New Kids on the Block: Living Radical Polymers

New Kids on the Block: Living Radical Polymers

by John Emsley

Rank	Paper	Citations This Period Nov- Dec 00	Rank Last Period Sep- Oct 00
1	T.C. Terwilliger, J. Berendzen, "Automated MAD and MIR structure solution," Acta Crystallograph. D - Biol. Crystallograph., 55:849-61, April 1999. [Los Alamos Natl. Lab., NM] *188NW	21	2
2	E. Meggers, M.E. Michel-Beyerle, B. Giese, "Sequence dependent long range hole transport in DNA," J. Amer. Chem. Soc., 120(49):12950-5, 16 December 1998. [U. Basel, Switzerland; Tech. U. Munich, Garching, Germany] *149MW	12	3
3	L.A. Curtiss, et al., "Gaussian-3 (G3) theory for molecules containing first and second-row atoms," J. Chem. Phys., 109(18):7764-76, 8 November 1998. [Argonne Natl. Lab., IL; Lucent Technol., Murray Hill, NJ; Northwestern U., Evanston, IL] *132ZZ	12	†
4	R.E. Stratmann, G.E. Scuseria, M.J. Frisch, "An efficient implementation of time-dependent density-functional theory for the calculation of excitation energies of large molecules," J. Chem. Phys., 109(19):8218-24, 15 November 1998. [Rice U., Houston, TX; Lorentzian Inc., New Haven, CT] *139AJ	11	†
5	D.J. Tozer, N.C. Handy, "Improving virtual Kohn-Sham orbitals and eigenvalues: Application to excitation energies and static polarizabilities," J. Chem. Phys., 109(23):10180-9, 15 December 1998. [U. Cambridge, U.K.] *147BZ	10	†
6	M. Scholl. et al., "Synthesis and activity of a new generation of ruthenium-based olefin metathesis catalysts coordinated with 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligands," Organic Lett., 1(6):953-6, 23 September 1999. [Caltech, Pasadena] *281Q	9	†
7	A. Altomare, et al., *"SIR97: a new tool for crystal structure determination and refinement,"* J. Appl. Cryst., 32:115-9, 1 February 1999. [U. Bari, Italy; Piazza U., Perugia, Italy; CNR, Inst. Struct. Chem. G. Giacomello, Rome, Italy] *173NK	8	7
8	Y.K. Chong, et al., "A more versatile route to block copolymers and other polymers of complex architecture by living radical polymerization: The RAFT process," Macromolecules, 32(6):2071-4, 23 March 1999. [CSIRO Mol. Sci., Victoria, Australia] *180MH	8	†
9	M. Scholl, et al., "Increased ring closing metathesis activity of ruthenium-based olefin metathesis catalysts coordinated with imidazolin-2-ylidene ligands," Tetrahedron Lett., 40(12):2247-50, March 1999. [Caltech, Pasadena] *173YM	7	10
10	B.J. Littler, Y. Ciringh, J.S. Lindsey, *"Investigation of conditions giving minimal scrambling in the synthesis of trans-porphyrins from dipyrromethanes and aldehydes,"* J. Org. Chem., 64(8):2864-72, 16 April 1999. [North Carolina St. U., Raleigh] *189VM	7	†

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

iving radical polymers last came to the attention of Science Watch in May/June of 1998, since when the subject has grown rapidly, to such an extent that it now justifies its enthusiasts holding six international conferences a year. What excites chemists about living radical polymers is that they offer the opportunity to make polymer chains of the same length, even when these may be several hundred units long. Today, living radical polymers can be made using sophisticated catalysts, which regulate the chain growth, so that a narrow range of polymer lengths is produced. (Traditional polymers are a tangled mass of chains of all lengths often with side chains growing from them in a random way.)

However, traditional polymers still had one advantage, in that they could be produced in the form of so-called block copolymers, in which chains are composed of blocks of different polymers linked together. It seemed unlikely that the processes that controlled the chemistry of living-radical-polymer formation would be able to produce these types of polymers, but, in 1999, this remarkable feat was achieved by a group of polymer chemists at Australia’s CSIRO Molecular Science Unit in Victoria. Their work is now acknowledged in the appearance of paper #8 on the current Hot Ten.

Block copolymers presented a tough challenge, but the rewards of being able to make them could be great. Such polymers can combine the advantages of two very different types of polymer and they are widely manufactured, for example by introducing flexibility into a chain that would otherwise be rigid. Block copolymers of styrene are extensively manufactured, and the combination of styrene with butadiene is particularly used because of its elastic properties.

Now Graeme Moad, Ezio Rizzardo, San Thang, and their CSIRO colleagues have found a way of making living radical block copolymers that provides polymers of predetermined molecular weight and narrow polydispersity. The secret is to add certain dithio compounds, which act as highly efficient chain transfer agents, and which convert a mixture of two kinds of monomer into block copolymer material. A typical dithio compound that they use is benzyl dithiobenzoate.

The polymerizations reported in #8 were carried out in bulk, in solution, in emulsion, or suspension, using standard reaction conditions. Seemingly incompatible polymers can be combined this way, and the paper reports results for diblock polymers made from various combinations of acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, benzyl methacrylate, dimethylacrylamide, styrene, and ethylene oxide. The combinations were chosen to synthesize a wide range of polymers combining those that are "hard" with those that are "soft," and those which attract water with those which repel it. Commercially, acrylate polymers make particularly good resin coatings.

To achieve maximum purity of the polymers, as low a concentration of initiator as possible is used, although this determines the rate of the radically driven polymerization. Moad and Rizzardo have been able to achieve block copolymers with no detectable homopolymer impurity in the final product.

The CSIRO team also extended their investigations to three-block polymer synthesis, which they achieved either by adding to an existing diblock polymer, or by starting with a difunctional transfer agent with two dithio groups attached at opposite ends of a benzene ring so that the polymers grow simultaneously.

"The technique has widespread application to products for biotechnology, nanotechnology and microelectronics," Moad tells Science Watch, adding: "The primary advantage of the technology over competitive technologies is that it is relatively cheap, compatible with a wide range of monomers and can be carried out in conventional reactors."

Although no commercial application has yet emerged from their work, the group at CSIRO Molecular Science has already developed a "strategic alliance" with one leading chemical company, DuPont’s High Performance Coating division.

Dr. John Emsley is science writer in residence at the
Department of Chemistry, Cambridge University, U.K.