Living Polymers: Dead Intresting to Chemists

Living Polymers: Dead Interesting to Chemists

by Dr. John Emsley

WHAT'S HOT IN CHEMISTRY...

Rank	Paper	Citations This Period Jan- Feb 98	Rank Last Period Nov- Dec 97
1	A.P. Scott, L. Radom, "Harmonic vibrational frequencies: an evaluation of Hartree-Fock, Mřller-Plesset, quadratic configuration interaction, density functional theory, and semiempirical scale factors," J. Phys. Chem., 100(41):16502-13, 10 October 1996. [Australian. Natl. U., Canberra] *VL579	28	10
2	A. Thess, et al., "Crystalline ropes of metallic carbon nanotubes," Science, 273(5274):483-7, 26 July 1996. [Rice U., Houston, TX; U. Pennsylvania, Philadelphia; Inst. Charles Sadron, Strasbourg, France; Michigan St. U., E. Lansing] *UY983	27	8
3	L.A. Curtiss, et al., "Assessment of Gaussian-2 and density functional theories for the computation of enthalpies of formation," J. Chem. Phys., 106(3):1063-79, 15 January 1997. [Argonne Natl. Lab., IL; Bell Labs, Lucent Technologies, Murray Hill, NJ; Northwestern U., Evanston, IL] *WD070	25	†
4	F. Leličvre, et al., "Capillary electrochromatography: operating characteristics and enantiomeric separations," J. Chromatography A, 723(1):145-56, 2 February 1996. [Stanford U., CA; Ecole Nationale Super. Chimie, Paris, France] *TW152	23	†
5	C. Granel, et al., "Controlled radical polymerization of methacrylic monomers in the presence of a Bis(ortho-chelated) arylnickel (II) complex and different activated aklyl halides," Macromolecules, 29(27):8576-82, 30 December 1996. [U. Ličge, Belgium] *WB741	22	†
6	K. Matyjaszewski, T.E. Patten, J. Xia, "Controlled/"living" radical polymerization. Kinetics of the homogeneous atom transfer radical polymerization of styrene," J. Amer. Chem. Soc., 119(4):674-80, 29 January 1997. [Carnegie Mellon University, Pittsburgh, PA] *WE826	21	†
7	P. Schwab, R.H. Grubbs, J.W. Ziller, "Synthesis and applications of RuCl₂(=CHR')(PR₃)₂: the influence of the alkylidene moiety on metathesis activity," J. Amer. Chem. Soc., 118(1):100-10, 10 January 1996. [Caltech, Pasadena; U. Calif., Irvine] *TQ161	21	†
8	A. Becke, "Density-functional thermochemistry. IV. A new dynamical correlation functional and implications for exact-exchange mixing," J. Chem. Phys., 104(3):1040-6, 15 January 1996. [Queen's U., Kingston, Ontario, Canada] *TQ801	20	†
9	D.A. Evans, J.A. Murry, M.C. Kozlowski, "C₂-symmetric copper(II) complexes as chiral Lewis acids. Catalytic enantioselective aldol additions of silylketene acetals to (benzyloxy)acetaldehyde," J. Amer. Chem. Soc., 118(24):5814-5, 19 June 1996. [Harvard U., Cambridge, MA] *UR839	19	†
10	M.S. Driver, J.F. Hartwig, "A second-generation catalyst for aryl halide amination: Mixed secondary amines from aryl halides and primary amines catalyzed by (DPPF)PdCl₂," J. Amer. Chem. Soc., 118(30):7217-8, 31 July 1996. [Yale U., New Haven, CT] *VA026	19	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

The polymers we encounter in our everyday life, such as plastic bags, polystyrene insulation, and PVC tubing, are easily made, but the long-chain molecules of which they consist are a chaotic mess of tangled threads of many lengths. Some are short, only a few hundred units long, while others are made up of chains consisting of thousands, sometimes millions, of units. From this confusion many wonderful properties arise, and there are polymers that surpass anything that occurs naturally in terms of strength, durability, clarity, and versatility.

Yet nature has achieved something rather remarkable, too, in the kinds of polymers that living things produce. The biological polymers that are needed in organisms are tailored to be just the right length for the job. No wasted material here no weak points due to random construction. Imagine what sophistication might be achieved if chemists could tailor synthetic polymers so that they were all the same length, and the optimum length for a particular application.

So-called "living" polymers are bringing this dream a step nearer, and in the current Hot Ten are two papers on this subject: #5 and #6. The first of these came from Phillipe Teyssié's group at the University of Liége, Belgium, and the second from Krzysztof Matyjaszewski and colleagues at Carnegie Mellon University, Pittsburgh, Pennsylvania. The previous Hot Ten (see Science Watch 9[2]:7, March/April 1998) also contained a paper at #5 from this latter team.

Paper #6 reports on polymerizations using a copper-based catalyst and a solublizing molecule derived from 2.2'-bipyridine (BP) that can attach itself to the copper to form the active unit. This brings about a process which Matyjaszewski calls atom transfer radical polymerization (ATRP). The system studied was styrene and the polymerization was done on the bulk material, or with this dissolved in diphenyl ether as a solvent. In both cases the product was polystyrene with a narrow range of molecular weights, indicating that all the chains were the same length. These workers investigated the mechanism by which this "living" polymer formed, noting for example that a 1:2 ratio of copper to BP gave the best results.

The role of the copper catalyst is to transfer atoms, but the essential feature of living polymerization is the maintaining of a steady concentration of the active species and a fast equilibrium between the growing polymers. The growing end of the polymer chain is continually activated and deactivated, and it is this fluid situation which causes the chains to adjust to being roughly the same length. But why should this happen? To uncover the mechanism of polymerization, Matyjaszewski looked closely at the rate at which the polymers grow and at the role of the copper catalyst. The result is an admirable piece of research that is now being highly cited.

Similarly, in their earlier paper (March/April issue, #5), the Carnegie Mellon chemists looked at the polymerization of various methacrylates, which also produce living polymers. In that paper they also studied the role of the copper catalyst, but focused mainly on the nature of the end of the growing chain and on proving that free radical intermediates were involved. More recent work from this source has been published in Macromolecules (30:7348 and 7697, 1997). The first of these papers reports on more efficient ligands for the catalyst, while the second observes that the amount of catalyst can be significantly reduced by adding a small amount of a zerovalent metal, such as iron. Recently Matyjaszewski has revealed that branched as well as linear polymers can be made by ATRP, and that these can take on some unusual forms in the shape of stars, combs, and hyperbranched chains (ACS Symposium Series, 685:396, 1998).

The Liége-based group behind paper #5, meanwhile, also studied methacrylate polymerization, this time using nickel catalysts in which the nickel atom was held in the center of a planar array of ligand molecules. They followed the growth of the polymers at temperatures of around 100^şC, noting that this occurred in a living fashion. Their use of a nickel catalyst, which is soluble in water, also permitted polymerization to be undertaken in this advantageous solvent.

Dr. John Emsley is Science Writer in Residence at the
Department of Chemistry, University of Cambridge, U.K.