Long-Range Kinetic Effects on the Alternating Ring Opening Metathesis of Bicyclo[4.2.0]oct-6-ene-7-carboxamides and Cyclohexene.

We report an investigation of rates of ruthenium-catalyzed alternating ring opening metathesis (AROM) of cyclohexene with two different Ru-cyclohexylidene carbenes derived from bicyclo[4.2.0]oct-6-ene-7-carboxamides (A monomer) that bear different side chains. These monomers are propylbicyclo[4.2.0]oct-6-ene-7-carboxamide and N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide. The amide substitution of these monomers directly affects both the rate of the bicyclo[4.2.0]oct-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 propylbicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening is 3-fold faster than that of the N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-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 bicyclo[4.2.0]oct-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 bicyclo[4.2.0]amide-cyclohexene provides prediction of copolymerization kinetics and gradient copolymer structures.

Gradient Copolymer Prepared from Alternating Ring-Opening Metathesis of Three Monomers.

Bicyclo[4.2.0]oct-6-ene-7-carboxamide is a simple but highly strained olefin monomer which forms an alternating copolymer with cyclohexene in the presence of N-heterocyclic carbene-ruthenium catalyst. [4.2.0] moiety with bulky substituent on C7 that chelate with the ruthenium center of the catalyst propagate more slowly than monomers that cannot chelate. Accordingly, the reactivity ratio of N-propylbicyclo[4.2.0]oct-6-ene-7-carboxamide with cyclohexene is significantly higher than that of N-(2-(2-ethoxyethoxy)ethan)-bicyclo[4.2.0]oct-6-ene-7-carboxamide with cyclohexene. A copolymerization involving the three monomers in a 1:1:2 (propyl:ethylene glycol:cyclohexene) molar ratio formed a gradient copolymer in a one-pot reaction. Surface hydrophobicity, topology, and thermal properties of the gradient copolymer were similar to those of a copolymer comprised of six microblocks prepared through multistep synthesis by alternately employing the same two bicyclo[4.2.0]oct-6-ene-7-carboxamides in each microblock. The properties of the gradient copolymer were distinct from a copolymer comprised of two larger blocks based on the same bicyclo[4.2.0]oct-6-ene-7-carboxamides.

Alternating Ring-Opening Metathesis Polymerization Provides Easy Access to Functional and Fully Degradable Polymers.

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 third-generation Grubbs ruthenium catalyst. Specifically, combining large-ring (7-10 atoms) cyclic acetal or lactone monomers with bicyclo[4.2.0]oct-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 (D M = 1.2-1.6). Copolymers containing ester and acetal backbones hydrolyzed to significant extent under basic condition (pH 13) and acidic conditions (pH ≤ 5) respectively to yield the expected by-products within 30 hours at moderate temperature. Unlike the copolymer with all-carbon backbone, copolymers with heteroatom-containing backbone exhibited 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.