Controlled Radical Polymerizations – Chemical Modification of Surfaces
Well-defined polymers with polydispersities below the theoretical limit for a conventional radical polymerization (1.5) can nowadays be prepared by living free radical polymerization. Nitroxide-mediated polymerizations (NMP), atom transfer radical polymerizations (ATRP) and reversible addition–fragmentation chain transfer (RAFT) polymerizations belong to this category. The NMP and ATRP processes are controlled by the persistent radical effect (PRE). The control of the polymerization in NMP is based on the reversible formation of a dormant alkoxyamine from the corresponding nitroxide and the chain growing polymer radical. The concentration of free radicals remains low during the entire polymerization, thus ensuring a very low fraction of irreversible termination via polymer radical dimerization/disproportionation.
During the past few years the Studer group has introduced several new alkoxyamines for the controlled NMP of styrene and of n-butyl acrylate. For example, alkoxyamines 3, 4 and 5 which are readily prepared are highly efficient initiators/regulators for the controlled polymerization of acrylates. In fact, alkoxyamine 5 is the best regulator known to date.
With these new alkoxyamine regulators new interesting polymers have been synthesized. For example, block copolymers and graft-copolymers or other complex polymer architectures with narrow polydispersities could be prepared using sterically highly hindered alkoxyamines.
Meanwhile, the Münster group is also working on the controlled nitroxide-mediated polymerization of N-isopropylacrylamide. Poly-N-isopropylacrylamide (PNIPAM) belongs to the class of smart thermoresponsive polymers. Peptide-PNIPAM conjugates have been prepared and the activity of these modified peptides towards enzymatic degradation could be regulated upon temperature change. At temperatures below 25 °C the PNIPAM-peptide conjugate is in its active state and amide hydrolysis using the enzyme is occurring. However, at 35 °C phase transition of the peptide conjugate is achieved and the peptide is stable towards hydrolysis. In collaboration with Prof. Watanabe at Nagoya University protein-PNIPAM conjugates are currently studied. It is believed that the activity of enzyme-PNIPAM conjugates can be regulated upon temperature change. Moreover, the self-assembly of various polymer-protein conjugates is currently studied in Münster. In a recently initiated collaboration with Prof. Luisa De Cola the synthesis and the aggregation behaviour of zeolite-polymer conjugates are investigated.
The controlled nitroxide mediated polymerization has meanwhile also been used for the modification of various surfaces (films, Si-wafers etc.). Surface polymerization affords so-called polymer brushes. Upon careful tuning of the monomer units, polymer coated surfaces with specific interactions with various molecules can be obtained.
Moreover, the radical carboaminoxylation has been shown to be highly useful for C-C-bond formations at olefin-terminated Si-wafers. PRE-mediated carboaminoxylations occurred with good to excellent yields (30-80% as determined by XPS) under rather mild conditions. Various functional groups could be introduced via this approach. Biologically interesting molecules such as unprotected sugar derivatives and peptides could be covalently bound to surfaces.
Recently, in collaboration with Prof. Fuchs and Prof. Chi (Physics department, Münster) Langmuir-Blodgett lithography has been used to covalently bind alkoxyamine regulators at defined positions of surfaces (control of the X- and Y-dimension). Via controlled radical polymerization, the Z-dimension could be controlled. This method has allowed the preparation of 3 dimensional polymer blocks on surfaces. Below an AFM image is presented showing structured polystyrene brushes. The width of the polystyrene is about 2 μm and the height is about 10 nm for this example. With the method developed, brushes with a width as small as 110 nm could be prepared.
Papers on this and related topics: No. 24, 34, 38, 40, 41, 44, 48, 53, 59, 60, 63, 67, 69, 72 and 77.