Precise synthesis of α,ω-chain-end functionalized poly(dimethylsiloxane) with azide groups based on metal-free ring-opening polymerization and a quantitative azidation reaction

Highlights

• Synthesis of α,ω-chain-end functionalized poly(dimethylsiloxane) with azide groups.

• Controlled M_n and narrow molar-mass distribution.

• Quantitative introduction of azido groups.

Abstract

We have so far developed a method for the controlled ring-opening polymerization of cyclotrisiloxane catalyzed by strong organobasic compounds to produce various chain-end functionalized polysiloxanes with controlled number-average molar mass (M_n) and narrow molar-mass dispersity (Ð_M) values. In this study, well-defined α,ω-chain-end functionalized poly(dimethylsiloxane) with azide groups (N₃-PDMS-N₃) possessing a narrow Ð_M range (Ð_M = 1.04) and a relatively high M_n of up to 28.2 kg mol⁻¹ was precisely synthesized in a mixed solvent of tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) in the presence of quaternary ammonium salts as catalysts; N₃-PDMS-N₃ was obtained by converting α,ω-chain-end functionalized PDMS with bromomethyl groups (Br-PDMS-Br), which was prepared by polymerizing hexamethylcyclotrisiloxane (D3), water, and 1,3-trimethylene-2-n-propylguanidine (TMnPG) as the monomer, the initiator, and the catalyst, respectively. It was necessary to choose proper solvents depending on the M_n of Br-PDMS-Br to achieve the complete conversion of the bromomethyl groups by the azidation reaction. Moreover, we demonstrate the model reaction of copper-catalyzed 1,3-dipolar cycloaddition between N₃-PDMS-N₃ and phenylacetylene. As a result, the cycloadducted compound was exclusively obtained while maintaining a low Ð_M value of 1.06 by employing copper(I) bromide and triethylamine as the catalyst and the ligand, respectively. This system was also effective for a polymer-polymer coupling reaction. Indeed, we successfully synthesized an ABA-type triblock copolymer where A and B are polystyrene (PS) and PDMS segments by reacting N₃-PDMS-N₃ with ω-chain-end functionalized PS with an alkynyl group.

Introduction

Polymer-chain-end transformation is one of the fundamental methodologies for obtaining various chain-end functionalized polymers with controlled architectures [1,2]. This method has been utilized to tune the properties of polymers, such as solubility, chain association, and rheology [[3], [4], [5]]. In addition, chain-end transformation is an accessible strategy for the synthesis of block copolymers by a polymerization reaction using chain-end functionalized polymers as macroinitiators or by a polymer-polymer coupling reaction [6]. However, these reactions are often not very efficient due to the occurrence of various undesirable side reactions. Accordingly, the use of chain-end functionalized polymers with precisely positioned and highly reactive functional groups is important, especially for performing efficient block copolymer synthesis. Since its discover, “click chemistry” has employed in polymer chemistry to overcome the above synthetic difficulties [7,8]. In particular, copper (Cu)-catalyzed azide-alkyne cycloaddition (CuAAC) has often been utilized as a method for synthesizing a wide range of terminally modified polymers and block copolymers [[9], [10], [11]]. Based on this background, the precise introduction of azide or alkyne groups at the chain-end(s) of polymers is quite important for the precise synthesis of various polymers with controlled structures.

Polysiloxanes have been utilized in a number of industrial fields due to their unique and useful chemical and physical characteristics, such as high flexibility and stretchability, high thermal stability, good solubility, and low surface energy [12]. Indeed, chain-end functionalized polysiloxanes and their block copolymers have been designed and developed for the production of polymers with special properties and functions [13,14]. For example, polysiloxane-based copolymers have been broadly applied to versatile commercial products, such as surfactants in polyurethane foams, artificial blood vessels, and gas separation membranes [14].

A chain-end functionalized polysiloxane is a potentially useful precursor for polymer-polymer coupling reactions to synthesize block copolymers or for polymer chain-end transformation to introduce specific functional groups at the chain-end(s). For instance, ω-chain-end functionalized polysiloxanes with an azide group could be synthesized by the reaction between sodium azide (NaN₃) and an ω-chain-end glycidyloxy or an ω-chain-end chloropropyl polysiloxane [[15], [16], [17], [18]]. Recently, the synthesis of α,ω-chain-end functionalized polysiloxanes with azide groups has been reported based on the acid-catalyzed ring-opening polymerization (ROP) of siloxanes using 1,3-bis(3-azidopropyl)-1,1,3,3-tetramethyldisiloxane in the presence of triflic acid [20]. In general, polysiloxanes synthesized by the above transformation reactions have at least one of the following disadvantages: 1) the starting ω-chain-end or α,ω-chain-end functionalized polysiloxanes have broad molar mass distributions; 2) the number of azide groups in a polymer is unclear; 3) the number-average molar mass (M_n < 10.0 kg mol⁻¹) is low. To date, the precise synthesis of chain-end functionalized polysiloxanes with a definite number of azide group(s) free of the disadvantages 1)–3) has never been reported.

In this study, we developed a simple methodology for the precise synthesis of α,ω-chain-end functionalized poly(dimethylsiloxane)s with azide groups (N₃-PDMS-N₃: 2) by the azidation of well-defined α,ω-chain-end functionalized PDMS with bromomethyl groups (Br-PDMS-Br: 1) which is synthesized by the organocatalytic ROP method, as shown in Scheme 1 [19]. In addition, the suitable conditions for azide-alkyne cycloaddition of N₃-PDMS-N₃ were investigated in detail. These novel techniques are being used in a new research field involving PDMS-based polymers with controlled primary structures.