[Rh(cycloolefin)(acac)] complexes as catalysts of polymerization of aryl- and alkylacetylenes: Influence of cycloolefin ligand and reaction conditions

• The mechanism of initiation of polymerization with [Rh(cycloolefin)(acac)] is proposed.
• The extent of initiation is influenced by solvent, cycloolefin ligand of catalyst.
• Solvent and cycloolefin ligand of catalyst allows the control over the polymers MW.
• Polymerization activity was achieved also in alkylacetylenes polymerization.
• New polymerization catalysts can be prepared by in situ ligand exchange of [Rh(ethylene)2(acac)].

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 polymerization of monosubstituted acetylenes into stereoregular cis-transoid poly(monosubstituted acetylene)s. All complexes (without cocatalyst) are highly active in phenylacetylene (PhA) polymerization in both coordinating (THF) and non-coordinating (CH2Cl2) solvents. [Rh(cot)(acac)] prepared in situ by ligand substitution in [Rh(ethylene)2(acac)] with cot is efficient in PhA polymerization even in systems with a high cot content. Selection of solvent and cycloolefin ligand of the catalyst allows the control over poly(phenylacetylene) molecular weight (Mw = 3 × 104 to 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%, Mw up to 7.3 × 104) and in copolymerization of alkylacetylenes with arylacetylenes. In situ 1H NMR monitoring of polymerization systems revealed monomer-assisted transformation of [Rh(cycloolefin)(acac)] complexes into precursors of polymerization centres consisting in proton transfer from 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]2 catalyst which is transformed into active centres precursors only in reaction with cocatalyst or a molecule of coordinating solvent and which is efficient only in presence of a proper cocatalyst and/or coordinating solvent.

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