Chemists Proceed in Perking Up Polymer Production

Chemists Proceed in Perking Up Polymer Production

by John Emsley

WHAT'S HOT IN CHEMISTRY...

Rank	Paper	Citations This Period Mar- Apr 98	Rank Last Period Jan- Feb 98
1	A.P. Scott, L. Radom, "Harmonic vibrational frequencies: an evaluation of Hartree-Fock, Mller-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	19	1
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	18	2
3	S.J. Tans, et al., "Individual single-wall carbon nanotubes as quantum wires," Nature, 386(6624):474-7, 3 April 1997. [Delft U. Technol., Netherlands; Rice U., Houston, TX] *WR256	13	†
4	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. Lige, Belgium] *WB741	10	5
5	A.D. Horton, et al., "Cationic alkylzirconium complexes based on a tridentate diamide ligand: New alkene polymerization catalysts," Organometallics, 15(12):2672-4, 11 June 1996. [Shell Res. Tech. Ctr., Amsterdam, Netherlands] *UQ719	10	†
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 U., Pittsburgh, PA] *WE826	10	6
7	J.D. Scollard, D.H. McConville, "Living polymerization of a-olefins by chelating diamide complexes of titanium," J. Amer. Chem. Soc., 118(41):10008-9, 16 October 1996. [U. Western Ontario, London, Canada] *VM937	9	†
8	T. Grimaud, K. Matyjaszewski, "Controlled/"living" radical polymerization of methyl methacrylate by atom transfer radical polymerization," Macromolecules, 30(7):2216-8, 7 April 1997. [Carnegie Mellon U., Pittsburgh, PA] *WT213	9	†
9	M.W. Wong, "Vibrational frequency using density functional theory," Chem. Phys. Lett., 256(4,5):391-9, 5 July 1996. [U. Queensland, Brisbane, Australia] *UX017	8	†
10	R.V. Slone, et al., "Self-assembly of luminescent molecular squares featuring octahedral rhenium corners," Inorg. Chem., 35(14):4096-7, 3 July 1996. [Northwestern U., Evanston, IL] *UY218	8	†

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

Many of the materials which make up the modern world are polymers, and most are manufactured by methods developed in industry many years ago. University researchers have shown relatively little interest in investigating ways to improve these processes. That is beginning to change, and is reflected in the current list of Hot Papers, five of which are on the subject of polymerization catalysts. Four of these papers (#4, #6, #7 and #8) report on catalysts that produce "living" polymers, which were featured in the previous issue (see Science Watch, 9[3]:7, May/June 1998); these have, as yet, no industrial application. The fifth paper, however (#5), deals with polymerization catalysts that are transforming the manufacture of poly-olefins such as polyethylene and polypropylene.

Walter Kaminsky of the University of Hamburg, Germany, first demonstrated the ability of titanium and zirconium compounds to catalyze oleins in the mid-1970s. In these the metal is surrounded by ligand molecules which bind strongly to it, and the complexes so formed are not only very soluble but have the ability to attach an ethylene or propylene molecule to the central metal atom, which then leads to polymerization.

Only group 4 metals, such as titanium and zirconium, appeared to work, but a year ago in Science Watch (8[3]:8, May/June 1997) I focused on a paper by Maurice Brookhart of the University of North Carolina at Chapel Hill, which was being heavily cited because it showed that compounds of other metals, notably palladium and nickel, would also work. Recently Brookhart's results in this area have made the pages of the leading chemical magazines.

Although it is reporting on a zirconium catalyst, paper #5 in the current Hot Ten caught my eye because it offers the unusual feature of being a positively charged species, and the catalyst was highly selective in polymerizing ethylene but not propylene. The paper comes from the Shell Research and Technology Centre in Amsterdam, The Netherlands, and is the work of a group headed by Andrew Horton.

The zirconium at the center of the catalyst molecule is bound to a ligand with three nitrogens, two of which are bonded to silicon groups. Also attached to the metal are either two benzyl or two methyl groups. When the benzyl version is exposed to a boron compound, one of the benzyls is lured away, creating a complex with a positive charge. When the methyl complex is exposed to boron it either refuses to give up its methyls and clings on to the boron molecule instead, or it gives up a methyl but then pairs off with another catalyst molecule to share one of its methyls.

Paper #5 reports that the new zirconium compound can polymerize ethylene at ambient temperatures (taking less than five minutes to do this at 25 Celsius) but with propylene very little reaction occurs. Whether these benefits can be exploited remains to be seen, but in any case they will have to compete with new olefin catalysts announced in March of this year. Within a few days of each other, two groups of researchers reported the ability of iron (or cobalt) molecules to catalyze the polymerization of ethylene. This cheap new catalyst is activated by a co-catalyst, methylalumoxane, which places a methyl group on the iron or cobalt and thereby primes it to act.

Vernon C. Gibson's group at Imperial College, London, working in cooperation with oil giant BP, reported the new catalysts in Journal of the Chemical Society-Chemical Communications, (see G.J.P Britovsek, et al., 7:849-50, 7 April 1998) describing how they are made from iron or cobalt chloride and aminopyridines. Meanwhile, Maurice Brookhart revealed similar compounds and his work is to be published in Journal of the American Chemical Society, coauthored with graduate student Brooke L. Small and Alison M.A. Bennett of DuPont. According to Brookhart, the selectivity for a-olefins is better than 99% and they exceed the activities of other catalysts, producing high-density polyethylene in high yield and at a high rate.

As governments withdraw from funding blue-skies chemistry research, industry is moving in to fill the gap. The poly-olefin work proves that such collaboration can yield both high quality fundamental chemistry, and commercial benefit.

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