Polymers with hydrolyzable groups in their backbones have numerous potential applications in biomedicine, lithography, energy storage, and electronics. In this study, acetal and ester functionalities ...were incorporated into the backbones of copolymers by means of alternating ring-opening metathesis polymerization catalyzed by the third-generation Grubbs ruthenium catalyst. Specifically, combining large-ring (7–10 atoms) cyclic acetal or lactone monomers with bicyclo4.2.0oct-1(8)-ene-8-carboxamide monomers provided perfectly alternating copolymers with acetal or ester functionality in the backbones and low to moderate molecular weight distribution (D̵ M = 1.2–1.6). Copolymers containing ester and acetal backbones hydrolyzed to significant extent under basic conditions (pH 13) and acidic conditions (pH ≤ 5), respectively, to yield the expected byproducts within 30 h at moderate temperature. Unlike the copolymer with an all-carbon backbone, copolymers with a heteroatom-containing backbone exhibited the viscoelastic behavior with crossover frequency, which decreases as the size of the R group on the acetal increases. In contrast, the glass transition temperature (T g) decreases as the size of the R group decreases. The rate of hydrolysis of the acetal copolymers was also dependent on the R group. Thus, ruthenium-catalyzed alternating ring-opening metathesis copolymerization provides heterofunctional copolymers whose degradation rates, glass transition temperatures, and viscoelastic moduli can be controlled.
We report an investigation of rates of ruthenium-catalyzed alternating ring opening metathesis (AROM) of cyclohexene with two different Ru-cyclohexylidene carbenes derived from ...bicyclo4.2.0oct-6-ene-7-carboxamides (A monomer) that bear different side chains. These monomers are propylbicyclo4.2.0oct-6-ene-7-carboxamide and N-(2-(2-ethoxyethoxy)ethanylbicyclo4.2.0oct-6-ene-7-carboxamide. The amide substitution of these monomers directly affects both the rate of the bicyclo4.2.0oct-6-ene-7-carboxamide ring opening and the rate of reaction of the resulting carbene with cyclohexene (B monomer). The resulting Ru-cyclohexylidenes underwent reversible ring opening metathesis with cyclohexene. However, the thermodynamic equilibrium disfavored cyclohexene ring opening. Utilization of triphenylphosphine forms a more stable PPh3 ligated complex, which suppresses the reverse ring closing reaction and allowed direct measurements of the forward rate constants for formation of various A-B and A-B-A′ complexes through carbene-catalyzed ring-opening metathesis and thus gradient polymer structure-determining steps. The relative rate of the propylbicyclo4.2.0oct-6-ene-7-carboxamide ring opening is 3-fold faster than that of the N-(2-(2-ethoxyethoxy)ethanylbicyclo4.2.0oct-6-ene-7-carboxamide. In addition, the rate of cyclohexene ring-opening catalyzed by the propyl bicyclooctene is 1.4 times faster than when catalyzed by the ethoxyethoxy bicyclooctene. Also, the subsequent rates of bicyclo4.2.0oct-6-ene-7-carboxamide ring opening by propyl-based Ru-hexylidene are 1.6-fold faster than ethoxyethoxy-based Ru-hexylidene. Incorporation of the rate constants into reactivity ratios of bicyclo4.2.0amide-cyclohexene provides prediction of copolymerization kinetics and gradient copolymer structures.
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