Negatively charged hyperbranched polyetherbased p

Authors: Emilie Barriau Holger Frey Anton Kiry Manfred Stamm Franziska Gröhn

Publish Date: 2006/06/21

Volume: 284, Issue: 11, Pages: 1293-1301

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Abstract

The preparation of carboxylated hyperbranched polyglycerols of narrow polydispersity was achieved by modification 78–90 of the hydroxyl end groups via Michael addition of acrylonitrile followed by hydrolysis High conversion could only be achieved for low molecular weight starting materials 520 and 1030 g mol−1 The solution properties of the resulting materials were investigated by dynamic light scattering DLS showing the formation of large aggregates with size depending on the pH value After deposition on a negatively charged mica surface the structures observed by atomic force microscope AFM show the coexistence of aggregates and single macromolecules Most interesting in the case of the lower molecular weight sample PG 520 g mol−1 extended and ordered terrace structures were formed which are unprecedented for hyperbranched polymers and are of interest for surface modification in generalPolyelectrolytes play an important role in numerous areas eg to control the stability and rheology of complex fluids for flocculation and biomineralization processes Polyelectrolytebased materials are also widely applied in coatings thickeners dispersants detergentadditives and ionexchange resins Thus the development of novel polyelectrolytes with unusual topology has become a topic of broad interest for both Colloid and Polymer Science as these polymers are likely to exhibit peculiar properties in aqueous solution that can strongly influence the fluid characteristics of aqueous suspensions and slurries permit the introduction of surface charges on neutral particles via adsorption and may also lead to strong interaction with ions or colloidal aggregates of opposite chargeIn several recent works highly branched polyelectrolyte structures have been introduced that are mostly based on perfectly branched dendrimer scaffolds 1 2 3 4 Müller et al have described a versatile strategy for hyperbranched polyelectrolytes based on the selfcondensing vinyl copolymerization of protected acrylate monomers and subsequent hydrolysis 5A longknown example of a highly branched polyelectrolyte is polyethyleneimine PEI For PEI pHvariation leads to strong variation of the properties in aqueous solution as the charge–density on the aminegroups is pHdependent 6 Furthermore a number of theoretical works has dealt with charged branched polyelectrolyte structures 7 8 9 Among the hyperbranched structures polyglycerol prepared by slow addition of glycidol to a partially deprotonated polyol core 10 represents a particularly interesting material as it is biocompatible 11 and can be prepared with controlled molecular weight and low polydispersity In an elegant recent work Schwab and Mecking reported the preparation of cationically charged polyglycerolbased polyelectrolytes based on the attachment of pyridinium and imidazolium functionalities to polyglycerol 12 Haag et al have recently introduced a synthesis for hyperbranched polyglycerolbased carboxylates based on conversion with sodium chloroacetate which however only led to a limited conversion of 26 of the hydroxyl end groups 12For the synthesis of polyaminedendrimers polypropylene imines 13 14 15 the Michael addition to the amino end groups plays a key role to obtain the polynitrile intermediates In this case even in high dendrimer generations the dendrimer periphery can be fully modified A subsequent hydrolysis step can also be employed to generate carboxylic acid end groups thus leading to dendritic polyelectrolytes This method has also been successfully employed for the preparation of amphiphilic lineardendritic block copolymers by Meijer et al 16In this paper a convenient pathway for the modification of the hydroxyl end groups of hyperbranched polyglycerol into carboxylic acid groups and thereby the formation of a hyperbranched anionically charged polyelectrolyte is presented The solution properties of these new hyperbranched polyelectrolytes will be discussed Furthermore the materials have been deposited on mica to generate ultrathin multilayers that have been investigated by atomic force microscope AFMHyperbranched polyglycerol 500 mmol was placed in a reactor and dissolved in 15ml absolute DMF under argon atmosphere Potassium tertbutoxide was added to the polymer solution to achieve 10 deprotonation of the hydroxyl groups The reaction mixture was stirred and heated up to 80 °C for 4 h and then cooled down to 0 °C Distilled acrylonitrile 61 mmol was slowly added via a syringe to the reactor and the temperature was kept at 0 °C After 4 h the reactor was heated up to 5 °C the solution became slightly red The reaction mixture was kept at 5 °C for another 20 h and finally for 48 h at room temperature Then 1 g of a cationic exchange resin was added to the reactor and that was stirred for 24 h After filtration DMF and the remaining acrylonitrile were removed by distillation The modified polyglycerol was first dissolved in acetone and then precipitated in methanol obtaining a brownish product Both fractions were analyzed The acetone fraction is a solid and the methanol soluble fraction is a highly viscous oil The 3Opolyglycerolpropanenitrile was dried in vacuum at 50 °C

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