Polymer Synthesis - The Polyhomologation Reaction
Carbon backbone polymers are prepared by the polymerization of olefins. The carbon backbone is built two carbon atoms at a time. In 1997 we reported a new sp3-sp3 carbon-carbon bond forming polymerization reaction that we termed polyhomologation. The reaction is fundamentally different from conventional olefin polymerizations in that the carbon backbone is built one carbon atom at a time. The first examples of the reaction involved the trialkyl and aryl organoborane catalyzed polymerization of ylides such as dimethylsulfoxonium methylide.
Polymerization of Ylides and Diazoalkanes
The product of the polymerization, a tris-polymethylene organoborane, is oxidized to afford linear telechelic α,ω-hydroxy polymethylene. The reaction offers a degree of control of molecular weight, linearity, polydispersity, functionality and topology that is difficult to achieve by other means. Star polymer organoboranes with molecular weights of over 1.5 million have been produced. The reaction has been used to synthesize novel polymer architectures including giant carbocyclic rings.
We have found that secondary ylides can be used as monomers in the polyhomologation reaction. This finding permits synthesis of substituted carbon backbone polymers. In one example the copolymerization of methylidine and ethylidine ylides resulted in a polymer with identical composition to one obtained by the random copolymerization of ethylene and propylene. This approach provides an opportunity to precisely control the molecular weight and composition of the two building blocks.
Co[polyethylidine-b- methylene]
Applying the polyhomologation reaction to cyclic and polycyclic organoboranes allows for the construction of unique oligomeric and polymeric architectures that are not readily accessible by standard olefin polymerization. The polyhomologation of 1-boraadamantane by dimethylsulfoxonium methylide generates giant tube-like structures. The oxidation of these macrocyclic organoboranes generates a three-armed star polymer incorporating a cis, cis-1, 3, 5-trisubstituted cyclohexane core. The very low polydispersity and molecular weight control of the polyhomologation reaction provides material for testing some of the most fundamental assumptions concerning solution behavior of branched polymers. The sp3-sp3 carbon-carbon bond-forming step is common to a number of synthetically important reactions in chemistry so we have initiated a detailed mechanistic investigation of this reaction.
Synthesis of Novel Polymethylene Architectures
