insoluble gels.
The authors suggest that 3-substituted cyclooctenes could be copolymerized with cyclooctene to yield copolymers of ethylene with polar vinyl monomers (PVMs) (acrylate esters, vinyl alcohol, etc.). These materials – especially those with low levels of PVMs – are of great interest in that they could retain the appreciated properties of polyethylene while improving properties imparted by PVMs (adhesion, impact strength, gas barrier, etc.) However, direct copolymerization of ethylene and most PVMs is complicated due to reactivity ratio differences and other factors.
Copolymerization of 5-substituted cyclooctenes with cyclooctene has been demonstrated 1 as a route to these copolymers, however, and it is unclear what benefit 3-substituted cyclooctenes would offer. 5-Substituted cyclooctenes and cyclooctene copolymerize at equal rates, giving homogeneous incorporation of the functional group. 3-Substituted cyclooctenes polymerize more slowly and therefore could lead to blocky microstructures. Since the copolymerization itself leads to a distribution in functional group placement, the precise spacing aspect of 3-substituted vs. 5-substiututed cyclooctene is of little relevance.
That being said, blocky microstructures and other architectures enabled by 3-substituted cyclooctenes could be interesting targets for certain commercial applications.
1Stephen E. Lehman, Kenneth B. Wagener, Lisa Saunders Baugh, Steven P. Rucker, Donald N. Schulz, Manika Varma-Nair, Enock Berluche, Macromolecules, 40, 2643-2656, 2007