Polymer CuAAC reaction

The use of click chemistry has also influenced the construction of more sophisticated star polymers, such as those with block copolymer arms. Maty-jaszewski has eloquently demonstrated the preparation of three-arm star block copolymers by again combining ATRP with CuAAC click couphng [109]. In these studies the ATRP of styrene, starting from a trifunctional initiator, yielded the three-arm star homopolymer bearing bromide end groups that subsequently were transformed by substitution with sodium azide. CuAAC reaction with PEO-alkyne... 

Meldal, M. (2008) Polymer clicking by CuAAC reactions. Macromol. Rapid Commun., 29, 1016-1051. 

Nurmi et have attached a functional thiol by Michael addition TEC at a polymer chain end, followed by CuAAC reaction of the pendant alkynes from the monomer units, further demonstrating orthogonality between CuAAC and TEC. Yu etal have synthesized alkene- and alkyne-terminated PNIPAAm for reaction by thiol-ene, or the closely related thiol-yne, chemistry. These polymers were then functionalized with commercially available thiols, and NMR spectroscopy showed complete consumption of the allyl or propargyl protons with concomitant appearance of unique resonances for the added thiols. Integration of these peaks demonstrated that thiol-yne chemistry provided full double functionalization, while thiol-ene provided full stoichiometric conversion. 

Three different strategies have been employed by various workers to combine ATRP and CuAAC reactions, namely, using (i) azide-telechelic macromonomers, (ii) alkyne-telechelic macromonomers, and (iii) azide or acetylenic moieties within side chains. ATRP has the advantage that the polymers produced by the method contain tu(temiinal)-halogen end groups, which can be substituted to contain azide groups. ATRP is thus an attractive technique for the synthesis of well-de ned end-functionalized polymers. 

The combination of ATRP and postpolymerization modi cation by CuAAC click chemistry can be employed to prepare well-de ned tu-(meth)acryloyl macromonomers in an ef cient manner. Thus, polystyrene (PSt) can be prepared by ATRP and subsequently derivatized to contain azide end groups. The azide-terminated polymers can then be reacted with alkyne-containing (meth)acrylate monomers to achieve near-quantitative chain-end functionalization by CuAAC reaction. An ef cient synthesis route is shown in Scheme P12.1.1. 

For modular synthesis of ABC-type triblock copolymer, two successive CuAAC reactions have to be performed on the central polymer chain (B block). To accomplish this, the B block polymer having both azide and acetylene end groups (heterotelechelic B) has to be used and, moreover, one of the termini has to be protected in order to prevent linear chain extension (cf Scheme P12.2.1) or formation of cyclic products (Scheme P12.4.1). In a straightforward methodology, the terminal acetylene moiety on B is protected and the azide terminus is used to carry out the rst coupling reaction to join the preformed A or C block. Next, the acetylene moiety is to be deprotected to make it available for the second coupling reaction to join the remaining C or A block. 

As the nal step in preparing the triblock terpolymer, TIPS-S-PMA-N3 (1.5 equiv) is coupled to the free end functionality of the H-=-PSt-(>-PtBA-Br diblock copolymer via CuAAC reaction, which is conducted at 50°C with CuBr/Me TREN as the Cu(I) source and DMF as the solvent (Opsteen and van Hest, 2007). The excess of azide functionalized PMA is again removed by reduction of the azide moiety with PPh3 and extraction of the amine terminated polymer with the mixed solvent CH2Cl2/MeOH (95 5). 

Scheme P12.6.1 (a) Structures of trialkyne containing (P6-I) and tetraalkyne containing (P6-II) coupling agents, (b) Schematic illustration of synthesis of 3-arm and 4-arm star PSt polymers using a combination of core- rst and coupling onto (CuAAC reactions) methodologies. (Adapted from Gao and Matyjaszewski, 2006.)...

As we have noted earlier, TEC reactions compare favorably with CuAAC reactions, including the use of readily available starting materials and catalysts. However, unlike CuAAC, the TEC reaction is free of metals and can be performed under photochemical initiation. The TEC is also very fast (often quantitative reaction is obseved within a period of seconds at ambient temperature) compared to extended reaction times and elevated temperatures occasionally required for the CuAAC reaction (Binder and Sachsenhofer, 2007). The use of TEC reaction has therefore attracted attention as a means of preparing star polymers and more eomplex polymers, such as dendrimers and other sterieaUy hindered structures. 

Various end-functionalized polymers can be synthesized by reacting alkynes with azide-derivatized polymers prepared by ATRR Accordingly, suggest a synthetic strategy to prepare o, tu-dihydroxy-terminated polystyrene by a combination of ATRP and subsequent modi cation via CuAAC reactions. 

Using a combination of ATRP and CuAAC reactions, suggest an ef cient route for the synthesis of narrow-disperse cyclic poly(iV-isopropylacrylamide) with an average of 80 monomer residues in the polymer ring. 

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