The Use of Well-Defined Molybdenum ROMP Initiators in the Synthesis of Fluorescent Poly(arylenevinylene) Homopolymers and Copolymers

Citation

Elder, Delwin Lerone (2000) The Use of Well-Defined Molybdenum ROMP Initiators in the Synthesis of Fluorescent Poly(arylenevinylene) Homopolymers and Copolymers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/sqjn-9q60. https://resolver.caltech.edu/CaltechTHESIS:08292024-221148036

Abstract

We have discovered that ring-opening metathesis polymerization (ROMP catalysts of the type Mo(CHMe₂Ph)(N-2,6-C₆H₃ⁱ/Pr₂)(OCMe(CF₃)₂)₂ (1) may be greatly activated by the addition of one or more equivalents of the cocatalyst HOCMe(CF₃)₂ (hexafluoro-t-butanol, HFB). In general, HFB increases the rate of initiation relative to propagation for the ROMP of low strain cyclic olefins. The ratio or propagation to initiation has been measured and found to decrease by up to three orders of magnitude upon the addition of HFB to polymerizations of cyclooctene, cyclooctadiene, and 3,4,6- trichloro-5-octyl-benzobarrelene (2) initiated by 1. With 2 we were able to measure k_p and k_i as well as k_p/k_i and found that HFB increases both k_p and k_i, but k_i increases to a greater degree. High strain cyclic olefins such as norbornene display no such enhancement of initiation. The mechanism of this "alcohol effect" is not yet well understood.

New conjugated poly(1,4-naphthalenevinylene)s (PNVs) disubstituted by electron-donating alkoxy substituents (3) have been synthesized by ROMP of new dialkoxy-benzobarrelene monomers (4) followed by aromatization with DDQ. Polymerizations were initiated with catalyst Mo(CHCMe₂Ph)(N-2,6-C₆H₃ ⁱPr₂)(OCMe₂(CF₃))₂ (5) whose performance was superior to initiator 1. This ROMP-Aromatization Route is a mild synthesis procedure and provides polymers of controlled molecular weight that are low in polydispersity and soluble in common organic solvents. The wavelengths of photoluminescence of 3 are from 534-546 nm with quantum efficiencies of (5-15%).

Block copolymers diester-PPV-block-dialkoxy-PNV (6) and dialkoxy-PNV-block- alkyl-PNV (7) and random copolymers diester-PPV-random-dialkoxy-PNV (8) have been synthesized by the ROMP-Aromatization Route utilizing initiator 2. The diester-PPV segments have a larger HOMO-LUMO gap than the dialkoxy-PNV segments allowing the polymer to be able to execute through-bond (non- Forster) energy transfer. Upon photoexcitation of the large band gap segments, the block copolymers 6 however do not display energy transfer, but the block copolymer 7 and the random copolymers 8 do display moderate to efficient energy transfer. Remarkably, in the case of 8c, photoluminescence is increased by a factor of 2 to 18%, and for 7, the efficiency is increased by a factor of 10 to 5%. This copolymer strategy should be a general technique to increase the quantum efficiency of polyarylenevinylenes.

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