CuAAC for the Synthesis of Star-Shaped Polymers

3-arm PS, poly(r rr-butyl acrylate) (PtBuA) or poly(ethylene glycol) (PEG) star-shaped polymer using the same strategy, with a 87% click efficiency (Table 8.1, Entry 2) (Altintas et ah, 2006a). 

Following the same arm-first approach, Schubert et al synthesized acetylene functionalized poly(e-caprolactones) (PCL), which were subsequently coupled onto a heptakis-azido-jS-cyclodextrin resulting in a 7-arm star-shaped heptakis-PCL-j8-cyclodextrin (Table 8.1, Entry 3) (Hoogenboom et al, 2006). However, harsh reaction conditions were required to drive the reaction to completion (microwave-assisted CuAAC at 100 C). NMR, SEC analysis (UV and RI) and MALDI-TOF MS spectra confirmed the quantitative character. 

When CuAAC was applied using standard conditions (Cu(I)Br/PMDETA, room temperature in THF), a maximum click efficiency of 64% was obtained. By changing the catalytic system, catalytic concentration or temperature, no positive trend was observed. This inefficient CuAAC of the highly branched PiBA star-shaped polymers with the linear polymers is ascribed to the entropic penalty, as also observed by Matyjaszewski (Gao and Matyjaszewski, 2006). In an attempt to improve this click efficiency, the effect of microwave heating on the Cu (I)-mediated click reaction of these bulky star-shaped polymers with other polymer segments 

Another approach to synthesize multiarm star block copolymers is based on a combination of CuAAC and the arm-first method (Durmaz et al, 2010). Protected alkyne PS polymers were prepared via ATRP and subsequently crosslinked by a divinyl containing compound. The formed 27-arm star-shaped polymers containing a protected alkyne periphery, was deprotected and subsequently coupled with azide-end-functionalized PEG and PtBuA to form star block or mixed block copolymers. The CuAAC reactions occurred at room temperature for 24 h, surprisingly leading to a full click efficiency. The quantitative character of the latter click reaction at ambient temperatures for such dense polymer structures is in contrast to those obtained by other research groups, as mentioned in previous paragraphs. 

To synthesize ABA star block copolymers, a combination of ROP, ATRP and CuAAC was used (Gou et al, 2009). First, a resorcinarene-centered 8-arm PCL star-shaped polymer was synthesized via ROP. After esterification of the hydroxyl end groups into 2-bromoesters, the star-shaped polymer is used as macroinitiator for the ATRP of tBuA. Then, the bromine end groups of the PCL-PtBuA-Br were converted into the azide end groups and subsequently coupled onto alkyne-containing PCL at 40 °C for 48 h, in an almost quantitative yield. 

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