n e w s of t h e w e e k breaking science that opens up a new field of research." Meijer believes the supramolecular polymers do have commercial potential. But he sees no reason to develop the lin ear variety because "we already have ex tremely useful covalently bonded linear polymers," he tells C&EN. "Why should we go to a hydrogen-bonded polymer," which is weaker? But for 3-D networks, Meijer says, the hydrogen-bonded polymers offer esearchers in the Netherlands have ed temperature, the hydrogen bonds dis advantages over covalent networks. found a way to make stable supra- sociate to yield the monomer, which ex When covalent 3-D networks are pro molecular polymers that can be hibits the typical characteristics of a small duced, the formation of some covalent disassembled and reassembled simply by molecule such as low viscosity and easy bonds blocks the formation of others, lead ing to defects such as unreacted function changing environmental conditions such processibility. "This reversible, chameleon-like behav al groups that are "pinned in place by as temperature or solvent. Unlike conventional polymers, which ior might facilitate polymer processing the topology of the network," Moore ex are built by covalent linking of mono and could be important for novel schemes plains. "You can never drive the reaction mers, the supramolecular polymers are in polymer recycling," comments Jeffrey to completion." created when molecules with "sticky S. Moore, professor of chemistry and of Meijer's polymers, on the other hand, ends" hydrogen-bond to each other to materials science and engineering at the form by reversible hydrogen bonding, so form linear chains or three-dimensional University of Illinois, Urbana-Champaign. that unattached groups can find a partner "It is an example of the practical use of as the network reorganizes itself several networks. This approach to self-assembling poly supramolecular chemistry to achieve times a second. The greater density of hy mers has been tried by other labs using something that is not possible with con drogen bonds and fewer defects that re single, double, or triple hydrogen bonds ventional—that is, covalent—polymers." sult lead to stiffer polymers. oriented in the same direction. The re Moore says Meijer's work is "groundWhen a conventional polymer is put sulting polymers generally under strain, covalent bonds have not held together very break irreversibly. But, Moore strongly. explains, when a supramo Through hydrogen bonding, afunctional lecular polymer is put un About two years ago, monomer forms linear polymer . . . der strain, hydrogen bonds however, chemistry profes temporarily break to re sor E. W. (Bert) Meijer of lease the strain, then re-form Eindhoven University of ο*Η>Αι elsewhere in an unstrained Technology and his cowork arrangement. "You might al ers accidentally discovered most think of it as a selfthat an array of four parallel repairing or self-healing net hydrogen bonds did hold work," he says. strongly. To explore the chemistry further, they setBecause of their proper tied on a relatively simple ties, the new polymers moiety—the 2-ureido-4-pyrcould be used for coatings imidone group—that forms and hot melts, where their the desired arrangement of reversible, strongly temper four hydrogen bonds in a ature-dependent changes in row. When molecules with form and flow would be R this "sticky" moiety at both highly advantageous. The ends are placed in solution, polymers have been patent . . . while molecule with three H-bond they link up via comple ed by DSM Research in arrays can form a 3-D network polymer mentary hydrogen bonding, Geleen, the Netherlands, and Meijer's group reports in a number of applications are Science [278, 1601 (1997)]. being explored. Under certain conditions, Meijer says his group's such as low temperature or ability to control the poly nonpolar solvents, this selfmer architecture opens the association is so strong that door to the study of a num (PEO/PPO) the supramolecular polymer ber of fundamental polymer exhibits conventional poly chemistry and polymer phys meric properties, such as ics properties. "It's only the Note: PEO/PPO is a block copolymer of poly(ethylene oxide) and polypropylene oxide) high viscosity. But under oth beginning," he adds. Bond into interior of cyclohexane ring indicates mixture of 3- and 4-substitution. er conditions, such as elevat Ron Dagani
SUPRAMOLECULAR POLYMERS
Structures held together by quadruple hydrogen bonding come apart reversibly
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DECEMBER 1, 1997 C&EN
