Synthesis of sequence-defined oligomers and polymers

The preparation of synthetic polymers with exact control over chain length and monomer sequence is one of the long-standing challenges in polymer science. Nature perfectly masters the synthesis of high molecular weight, perfectly monodisperse polypeptides (proteins), which outperform synthetic polymers, both in terms of structural complexity as well as with respect to functional properties. The availability of synthetic strategies that would allow the preparation of polymers with a defined primary structure and chain length, and consequently, a controlled folding behavior in solution, may lead to polymeric materials with new or improved properties and functions. Over the past decades, several "living" or controlled polymerization techniques have been developed, which significantly enhanced the ability to control chain length, monomer sequence and composition: nowadays, synthetic polymers can be prepared with relatively narrow chain length distributions. However, in spite of all these advances, these methods still do not allow the preparation of polymers with perfectly uniform chain lengths and strictly defined monomer sequences, which would represent the synthetic counterpart of the proteins produced by Nature. An alternative approach towards precisely-defined polymers relies on multistep synthetic protocols and has already allowed the successful preparation of a wide variety of uniform oligomers. The synthesis of monodisperse oligomers with the propensity to fold (so called "foldamers") provides a useful stepping-stone for the synthesis and establishment of structure-property relationships with the corresponding higher molecular weight polymers. This Thesis describes the synthesis of oligomers and polymers with strictly defined monomer sequences, with the ultimate aim to direct and control their folding behavior, supramolecular organization and properties in solution. The design of the proposed oligomers/polymers is based on the principle of hydrophilic/hydrophobic patterning, which is a well-established and powerful strategy to guide secondary structure formation of both natural as well as non-natural oligomers and polymers. For example, alternating sequences of hydrophilic and hydrophobic residues are expected to lead to β-sheet type secondary structures. Three approaches have thus been investigated to obtain strictly alternating hydrophilic/hydrophobic oligomers and polymers. The different synthetic strategies that have been used for the preparation of these non-natural oligomers and polymers are reviewed in Chapter 1. The first approach explores the feasibility for the synthesis of uniform, sequence-defined, hydrophilic/hydrophobic patterned oligo(α-hydroxy acid)s (Chapter 2). This strategy is based on post-modification of a reactive oligoester scaffold composed of an alternating sequence of hydrophobic ((2S)-2-hydroxy-4-methylpentanoic acid) and masked hydrophilic ((2S)-2-hydroxypent-4-enoic acid) α-hydroxy acids. In a subsequent post-modification step, the allyl side chains could be quantitatively modified via free-radical addition of different ω-functional thiols to afford hydrophilic/hydrophobic patterned oligoesters. This synthetic route provides an interesting method to generate libraries of foldamers with identical chain length and monomer sequence but different side chain functionalities. The second approach relies on the preparation of the corresponding oligopeptide analogues (Chapter 3). Alternating hydrophilic/hydrophobic patterned peptide foldamers were obtained via post-modification of a reactive peptide precursor, synthesized via solid phase peptide synthesis (SPPS). The peptide scaffolds consisted of an alternating sequence of L-leucine and L-allylglycine residues. Thiol-ene coupling chemistry allowed the post-modification of the allylglycine residues with cysteamine hydrochloride, thioglycolic acid, 1-thioglycerol and 2,3,4,6-tetrα-O-acetyl-thio-β-d-glucopyranose. The dilute solution folding properties of the cysteamine and thioglycolic acid post-modified octapeptides were studied by circular dichroism (CD) spectroscopy. In agreement with the alternating hydrophilic/hydrophobic patterned primary structure, these peptides were found to adopt a β-sheet secondary structure in basic and acidic aqueous media, respectively. The preparation of high molecular weight polydepsipeptides, which are structurally related to oligoesters and oligopeptides, is discussed in Chapter 4. Polydepsipeptides were obtained via tin(II) 2-ethylhexanoate catalyzed ring-opening polymerization (ROP) of morpholine-2,5-dione derivatives. To allow flexibility in the choice of the side chain functional groups, morpholine-2,5-dione derivatives were constructed using L-allylglycine. Introduction of diverse functional groups in the side chains was achieved via radical addition of ω-functional thiols. This approach can be regarded as a convenient and rapid method for the synthesis of high molecular weight functional polymers, with strictly alternating monomer sequences.