Controlled one-step synthesis of a diblock copolymer
Introduction
The synthesis of block copolymers from monomers which polymerize by different mechanisms still remains difficult and constraining. For this purpose, the dual initiator route seems to be promising. A dual initiator (also called double-headed or heterobifunctional) bears two functional groups able to initiate independently and selectively two different polymerizations. Two approaches can be used:
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The sequential (two-step) process in which at least one initiating group can initiate a polymerization without interferences with the second initiating group, leading so to an intermediate macroinitiator.
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The one-step process, much more constraining, in which the two initiating groups must initiate selectively and simultaneously two independent polymerizations. It implies (i) no interactions between the initiating groups, the eventual catalysts or coinitiators and the monomers (ii) the two polymerization rates must be similar in the same experimental conditions (solvent, temperature).
Several articles have reported the use of a dual initiator in the two-step process for the synthesis of well-defined diblock copolymers, using combinations such as ATRP-NMP [1], [2], ROP-ATRP [3], [4], [5], NMP-ROP [6], [7], [8], cationic-NMP [9], cationic-ATRP [10], cationic-anionic [11], ROP-photoinduced CTC [12]. We have also used this strategy to prepare triarm (or miktoarm) star block copolymers from a heterobifunctional macroinitiator [13], [14], [15].
Only few works have been done on the synthesis of block copolymers in a one-step process. Sogah et al. [9] (cationic and NMP), Hedrick et al. [5] (ROP and ATRP) and most recently Heise et al. [3] (enzymatic ROP and ATRP) have proved the feasibility of such syntheses. In these one-step process, as the two polymerization rates are very different, the final composition of the copolymer can only be modulated by the initial feed.
In this paper we report the one-step controlled synthesis of a block copolymer by using a dual initiator able to initiate the polymerizations of ε-caprolactone (CL) (through a hydroxyl function, in the presence of triethylaluminium in a catalytic process) and tert-butylmethacrylate (tBuMA) (through a tertiary bromide function, in the presence of CuBr/bipy2). As the ROP rate in this catalytic process can be easily modified by varying the ratio AlEt3/initiator, the final copolymers compositions can be so, for the first time, controlled. Slight interferences between the two catalyst systems were nevertheless observed.
Section snippets
Characterizations
Size exclusion chromatography was carried out using a Waters 2690 liquid chromatograph equipped with three columns, Waters Styragel 5 μm, 104, 500 and 100 Å (columns, injection and refractometer temperature, 35 °C; injection volume, 100 μl; solvent, THF at 1 ml/min), a refractive index detector (Waters 410) coupled with a UV/vis photodiode array detector (Waters 996). It was calibrated with PS standards. 1H NMR spectra were recorded on a 400 MHz spectrometer (Brucker AC 400) using CDCl3 as a solvent.
Materials
Synthesis of the dual initiator
The dual initiator (4-hydroxy-butyl-2-bromoisobutyrate) was synthesized by esterification of an excess of 1,4-butanediol with 2-bromo-2-methylpropionyl bromide in the presence of triethyl amine. This dual initiator has recently been obtained by other authors, using a different synthetic approach [10]. It contains a primary alcohol used as the initiating site for the living ROP of CL in the presence of TEA (by formation in situ of aluminum alkoxides which are the actual active center), as well
Conclusion
We have studied a dual initiator bearing a hydroxyl and a tertiary bromide functions able to initiate the CL ROP and the tBuMA ATRP in a one-step process. As we have used a catalytic process for the ROP, we are able to modulate the ROP rate by varying the OH/Al ratio, leading to the highest rate for ROP (ratio 10) or for ATRP (ratio 20) or equal polymerization rates (ratio 15).
So, starting from an initial equimolar feed, the final block copolymer composition will depend on the OH/Al ratio used.
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