Block Copolymer Synthesis Using a Nitroxide-mediated Radical Polymerization (NMP) Approach

Introduction
Synthetic Protocol
Conclusions
References

Introduction

Controlled radical polymerization has become one of the most indispensible tools for polymer chemists. The living character of this type of polymerization provides an ability to produce polymers with controlled molecular weight and narrow molecular weight distribution and to extend chains with different monomers to obtain multi-block copolymers. Nitroxide-mediated radical polymerization (NMP) is one of these controlled radical polymerizations, and stands out due to its simplicity: the polymerization is thermally initiated in the absence of an external radical source or a metal catalyst.

One of the most significant advances with NMP was the identification of an alkoxyamine that could act as a unimolecular agent, providing both the reactive initiating radical and the stable mediating nitroxide radical.¹ Hawker developed a universal NMP initiator,² which has received a broad application in laboratories around the world. The universal NMP initiator together with the corresponding nitroxide are commercially available, making NMP an accessible technique for all polymer scientists.

To verify utility of the commercially available universal initiator (Product No. 700703), we prepared an amphiphilic diblock copolymer precursor,³ poly(tert-butyl acrylate)-b-poly(4-acetoxystyrene) with a controlled molecular weight and a narrow molecular weight distribution. We added 5 equivalent percent of the corresponding nitroxide (Product No. 710733) to assist with capping the propagating chain ends during the polymerization.

Synthetic Protocol

¹H NMR and ¹³C NMR spectra were recorded at 300-MHz and 75-MHz, respectively, as solutions with the solvent proton as a standard.

Poly(tert-butyl acrylate)₁₄₀ (I). To a flame-dried 50-mL Schlenk flask equipped with a magnetic stir bar and under N₂ atmosphere, at room temperature, was added Product No. 700703 (124 mg, 0.381 mmol), 2,2,5-trimethyl-4-phenl-3-azahexane-3-nitroxide (Product No. 710733) (4.19 mg, 0.019 mmol), and tert-butyl acrylate (10.16 g, 79.6 mmol) (e.g. Product No. 327182). The reaction flask was sealed and stirred for 10 min at rt. The reaction mixture was degassed through three cycles of freeze-pump-thaw. After the last cycle, the reaction mixture was recovered back to rt and stirred for 10 min before being immersed into a pre-heated oil bath at 125 °C to start the polymerization. After 36 h, ¹H NMR analysis showed 50% monomer conversion had been reached. The polymerization was quenched by quick immersion of the reaction flask into liquid N₂. The reaction mixture was dissolved in THF and precipitated into H₂O/MeOH (v:v, 1:4) three times to afford PtBA as a white powder, (4.1 g, 80% yield based upon monomer conversion); M_n^NMR = 19,130 g/mol, M_n^GPC = 18,220 g/mol, M_w/M_n = 1.10. ¹H NMR (CD₂Cl₂, ppm): δ 1.43 (br, 1290 H), 1.80 (br, 70 H), 2.21 (br, 160 H), 7.14-7.26 (m, 10 H). ¹³C NMR (CD₂Cl₂, ppm): δ 28.4, 36.5, 38.0, 42.5, 80.9, 174.4.

Poly(tert-butyl acrylate)₁₄₀-b-poly(acetoxystyrene)₅₀ (II). To a flame-dried 50-mL Schlenk flask equipped with a magnetic stir bar and under N₂ atmosphere, at room temperature, was added I (124 mg, 0.381 mmol), 2,2,5-trimethyl-4-phenl-3-azahexane-3-nitroxide (Product No. 710733) (4.19 mg, 0.019 mmol), and 4-acetoxystyrene (10.16 g, 79.6 mmol) (e.g. Product No. 380574). The reaction flask was sealed and stirred for 10 min at rt. The reaction mixture was degassed through three cycles of freeze-pump-thaw. After the last cycle, the reaction mixture was recovered back to rt and stirred for 10 min before being immersed into a pre-heated oil bath at 125°C to start the polymerization. After 4 h, ¹H NMR analysis showed 25% monomer conversion had been reached. The polymerization was quenched by quick immersion of the reaction flask into liquid N₂. The reaction mixture was dissolved in THF and precipitated into H₂O/MeOH (v:v, 1:4) three times to afford PtBA-b-PAS as a white powder, (4.62 g, 87% yield based upon monomer conversion); M_n^NMR = 26,620 g/mol, M_n^GPC = 26,330 g/mol, M_w/M_n = 1.12. ¹H NMR (CD₂Cl₂, ppm): δ 1.43 (br, 1500 H), 1.80 (br, 100 H), 2.21 (br, 290 H), 6.36-6.82 (m, 190 H), 7.14-7.26 (m, 10 H). ¹³C NMR (CD₂Cl₂, ppm): δ 21.5, 28.4, 36.5, 38.0, 40.5, 42.6, 80.9, 121.8, 128.9, 143.0, 149.4, 169.7, 174.7.

Conclusions

The facile preparation of an amphiphilic diblock copolymer precursor with a controlled molecular weight and a low PDI using the universal NMP initiator (Product No. 700703) in combination with the nitroxide (Product No. 710733) has been demonstrated. No special apparatus or technique, beyond those employed for standard radical polymerizations were required. The final block copolymer was purified by precipitation to remove excess monomers, and was then deprotected. The morphology and size of the subsequent supramolecularly-assembled nanostructures in water depend on the polymer block length and the ratio of the block lengths, each carefully manipulated through monomer conversions, the control over which arises from the universal NMP initiator. With the simplicity of this system together with the commercial availability of NMP products, we expect to see an increased breadth of application in the field of NMP.

Acknowledgements. This material is based upon work supported by the National Heart Lung and Blood Institute of the National Institutes of Health as a Program of Excellence in Nanotechnology (HL080729). N. S. Lee thanks GlaxoSmithKline for their financial support through the ACS Division of Organic Chemistry Graduate Fellowship 2008 - 2009.