Hybrid Brush Copolymers
As noted elsewhere on this web site one advantage of ATRP is the ease with which targeted substrates can be functionalized using commercially available, or easily synthesized functional α-haloesters or benzyl halides. Functional ATRP initiators have been successfully tethered to both organic and inorganic materials with either flat surfaces (2 dimensional) or curved (3 dimensional) surfaces, particles. As a result well-defined flat surface and spherical polymer brushes of varying composition and dimensions have been synthesized by the ATRP of organic vinyl monomers from various surfaces and colloidal particles.(1-5) Control over the degree of polymerization (DP) of each tethered segment, as well as of the functionality of the selected monomers enabled precise engineering of both surface properties and colloidal composite structures and the properties of the resulting hybrid nano-structures.
One of the problems associated with use of multifunctional initiators in an ATRP is the impact of termination reactions on the properties of the final material. Termination can occur either inter-molecularly resulting in crosslinking or intra-molecularly which affects functionality and distribution of tethered chains.(2) Accordingly reaction conditions should be selected to minimize termination reactions including:
- addition of the redox conjugate to minimize initial termination reactions that normally build up the persistent radical and to slow down propagation
- stopping the reactions at low conversion
- conducting the reactions in dilute solution or
- optionally conducting the reaction in a miniemulsion system
Tethered brush molecules have even been prepared by individual manipulation of molecules.(6) Scanning probe microscopy-based techniques were used to manipulate single molecules and deliver them in a precisely controlled manner to a specific target. The ultimate physical limit in the design and fabrication of organic surfaces can be reached using this approach. This article showed that the atomic force microscope (AFM), which has been used extensively to investigate the stretching of individual molecules, can deliver and immobilize single molecules, one at a time, on a surface. Reactive polymer molecules, attached at one end to an AFM tip, are brought into contact with a modified silicon substrate to which they become linked by a chemical reaction. When the AFM tip is pulled away from the surface, the resulting mechanical force causes the weakest bond - the one between the tip and polymer - to break. This process transfers the polymer molecule by molecule to the substrate where it can be tethered by further chemical reactions.
(1) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716-8724.
(2) Pyun, J.; Matyjaszewski, K.; Kowalewski, T.; Savin, D.; Patterson, G.; Kickelbick, G.; Huesing, N. J. Am. Chem. Soc. 2001, 123, 9445-9446.
(3) Pyun, J.; Jia, S.; Kowalewski, T.; Patterson, G. D.; Matyjaszewski, K. Macromolecules 2003, 36, 5094-5104.
(4) Liu, T.; Jia, S.; Kowalewski, T.; Matyjaszewski, K.; Casado-Portilla, R.; Belmont, J. Langmuir 2003, 19, 6342-6345.
(5) Pyun, J.; Jia, S.; Kowalewski, T.; Matyjaszewski, K. Macromolecular Chemistry and Physics 2004, 205, 411-417.
(6) Duwez, A.-S.; Cuenot, S.; Jerome, C.; Gabriel, S.; Jerome, R.; Rapino, S.; Zerbetto, F. Nature Nanotechnology 2006, 1, 122-125.