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Single Chain Polymer Nanoparticles

Final Report Summary - SCPNANOPART (Single chain polymer nanoparticles)

In this project we are exploring the possibility of combining the attractive features of self-assembly with classic polymer chemistry. From this, an exciting new field of chemistry to study emerges namely the field of single chain polymer nanoparticles (SCPN). The project deals with the synthesis of polymer chains that are able to collapse into nanoparticles from a supramolecular approach. To date, collapse of random coil polymer is done through covalent cross-linking of the chain. In our approach, we are first synthesising copolymers with suitable functionalisation. Two different approaches are being followed:

(1) copolymerisation of monomers to obtain polymers with the desired functionality;
(2) post-funtionalisation strategy, in which a given functionality can be attached to the polymer.

The different supramolecular units attached (via the two methodologies described) promote, when triggered, the folding and collapse of a random coil into a well-defined nanoparticle. The folding process is being studied with a large variety of techniques such as gel permeation chromatography (GPC), several nuclear magnetic resonance (NMR) techniques, dynamic light scattering (DLS), ultraviolet (UV) and circular dichroism (CD) spectroscopies and / or atomic force microscopy (AFM) imaging. Moreover, we are aiming to control the folding process, as nature does with biomacromolecules, in order to create well-defined architectures.

At the same time, we are exploring the potential applications of these new compartmentalised systems. Thus, catalysis is one of the most challenging topics, due to folding promotes the existence of different environments to perform reactions. Inspired by enzymes, which are the ultimate (enantio)selective catalysts, we have prepared polymers with (protected) recognition moieties that can fold on demand when triggered with an external stimulus. By incorporating a catalytically active species in water soluble polymers, and by compartmentalising the system by secondary interactions, such as hydrogen bonding, it is possible to create a well-defined three-dimensional structure with a hydrophobic interior in which catalysis can occur. We have developed different strategies to incorporate catalytically active sites in our system and thus perform catalysis in aqueous media, namely:

(1) copolymerisation of catalytically active units;
(2) post-funtionalisation of reactive polymers;
(3) incorporation of catalytically active site through supramolecular interactions.

These three different strategies have been successfully employed in the design and synthesis of catalytically active SCPN. Different catalytic systems have been investigated in parallel. We have paid special attention to two different systems:

(1) L-Proline functionalised polymers for aldol reaction;
(2) phenyl boronic acid functionalised polymers for catalysis in water and organic solvents.

To the date, L-Proline functionalised polymers bearing benzene-1,3,5-ticarboxamide (BTA) as recognition units has been synthesised via copolymerisation strategy (strategy 1). The folding of these catalytically active polymers has been thoroughly studied using techniques such as CD, UV, DLS, GPC, NMR amongst others. Besides, the catalytic activity has been tested against the aldol reaction in aqueous environment. These new compartmentalised systems exhibit a remarkable activity in water, resembling to that found in natural enzymes: in both cases activity is only expressed in the folded state. However, due to the randomicity of these polymers, full control over L-Pro positioning is not achieved. The diastero and enantioselectivity observed in the aldol reaction is moderate as a result of the different environments surrounding the catalytic sites. In order to gain control over catalyst positioning we attached a proline unit to a BTA scaffold (L-Pro-BTA). The BTAs in the polymer chain can recognise and incorporate the L-Pro-BTA in the stack (strategy 3). In this way we gained control over the selectivity of the aldol reaction, with ee ranging 97 - 98 % and de 90 - 94 %.

On the other hand, boronic acid derivatives are known to catalyse a wide number of reactions. Polymers bearing phenyl boronic acid derivatives as catalytic unit and BTA as supramolecular structuring unit were synthesised using either strategies 1 or 2. The folding of these polymers was studied in organic solvents and in water. In one of the cases, very high activities towards direct amidation reaction, i.e. the reaction between a carboxylic acid and an amine, were found in organic solvent (tetrachloroethane). A water soluble version of this polymer displayed no activity towards the same reaction in water. However, this polymer showed to be active towards Diels-Alder reactions in water using acrylic acid as dienophile. We are currently working on different versions of this polymer with the final goal of catalysing direct amidation reaction in water.

These new compartmentalised systems in which the catalytic site is isolated into a hydrophobic cavity are very promising to perform cascade reactions.

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