Preparation of poly(ethylene imine) derivatives with precisely controlled molecular weight
Graphical abstract
The synthesis of poly(ethylenimine)s derivatives (PEId) with well-defined structure and precisely controlled molecular weight is reported. Two strategies involving the use of solid- and soluble-polymer-supports are compared. The supports are utilized for the iterative coupling of a modified tetraethylene pentamine followed by a successful PEGylation. Compared to Wang resin, the PS soluble support allow the precise control of PEId block length as well as PEGylation.
Introduction
Despite the development of controlled polymerization techniques, the precise control of molecular weight remains a challenge. The dispersity observed in polymer chains, although more and more minimized, could completely change the final (co)polymer properties [1], [2]. Precision polymer science has attracted much consideration in the last few years [3], [4], [5]. Polymers with well-defined structures and architecture have emerged and the challenge is nowadays to use the newly developed approaches for the insertion of information on a polymer chain and the development of more and more sophisticated materials [6], [7], [8]. However, could these tools be simply used to eliminate dispersity and therefore eventual data interpretation problems? Controlling precisely the molecular weight and the molecular weight distribution of polymers is really challenging and will allow a better understanding of the relationship between polymer size and final properties. In that direction, the work of Davis and coll., describing a “chain doubling” strategy to form highly pure and monodisperse PEG-oligomers, is of particular interest [9]. More recently, Hawker and coll. developed an elegant strategy for discrete oligomers preparation (Đ = 1.0) [10]. They combined controlled polymerization techniques with automated chromatography to obtain large scale oligomers from commercially available monomers which show no molecular weight distribution. With their technique, they can even separate oligomers according to their tacticity. In the present paper, the choice fell on the preparation of poly(ethylene imine) derivatives (PEId) with a discrete molecular weight. Poly(ethylene imine) (PEI) was considered as one of the most efficient synthetic vectors for gene delivery. Since the precursor work of Behr and coll., many studies have been dedicated to these cationic polymers [11], [12]. Research groups have highlighted the relationship between PEIs structure and architecture on their transfection efficiency [13], [14], [15], [16]. Nevertheless, few studies consider the effect of molecular weight, while as emphasized by Godbey et al. the molecular weight is an important parameter to take into account when considering the efficiency of a PEI as vector for gene delivery [17]. This point, as well as the importance of dispersity were also highlighted by other groups using fractionation chromatography [18], [19]. Indeed, linear PEI-22KDa was often referred to as the “gold standard” of cationic non-viral vectors for gene delivery by many authors [20], [21]. However, some studies proved that low molecular weight and therefore less cytotoxic PEIs give comparable efficiency while others show the opposite [18], [19], [22], [23]. Those polymers have been prepared by cationic ring opening polymerization of 2-ethyl-2-oxazoline followed by an acid or basic hydrolysis. Despite the controlled character of the polymerization; dispersity remains. This slight dispersity can have an effect if in vivo applications are targeted. In this context, preparation of (co-)polymers with purely monomolecular molar masses are of particular interest and attempts in this direction have been already realized [24]. However, a purely unimodal distribution was never achieved although, this could help accurate data interpretation.
The aim of the present work is not to provide a full study about the relation between chain length and transfection efficiency but to develop a chemical platform for the synthesis of polymers with well-defined structures and discrete molecular weights. Two strategies involving the use of solid- and soluble supports have been compared. Solid-supports, also known as resins, are well-known in solid-phase peptide synthesis. Since the pioneer work of Merrifield on that domain, resins with different attached linker were developed and utilized in order to allow different types of chemistry [25]. Polymeric soluble supports, although less widely studied, were also considered since the 60s for the preparation of peptides, or for the preparation of oligomers (∼1000 g mol−1) [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. However, to our knowledge, they were never successfully used for the preparation of long, monodisperse polymers.
Section snippets
Chemicals
Tetraethylenepentamine pentahydrochloride (98%, Sigma Aldrich), potassium tert-butoxide (97%, Alfa Aesar), 4-methoxytritylchloride (MMT-Cl, 97%, Alfa Aesar), 2-mesitylenesulfonyl chloride (SO2Mes-Cl 99%, Alfa Aesar), methyl bromoacetate (98%, Alfa Aesar), N,N-diisopropylethylamine, (DIPEA, 99%, Alfa Aesar), sodium hydroxyde (NaOH, pellets, 99%, VWR), N,N′-dicyclohexylcarbodiimide (DCC, 99%, Alfa Aesar), 4-(dimethylamino)pyridine (DMAP, 99%, Sigma Aldrich), diodopropane (Sigma Aldrich);
Results and discussion
At first, a solid-support strategy has been used. The aim was to prepare a well-define poly(ethylene imine) bearing an exact number of ethylene imine units. For that purpose several resins possessing different functional groups as well as various loading amounts have been tested in order to couple the compound N5-I. These tests include the use of carboxylic resin (Novabiochem, loading 1 mmol/g), 1,3-diaminopropane trityl resin (Novabiochem, 1.1 mmol/g), aminomethylated polystyrene (Novabiochem,
Conclusions
We efficiently combined organic and controlled polymer chemistry tools for the precise synthesis of PEI derivatives bearing an exact number of monomer units and their efficient PEGylation. Polymer soluble supports are efficient tools for the synthesis of copolymer blocks with discrete molecular weights. This point is of particular interest for studying the relationship between block length and final properties without contradictory results due to dispersity. This strategy can be extended to any
Acknowledgements
The CNRS and the University of Strasbourg, are acknowledged for financial support. The authors thank Laurence Oswald (Institut Charles Sadron, Strasbourg) for the synthesis of the PS-soluble support, Catherine Foussat and Dr. Mélanie Legros (Institut Charles Sadron, Strasbourg) for the SEC measurements, Dr. Jean-Marc Strub (ECPM, Strasbourg) for mass spectrometry analyses.
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