Synthesis of Functionalized Polymers by Ring-Opening Metathesis Polymerization (ROMP)

In Chapter 1, the ROMP of 5-methacrylate-1-cyclooctene and the copolymerization of this monomer with cyclooctadiene using the initiator (PCy3)2Cl2Ru=CHCH=CPh2 were investigated to produce polymers with cross-linkable side-chains. The impact of concentration, monomer to initiator ratio, and the amount of inhibitor in the polymerization was examined. These polymers were cross-linked through the methacrylate side-chains with either thermal or photochemical initiation, and the incorporation of these polymers into poly(methyl methacrylate) (PMMA) to produce AB cross-linked materials was accomplished. A comparison of the physical properties of PMMA and these new materials demonstrated that these materials had higher thermal stability and solvent resistance than pure PMMA. As an extension of the work presented in Chapter 1, Chapter 2 illustrated an alternative approach for the preparation of cross-linkable polymers by ROMP. The synthesis of ring-opening metathesis polymerization chain transfer agents bearing methacrylate and epoxide end-functionality was accomplished. In the presence of these chain transfer agents, cyclooctadiene was polymerized via a ruthenium benzylidene initiator, (PCy3)Cl2Ru=CHPh, to produce telechelic poly(butadiene)s with either methacrylate or epoxide end groups. The impact of initiator concentration, reaction time, and temperature on the polymer yield and chain transfer agent incorporation was examined. Control over the polymer molecular weight through the cyclooctadiene/chain transfer agent ratio was demonstrated providing for a range of telechelic poly(butadiene) molecular weights. Successful cross-linking of these polymers by thermal or photochemical initiation in the case of the bis(methacrylate)-functionalized telechelic poly(butadiene)s or through acid catalysis in the case of the bis(epoxide)-functionalized telechelic poly(butadiene)s was accomplished. In an effort to further explore the functional group tolerance of the ruthenium-based metathesis initiators developed in our group, the investigation presented in Chapter 3 encompassed the synthesis and living ring-opening metathesis polymerization (ROMP) of substituted cyclobutenes with the functional group tolerant polymerization initiators (PCy3)2Cl2Ru=CHCH=CPh2 and (PCy3)2Cl2Ru=CHPh. Synthetic methodology was developed for the synthesis of a wide variety of 3-functionalized cyclobutenes containing ether, ester, alcohol, amine, amide, and carboxylic acid substituents. Coordination of these functional groups to the propagating carbene was observed resulting in the formation of a chelated propagating species with concomitant loss of one phosphine ligand from the metal center. Studies aimed at understanding this chelation and its effect on the polymerization were undertaken. Based on these results, the synthesis of a series of functionalized cyclobutenes was accomplished which minimized this chelation and allowed for living polymerizations. A new class of functionalized poly(butadiene) homopolymers and diblock copolymers was synthesized and the thermal properties analyzed by thermogravimetric analysis and differential scanning calorimetry. In Chapter 4, the effect of backbone flexibility on the mesomorphic behavior of side-chain liquid crystalline polymers synthesized by ring-opening metathesis polymerization was investigated. The synthesis of norbornene and cyclobutene monomers containing a p-nitrostilbene moiety as the mesogenic group and polymerization of these monomers with the metathesis initiator (PCy3)2Cl2Ru=CHPh to produce side-chain liquid crystalline polymers with low polydispersities and defined molecular weights was accomplished. The relatively rigid poly(norbornene)s displayed enantiotropic nematic mesomorphism with glass transitions from 44-64°C and isotropization temperatures between 108-121°C, whereas the more flexible poly(butadiene)s showed enantiotropic smectic A mesomorphism with glass transition temperatures from 14-31°C and isotropization temperatures between 74-111°C. A diblock copolymer containing a 1:1 mixture of the poly(norbornene) and poly(butadiene) backbones also exhibited a smectic A mesophase. The dependence of the degree of polymerization and flexible spacer length on the phase transitions of these systems was determined demonstrating stabilization of the mesophase by both increasing molecular weight and flexible spacer length. A short chapter on the development of methodology for an improved synthesis of 3-methyl-3-phenylcyclopropene was included in Appendix 1. This research was investigated in hopes of developing a more facile and inexpensive procedure for the preparation of this compound than has been previously reported. Phase transfer catalyzed dichlorocarbene addition to α-methylstyrene followed by a selective catalytic Bu3SnH reduction resulted in the 1-chloro-2-methyl-2-phenylcyclopropane intermediate in excellent yield. Base-induced elimination of this compound resulted in the desired 3-methyl-3-phenylcyclopropene. This approach allowed for the preparation of this cyclopropene on large scale utilizing inexpensive reagents.