Living Polymerization of Alkynes by Molybdenum Imido Alkylidene Complexes

Abstract

Organic polymers and oligomers that have conjugated π-systems possess a variety of interesting optical and electrical properties.1 For example, polyacetylene, one of the simplest of such polymers, is highly conductive when doped, particularly when oriented, and possesses a high bulk third-order hyperpolarizability (χ⁽³⁾).2,3 The discovery of polyacetylene initiated a search for more soluble polyenes that have most of the desirable properties of polyacetylene, but few of its undesirable properties (among them insolubility and air sensitivity). The most direct way to such molecules is to polymerize substituted alkynes.4 – 6 (Indirect methods include the ring opening polymerization of TCDTF6 (TCDTF6 = 7,8-bis(trifluoromethyl)tricyclo[4.2.2.0^2,5]deca-3,7,9-triene),8 or substituted cyclooctatetraenes.9,10) Disubstituted alkynes not only are relatively difficult to polymerize, but the resulting polyenes are often white and have few to none of the desirable electrical or optical properties of highly conjugated π systems. Monosubstituted alkynes can be polymerized to polyenes that are sometimes conjugated and soluble. Curiously, the inherent regiochemical problem associated with this polymerization has rarely been addressed in the literature; such polyenes are commonly assumed to be head-to-tail polymers and differences in the degree of conjugation (as measured by λ_max) are ascribed to different possible structures (cistransoid, trans-transoid, etc.), rather than regiochemical irregularity. IR and NMR spectroscopy have been the main tools for assigning polymer structure.