Formation of Alternating trans-A-alt-B Copolymers through Ring-Opening Metathesis Polymerization Initiated by Molybdenum Imido Alkylidene Complexes

Abstract

Ring-opening metathesis polymerization (ROMP) is used to prepare trans-poly(A-alt-B) polymers from a 1:1 mixture of A and B where A is a cyclic olefin such as cyclooctene (A 1) or cycloheptene (A 2) and B is a large norbornadiene or norbornene derivative such as 2,3dicarbomethoxy-7-isopropylidenenorbornadiene (B 1) or dimethylspirobicyclo[2.2.1]hepta-2,5diene-2,3-dicarboxylate-7,1'-cyclopropane (B 2). The most successful initiators that were examined are of the type Mo(NR)(CHCMe 2 Ph)[OCMe(CF 3) 2 ] 2 (R = 2,6-Me 2 C 6 H 3 (1) or 2,6-i-Pr 2 C 6 H 3 (2)). The trans configuration of the AB linkages is proposed to result from the steric demand of B. Both anti-MB and syn-MB alkylidenes are observed during the copolymerization, where B was last inserted into a Mo=C bond, although anti-MB dominates as the reaction proceeds. Anti-MB is lower in energy than syn-MB, does not react readily with either A or B, and interconverts slowly with syn-MB through rotation about the Mo=C bond. Syn-MB does not readily react with B, but it does react slowly with A (rate constant ~1 M-1 s-1) to give anti-MA and one trans-AB linkage. Anti-MA then reacts with B (rate constant ~300 M-1 s-1 or larger) to give syn-MB and the second trans-AB linkage. The reaction has been modeled using experimental data in order to obtain the estimated rate constants above. The reaction between anti-MA and A is proposed to give rise to AA linkages, but AA dyads can amount to <5%. Several other possible A and B monomers, initiators, and conditions were explored.

Ring‐Closing Olefin Metathesis Catalyzed by Well‐Defined Vanadium Alkylidene Complexes

“…All prepared complexes are a mixture of syn ‐ and anti ‐isomers in solution. NMR studies show that syn ‐isomers do not strongly bind phosphines, presumably due to an agostic interaction between H‐alkylidene and V; this may result in the difference of initiation, selectivity, and catalytic activity of two isomers [32, 37] . The catalytic activity in RCM reactions strongly depends on the size and electron‐donating properties of the imido group, as well as the size and σ‐donating properties of the phosphine.…”

Oxanorbornenes: promising new single addition monomers for the metathesis polymerization

“…[15][16][17][18][19][20][21]66 Several approaches have been made to achieve alternating ring-opening metathesis polymerization (AROMP) either via catalyst modication or through monomer design. [22][23][24][25][26][27][28][29][30] In 2009, Sampson and co-workers reported an AROMP strategy where cyclobutene-1-carboxylic esters undergo ring-opening metathesis with a ruthenium catalyst giving an enoic carbene incapable of homopropagation. [31][32][33][34][35][36][37][38][39] However, alternating copolymerization of cyclobutene-1carboxylic esters with cycloalkenes lead to a sterically accessible double bond in the polymer backbone, resulting in secondary metathesis (chain transfer to the polymer).…”

The Nature of Secondary Interactions at Electrophilic Metal Sites of Molecular and Silica-Supported Organolutetium Complexes from Solid-State NMR Spectroscopy

“…These secondary interactions are often encountered in 3-center-2-electron (3c-2e) bond interactions between an empty orbital on the metal with the pair of electrons in a σ-C–H bond and are labeled agostic C−H interactions (Figure a). − The presence of α-, β-, and/or γ-C–H agostic interactions is often postulated in the transition states for insertion of olefins. − For example, the α-CH agostic interaction found in the transition state in metal-catalyzed polymerization of polypropylene is thought to direct the stereoselectivity in the polymer products. , The insertion of an olefin into an early metal–alkyl bond can generate a γ-CH agostic interaction, which are proposed intermediates in metal-catalyzed olefin polymerization reactions. , β-C–H Agostic interactions are intermediates in late transition-metal olefin polymerization catalysts that undergo chain-walking to form hyper-branched polyolefins . The agostic interaction is also an important component in stabilizing the syn -configuration of Schrock-type alkylidene complexes, essential for the stereoselective production of alkenes in metathesis reactions. − This brief outline illustrates that agostic interactions are conceptually important in many metal-catalyzed reactions. , …”