SG1-based alkoxyamine bearing a N-succinimidyl ester: A versatile tool for advanced polymer synthesis
Graphical abstract
Introduction
Nitroxide mediated polymerization (NMP) is a controlled radical polymerization (CRP) technique, which already offers the ability to prepare a wide variety of well-defined polymer architectures [1]. However, emerging technologies such as optics, microelectronics, and biomaterials are driving for new materials exhibiting continuously more sophisticated properties and performance [2]. Despite the significant improvements brought to CRP techniques such as NMP, atom transfer radical polymerization (ATRP) or reversible addition fragmentation chain transfer (RAFT), the preparation of complex architectures by CRP is restricted by the exclusion of monomer systems that are polymerized by fundamentally different mechanisms, like lactides, ethylene/propylene oxide. The most promising approaches to extend the range of polymer compositions are based on the use of heterofunctional initiators [3] allowing the combination of mechanistically distinct polymerization reactions without the need for intermediate transformation and protection steps.
In the field of NMP, the development of multifunctional initiators was focused on TEMPO and TIPNO based alkoxyamine derivatives (Fig. 1) and the combination of either ring-opening polymerization (ROP), ring-opening metathesis polymerization (ROMP) or ATRP techniques with NMP [4], [4](a), [4](b), [4](c), [4](d), [4](e), [4](f), [4](g), [4](h), [4](i), [4](j), [4](k).
However, these alkoxyamines exhibit several drawbacks such as the restriction to styrenic monomers when using TEMPO and the multi-step synthesis required to prepare TIPNO derivative alkoxyamines [5]. To overcome these drawbacks the aim of this work was to develop an initiator based on the commercially available BlocBuilder™ alkoxyamine also called in this paper MAMA-SG1 bearing a succinimidyl activated ester moiety, 1, Scheme 1. As already reported for ATRP [6] and RAFT [7] techniques but, from the best of our knowledge, not yet for NMP, such NHS-activated ester CRP precursors are indeed particularly convenient for the functionalization of polymer chain ends.
MAMA-SG1 has been developed in our group in collaboration with Arkema company [8]. Thanks to a particularly high dissociation rate constant value kd1, up to now this alkoxyamine has proved to be one of the most potent alkoxyamines reported in the field of NMP [9], [10], [11].
Another potential advantage of MAMA-SG1 is the presence of the terminal carboxylic acid group, which could be further transformed to offer new possibilities for complex macromolecular architecture synthesis. However, attempts to transform by esterification (using allyl alcohol for example) or amidification (using allyl amine or aminoethanol) this carboxylic acid group, in presence of either N,N′-dicyclohexylcarbodiimide (DCC) and catalytic amount of 4-dimethylaminopyridine (DMAP) or via reaction with thionyl chloride, were unsuccessful. Indeed in each case the reactions lead to a complex reaction mixture difficult to work up. These results could be explained by the combination of several reasons, like steric hindrance, decomposition reaction during the synthesis due to the formation of highly labile intermediates or undesired side reactions. In order to partially overcome these drawbacks, we then decided to focus our attention on the preparation of an isolable SG1-based alkoxyamine bearing an activated ester which can be further easily transformed.
Due to the high homolysis rate constant of SG1-alkoxyamine derivatives bearing a stabilized tertiary alkyl moiety, it was necessary to develop a strategy where the synthesis and reaction of the corresponding activated ester could be performed rapidly and at room temperature or lower. Among a number of possibilities [12], the introduction of a N-succinimidyl ester was chosen because of the easy preparation of such esters and for their high reactivity towards nucleophiles and particularly amines [13]. Many examples in the literature show the formation of bulky amides by this mean in a short reaction time at room temperature [14]. Such reactivity can thus be used for the preparation of a large range of heterofunctional initiators and advanced polymers.
According to specific experimental requirements, this strategy enables the synthesis of α-functional (Y group in Scheme 2) polymers, either by chemical transformation of the α-NHS chain ends of 1-derived NMP polymers (path A, so-called post-functionalization approach) or by functionalization of alkoxyamine 1 prior to NMP (path B, so-called pre-functionalization approach). Depending on the nature of the Y group (functional molecule, biomolecule or macromolecule) specific properties can be easily introduced in the polymer and preparation of block copolymers that would be difficult to obtain by other methods becomes accessible.
In this article, the synthesis of 1 and its use as an efficient initiator for NMP of styrene and n-butyl acrylate are described. In addition, several examples are given to illustrate the potential and versatility of this novel alkoxyamine for the preparation, following path A and path B, of α-functional polymers (NHS-, OH-polystyrenes), and advanced polymer architectures (polystyrene-b-polylactide, PS-b-PLA, and polypropylene oxide-b-polystyrene, PPO-b-PS, block copolymers).
Section snippets
Materials
BlocBuilder™ (MAMA-SG1) was kindly provided by Arkema (France). Styrene, n-butyl acrylate, ethanolamine polypropylene oxide, dicyclohexylcarbodiimide, N-hydroxysuccinimide and tin(II)-2-ethylhexanoate (Sn(Oct)2) were purchased from Aldrich and used as-received. d,l-Lactide was purchased under vacuum from Purac Biochem (The Netherlands).
Analytical techniques
13C, 31P, 1H NMR analyses were performed on a Bruker Advance 300 spectrometer in CDCl3 or DMSO-d6. Mass analyses were performed using a mass spectrometer 3200
Synthesis and characterization of alkoxyamine 1
Alkoxyamine 1 was obtained after a simple one step procedure in 72% yield. Typically, 1 was prepared upon the reaction of MAMA-SG1 with NHS in the presence of DCC for 1.5 h at 0 °C in THF (Scheme 3). Alkoxyamine was isolated as a white powder easy to store and handle after precipitation in pentane.
Dissociation rate constant of 1 was measured by ESR following a described procedure [15] in tert-butyl benzene at 120 °C and the activation energy Ea was estimated using the average frequency factor 2.4 ×
Conclusion
After the synthesis of the NHS-activated ester alkoxyamine 1, several examples were used to demonstrate its potential and versatility using both paths A and B, Scheme 2.
First of all, we showed that 1 was an efficient alkoxyamine for NMP of styrene and n-butyl acrylate and for achieving well-defined α-NHS functional polymers (NHS–PS–SG1). Then the post-functionalization approach (path A) was illustrated by derivatization of the NHS–PS–SG1 with ethanolamine chosen as a model compound. The
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