Polymer Synthesis
Ayusman SenDepartment of Chemistry
The Pennsylvania State University
University Park, Pennsylvania 16802, USA
E-mail: asen@psu.edu
Metal Catalysis:
We have an extensive research program in metal-catalyzed polymer synthesis. Several years ago, we discovered catalysts for the alternating copolymerization of carbon monoxide with alkenes under unusually mild conditions. These polymers are of great interest from at least four standpoints. First, as a monomer, carbon monoxide is particularly plentiful and inexpensive. Second, the presence of the carbonyl chromophore in the backbone makes them photodegradable. A third reason for the interest in alkene-carbon monoxide copolymers is that, because of the ease with which the carbonyl group can be chemically modified, the polyketones serve as excellent starting materials for other classes of functionalized polymers. Finally, specific interest in the alternating ethylene-carbon monoxide copolymer stems from its high mechanical strength and melting point which result from its high crystallinity. Indeed, Shell started the commercial production of the alternating ethylene-carbon monoxide copolymer using a catalyst system closely analogous to those developed by our group. Mechanistic studies on the copolymerization reaction revealed a chain growth sequence involving alternate insertions of carbon monoxide and alkene into a preformed metal-carbon bond. These investigations also led to the discovery of methods to control the chain-length and the end-groups of the copolymers.
Fig. 1. Alternating copolymerization of ethene and carbon monoxide.
More recently, we have achieved the synthesis of chiral, isotactic, alternating 1-alkene-carbon monoxide copolymers using appropriate chiral catalysts. The resultant polymers are very rare examples of polymers possessing main chain chirality that are derived from achiral monomers. Moreover, these polymers exhibit chiral recognition. Thus, an enantiomerically pure copolymer sample with a given chiral sense for the tertiary carbons in the main chain strongly prefers to form a stereocomplex with a second 1-alkene-carbon monoxide copolymer with opposite chiral sense for the tertiary carbons in the main chain.
One major goal of our ongoing research is the design of metal-catalyzed systems for the homo and copolymerization of functional polar vinyl monomers. Currently, both electron-rich (e. g., vinyl esters and ethers) and electron-deficient (e. g., acrylates, acrylonitrile, vinyl and vinylidene chlorides, and perfluoro alkenes) polar vinyl monomers are commercially produced by free-radical polymerization. As such, there is very little control over tacticity and molecular weight. While alternative procedures exist that allow for greater control over these features, these are not very convenient to carry out. Clearly, the discovery of general metal-catalyzed pathways for the homo and copolymerization of polar vinyl monomers would constitute a major breakthrough in polymer synthesis.
Another of one our goals is the design of a metal-catalyzed system for polypeptide synthesis through the alternating copolymerization of imines with carbon monoxide. Polypeptides are currently synthesized either directly or indirectly through the condensation polymerization of available chiral amino acids. As such, it is difficult to make polypeptides derived from D-amino acids or other unnatural amino acids that are not easily accessible. Additionally, the present methods do not allow the very large scale (commercial) production of polypeptides and, given the high interest in potentially biodegradable materials with diverse physical properties, the discovery of such a procedure would be of considerable importance. If the synthesis of chiral alternating 1-alkene-carbon monoxide copolymers are any guide, inexpensive achiral imines can be employed as monomers and the chirality in the polypeptide formed can be induced through the use of chiral ligands on the metal.
Fig. 2. Copolymerization of imines and carbon monoxide to form polypeptides.
The final area of metal-catalyzed polymerization that we have been involved in is the homo and copolymerization of functional norbornene derivatives. We have developed a number of highly active palladium and nickel-based catalysts for the vinyl addition polymerization of norbornene derivatives with a wide range of functionalities. Furthermore, we have examined the underlying reasons for the preferred uptake of exo versus endo-substituted norbornene monomers in the polymerization reactions and shown how this preference can be altered by appropriate choice of catalyst.
Radical Initiation:
We have been the first to describe the controlled radical copolymerization of polar vinyl monomers with simple alkenes, fluoroalkenes, and norbornene derivatives. This has led to the synthesis of unique random and block copolymers. Additionally, some of the copolymers of fluoroalkenes with polar vinyl monomers form strong adherent, yet hydrophobic, coatings on a variety of surfaces. We are also elucidating the detailed mechanism of the controlled copolymerization reactions - for example, how ethene and 1-alkenes, which form less stable primary and secondary radicals, can participate in such copolymerizations. More recently, we have discovered that simple recyclable Lewis acids can be employed to significantly increase the polymerization rate, as well as the level of incorporation of the non-functionalized alkene.