Azide telechelics chain extended by click reaction: Synthesis, characterization, and cross‐linking

Abstract

Azide telechelics of poly(dimethylsiloxane) (PDMS), polypropylene oxide (PPO), and polyethylene oxide (PEO) were synthesized from the corresponding epoxy telechelics and characterized. These oligomeric azides were chain extended by reaction with bispropargyl ether of bisphenol A (BPEBA) through a copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. PDMS manifested a faster reaction in contrast to PPO or PEO. The chain-extended polymers underwent cross-linking above 170°C through thermal cleavage of residual (terminal) azide groups. This was manifested in their rheograms and was further substantiated by FTIR and NMR spectroscopic analyses. Dynamic mechanical analyses of the cross-linked polymers exhibited characteristic transitions of hard and soft segments, implying microphase separation in the system. Microscopic evaluation of the thermally cross-linked sample revealed a porous morphology with microsized to nanosized pores. KEYWORDS azide telechelics, propargyl ether, thermal cross-linking, triazole, click reaction 1 | INTRODUCTION Telechelic polymers evince a lot of interest because of their various applications such as precursors for the synthesis of multiblock copolymers and synthesis aid for a large number of branched and complex polymeric structures. 1,2 For example, hydroxyl-terminated polybutadiene (HTPB) is a well-known telechelic system used as a propellant binder and as adhesive, sealant, elastomer, 3 etc. End-functional poly(dimethylsiloxane) (PDMS) is another example of a very useful synthesis intermediate widely used in adhesives, sealants, and coatings formulations 4 in space research. The possibility to externally steer the morphology and the associated physical properties of telechelic polymers renders them important and versatile building blocks for modern materials science. 2,3Diverse methods such as anionic, cationic, ring opening, group transfer, free radical, step-growth polymerization, and chain scission processes are adopted for the syntheses of these telechelic polymers. [5][6][7][8] The criticality in synthesis of telechelic polymers lies in the quantitative introduction and subsequent conversion of the functional group at the chain ends. Living polymerization technique is a good technique for getting telechelic polymers with narrow molecular weight distribution. 9,10 However, the properties of polymers obtained by end-linking of telechelic oligomers depend strongly on stoichiometry, side reactions, extent of conversion, etc. The relative reactivities of the groups, substitution effects, side groups, etc play a major role 11,12 in deciding the success factor. Different types of synthesis strategies such as Diels-Alder, [13][14][15] thiol-ene, [16][17][18][19] and azide-alkyne 20,21 figure under the title of "click chemistry." Of these, the Diels-Alder belongs to a class of thermoreversible reactions. 22 Copper-catalyzed azide-alkyne cyclo addition (CuAAC) is shown to be the most effective and versatile, since it is highly specific and efficient, requires mild conditions, 23 has ...