Bio-inspired polymers (peptoids)
Biological polypeptides are complex copolymers that derive their phenomenal properties from precisely controlled sequences and compositions of the constituent amino acid monomers, which in turn lead to precisely controlled chain shapes and self-assembled structures. While these materials can be synthetically produced, they are made in notoriously low yield and small quantities.
Polypeptoids are a class of non-natural biomimetic oligomers based on an N-substituted glycine backbone (Fig. 1) that combine many of the advantageous properties of bulk polymers with those of synthetically produced proteins. As opposed to polypeptides, polypeptoids have an achiral backbone devoid of hydrogen bond donors, which makes them readily processable while still being able to form secondary structures such as helices. Furthermore, they are protease resistant, chemically and thermally stable, and have a similar polarity and side chain spacing to that of proteins.
These polymers can easily be synthesized via a two step submonomer synthesis method that readily incorporates primary amines to obtain side chain functionalities. As a result, literally hundreds of side chain moieties are available for inclusion into the polymer chain. In addition, a rapid, automated synthesizer has recently been developed, making large, scalable batch sizes achievable.
Our group has created self-assembling polypeptoid systems which have hierarchical levels of control reminiscent of biological molecules. We are currently studying the effect of monomer sequence on the coil-globule transition of peptoid copolymers in solution. In addition, the material properties of polypeptoids are largely unexplored; thus, we are also studying the self-assembly of peptoid-polymer hybrids in the bulk. Polypeptoids are an ideal model system for exploring the effect of monomer sequence on polymer and self-assembly properties.
Peptoids for anti-fouling applications
Biofouling of ship hulls results in highly increased fuel consumption, which is why antifouling coatings are standardly applied on most ships. The first generation antifouling coatings were copper-based, and had a highly negative impact on the marine environment. Because of this, more environmentally friendly alternatives to these coatings have been widely investigated. Present antifouling strategies include the use of highly hydrophilic and even zwitterionic polymers. These polymers inhibit the adsorption of fouling organisms due to the presence of a water hydration layer. On the other hand highly hydrophobic coatings from which foulants can easily be removed by shear forces (so-called fouling release coatings) are also used. Furthermore, three dimensionally patterned surfaces, and even surfaces with mixed functionalities have been proven to discourage attachment. This diversity of strategies results in a large number of vastly different polymer chemistries which are difficult to directly compare, presenting a fundamental problem in gaining understanding and development of coatings.
The high yield and low cost of polypeptoids suggest that they will be ideal components to large scale materials applications including coatings. Many known antifouling functionalities can be incorporated using a single backbone chemistry, making a direct comparison of the effects of different functionalities possible. Moreover, due to the sequence specificity of polypeptoids, different functionalities can be inserted at precisely known locations along the polymer chain, opening up the opportunity to access many sequence specific variants with ease. In this way we can design a coating with optimum performance, while at the same time exploring the chemical and physical properties of these new materials.
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