Arborescent Unimolecular Micelles: Poly(γ-Benzyl l-Glutamate) Core Grafted with a Hydrophilic Shell by Copper(I)-Catalyzed Azide–Alkyne Cycloaddition Coupling

Amphiphilic copolymers were obtained by grafting azide-terminated polyglycidol, poly(ethylene oxide), or poly(2-hydroxyethyl acrylate) chain segments onto alkyne-functionalized arborescent poly(γ-benzyl l-glutamate) (PBG) cores of generations G1–G3 via copper(I)-catalyzed azide–alkyne Huisgen cycloaddition (CuAAC) coupling. The alkyne functional groups on the arborescent PBG substrates were either distributed randomly or located exclusively at the end of the chains added in the last grafting cycle of the core synthesis. The location of these coupling sites influenced the ability of the arborescent copolymers to form unimolecular micelles in aqueous environments: The chain end grafting approach provided enhanced dispersibility in aqueous media and favored the formation of unimolecular micelles in comparison to random grafting. This is attributed to a better defined core-shell morphology for the copolymers with end-grafted shell segments. Aqueous solubility also depended on the type of material used for the shell chains. Coupling by CuAAC opens up possibilities for grafting a broad range of polymers on the arborescent substrates under mild conditions.


Synthesis of α-Azido Polyglycidol
The actual Mn determined by size exclusion chromatography was 14,100 g/mol, higher than the target Mn = 10,000 g/mol. This is attributed to tetrabutylammonium azide losses during azeotropic drying of the initiator prior to the polymerization; the molecular weight distribution of the polymer was nevertheless narrow (Mw/Mn = 1.06). Due to overlapping signals in the spectrum, an accurate estimate of the degree of polymerization could not be obtained by 1 H NMR analysis. One advantage of CuAAC coupling is that many functional groups, other than azides or alkynes, do not interfere with the reaction. This allowed removal of the acetal protecting groups prior to the coupling reaction. The corresponding α-azido polyglycidol sample (α-azido PGly, Mn = 7100 g/mol) was characterized by IR spectroscopy to verify that the azide functionality was still present. The IR spectrum obtained for PGlyAc had a sharp azide stretch near 2100 cm -1 , that remained strong in the α-azido PGly spectrum, in addition to a broad peak between 3500-3000 cm -1 for the free hydroxyl groups in α-azido PGly.

Synthesis of ω-Azido Poly(ethylene oxide)
A commercial poly(ethylene oxide) (PEO) monomethyl ether sample with Mn = 5000 g/mol, with one ω-hydroxyl group, was converted to ω-azido PEO via an ω-tosyl PEO intermediate. In addition to anhydrous conditions, a large excess of p-toluenesulfonyl chloride (20 equivalents) and an extended reaction time were used to achieve a high conversion. Unfortunately even under these conditions only 85% conversion was achieved, so a second reaction step was necessary for full conversion. The 1 H NMR spectrum obtained for ω-tosyl PEO had a peak at 4.15 ppm for to the -CH2-protons adjacent to the ωtosyl functional group. The conversion level calculated by integration of the signal at 4.15 ppm and the peak for the backbone protons at 3.6 ppm yielded a ratio of 1:113, corresponding to 100% conversion of ω-hydroxyl PEO to ω-tosyl PEO. Sodium azide was then used to convert the tosylated polymer to ωazido PEO. The reaction was allowed to proceed for 48 h to ensure complete conversion. The signal in the 1 H NMR spectrum for the protons adjacent to the tosyl group (4.15 ppm) disappeared after the reaction, confirming that the group had been displaced. No signals were visible for the protons adjacent to the azido group, due to overlapping signals from the protons in the polymer backbone near 3.6 ppm.
The presence of the azide functionality on the PEO chains was confirmed by IR analysis, through an azide stretch clearly visible at 2116 cm -1 .

Synthesis of ω-Azido Poly(2-trimethylsilylethyl acrylate)
While the ATRP of 2-hydroxyethyl acrylate (HEA) in its unprotected form has been reported, it was proposed that better control over the polymerization reaction and higher conversions could be achieved if the hydroxyl group were protected [20]. The HEA monomer was therefore protected with a labile trimethylsilyl (TMS) group in this work. The polymerization of HEA-TMS was monitored by 1 H NMR analysis as the monomer conversion, calculated from the peak integration ratio for the alkene protons of the monomer (6.5-5.7 ppm) and the trimethylsilyl protons present on both the monomer and the polymer at 0.1 ppm. After 20 min, 49% monomer conversion had been reached. Based on a monomer to initiator ratio (M/I) of 80, 49% monomer conversion should correspond to a degree of polymerization Xn = 39. The ratio of the polymer backbone -CH2-protons (2.0-1.3 ppm) to the methyl initiator protons (1.1 ppm) provided an Xn value of 35. The polymerization was stopped after 35 min based on the results from the t = 20 min sample. At t = 35 min the monomer conversion (74%) corresponded to Xn = 59, whereas NMR analysis yielded Xn = 57. After sample purification Xn = 57.3 was obtained, corresponding to Mn = 10,800 g/mol.
The conversion of ω-bromo P(HEA-TMS) to ω-azido P(HEA-TMS) was performed with NaN3 in DMF over 48 h. The presence of the azide functionality could not be confirmed by 1 H NMR analysis due to overlapping peaks from the polymer that interfered with the protons adjacent to the ω-bromo and ωazido functionalities. IR analysis nevertheless confirmed the presence of an azide group on the polymer with a small peak for the azide stretch at 2117 cm -1 . The 1 H NMR spectrum for ω-azido P(HEA-TMS) revealed that approximately 7% of the TMS protecting groups were cleaved during the azidation reaction. This is attributed to the presence of hydrazoic acid, produced by trace amounts of water in the DMF used for the reaction. ω-Azido P(HEA-TMS) was stored in diethyl ether until it was used in the coupling reaction, to avoid potential cross-linking of the unprotected 2-hydroxyethyl acrylate repeating units. Figure S1: CONTIN size distributions -G1PBG and copolymers.