Tactiticy and stereochemistry in the ring-opening polymerization of 5,5-dimethylbicyclo[2.2.1]hept-2-ene initiated by metathesis catalysts

Abstract

Ring-opened polymers of optically active 5,5-dimethylbicyclo[2.2.1]-hept-2-ene (5,5-dimethylnorbornene DMNBE), having cis double bond contents of 0 – 100%, were prepared using various olefin metathesis catalysts based on Mo, W, Re, OS, Ru and Ir compounds. The ring dyad tacticities in these polymers, with respect to both cis and trans double bonds, were determined from 13C NMR spectra. Cis double bonds are always asociated with 50 – 100% r dyads (syndiotacticity (σ r ) c = 0.5 – 1.0), while trans double bonds are always associated with 50 – 100% m dyads (isotacticity (σ m ) t = 0.5 – 1.0). The polymers may be divided into four groups: I, those of high tacticity, with (σ r ) c ) ∼ (σ m ) t ∼ 1.0; II, those of intermedia tacticity, with (σ r ) c ∼ (σ m ) t ∼ 0.6 – 0.9; III, those with (σ r ) c > (σ m ) t ; IV, those of low tacticity, with (σ r ) c ∼ (σ m ) t ∼ 0.5. These is no correlation between tacticity and cis content, but all- cis polymers are generally highly syndiotactic and all- trans polymers slightly isotactic when prepared from 3 M monomer at 20 °C. The tacticity falls with increasing preparation temperature but not always with decreasing monomer concentration. The results are interpreted in terms of sequential formation of a metallacarbene-monomer complex, 4, a metallacyclobutane, 5, a metallacarbene with the newly-formed double bond first coordinated to, 6, and achiral ‘symmetrical’ form of 6; there are corresponding mirror-image and symmetrical forms of 3. The first type of rupture accounts for cases of high tacticity or tacticity which falls with monomer dilution; the second explains how the tacticity can be independent of monomer dilution. Propagation is assumed to occur either by addition of monomer to 3 to form a cis or trans double bond, or by displacement of coordinated cis double bond from 6 by monomer to form a new cis doubled bond. With these assumptions a complete mechanism is developed (Scheme 6) to account not only for the results presented here but also for some more general features of olefin metathesis, especially stereospecificity and selectivity in cross-methathesis.