[Rh(cycloolefin)(acac)] complexes as catalysts of polymerization of aryl- and alkylacetylenes: influence of cycloolefin ligand and reaction conditions
Abstrakt
Complexes [Rh(nbd)(acac)], [Rh(cod)(acac)] and [Rh(cot)(acac)] (nbd = norborna-2,5-diene,cod = cycloocta-1,5-diene, cot = cyclooctatetraene) were investigated as catalysts of polymerizationof monosubstituted acetylenes into stereoregular cis-transoid poly(monosubstituted acetylene)s. Allcomplexes (without cocatalyst) are highly active in phenylacetylene (PhA) polymerization in bothcoordinating (THF) and non-coordinating (CH2Cl2) solvents. [Rh(cot)(acac)] prepared in situ by ligandsubstitution in [Rh(ethylene)2(acac)] with cot is efficient in PhA polymerization even in systems witha high cot content.
Selection of solvent and cycloolefin ligand of the catalyst allows the control overpoly(phenylacetylene) molecular weight (Mw= 3 × 104to 4 × 105). [Rh(nbd)(acac)] exhibit (moderate)activity also in polymerization of alkylacetylenes (alkyl = n-butyl, tert-butyl, 4-chlorobutyl, cyclopropyl,yields up to 28%, Mwup to 7.3 × 104) and in copolymerization of alkylacetylenes with arylacetylenes.In situ1H NMR monitoring of polymerization systems revealed monomer-assisted transformation of[Rh(cycloolefin)(acac)] complexes into precursors of polymerization centres consisting in proton transferfrom the monomer to acac ligand of the catalyst under release of acetylacetone. [Rh(cycloolefin)(acac)]catalysts thus differ from the widely used [Rh(nbd)Cl]2catalyst which is transformed into active centresprecursors only in reaction with cocatalyst or a molecule of coordinating solvent and which is efficientonly in presence of a proper cocatalyst and/or coordinating solvent.