Accelerated REMP with Double Tethered Metallacyclobutanes

Courtney Sever^a, Alec Esper ^a, Ion Ghiviriga ^a, ChristiAnna Brantley ^a, Daniel W. Lester ^b, Christian Ehm ^c, Adam S. Veige ^a

aUniversity of Florida, Department of Chemistry, Center for Catalysis, P.O. Box 117200, Gainesville, FL, 32611.
bPolymer Characterization Research Technology Platform, University of Warwick, Coventry CV4 7AL, United Kingdom.
c Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Via Cintia, 80126 Napoli, Italy

Combining a commercially available molybdenum-alkylidene with two equivalents of ene-ol proligands produces a double tethered metallacyclobutane in quantitative yield, a new class of pre-catalyst. These pre-catalysts exhibit high activity in the polymerization of norbornene, yielding cyclic polynorbornene. Active species forms after the addition of norbornene to pre-catalyst, replacing one tethered arm, creating a new metallacyclobutane. The quantification of the active catalysts is achieved through the measurement of vinyl resonances in the 1H NMR, showing roughly 1% of active catalyst species. With rational design choices, significant progress has been made in improving the activity and selectivity. The adamantyl imido derivative (2) is 10% more cis selective than the 2,6-diisopropylphenyl imido derivative (1) in generating cyclic polynorbornene. Introduction of chirality in the tethered arms (3) improves the activity of the catalyst and allows for loadings as small as 1:20,000 (catalyst:monomer). DFT calculations suggest that all three complexes have slow initiation and fast propagation. The calculated half-lives of 1, 2, and 3 are 14.4 days, 7.1 hours, and 37.5 seconds at STP, respectively. However, 3 is the most active due to influences from lack of steric bulk and a more favorable trigonal bipyramidal geometry around the metal center. The insertion of norbornene monomer units into the active site of 3 accelerates the REMP cycle, as each subsequent monomer addition lowers the activation energy of the following enchainment. With this insight into the mechanism, a plethora of other ene-ol proligands can be designed and screened, thus allowing for synthesis of pre-catalysts with different activity and stability.