Control in Both Backbone and Branch Segment Lengths of Poly(tetrahydrofuran) Polymacromonomers by Electrostatic Self-Assembly and Covalent Fixation

Abstract

Polymacromonomers are a novel class of branched polymers, composed of a linear backbone segment with densely substituted side chains of either linear, flexible polymer segments1,2 or highly branched, dendritic segments (dendron-jacketed polymers).3-5 They are unique in forming anisotropic cylindrical three-dimensional structures, as observed directly by means of an AFM or SFM technique, where the otherwise flexible backbone segment is forced to adopt an extended rodlike form rather than its entropically favored random-coil form.6-10 In consequence, polymacromonomers can produce, through self-assembly, a variety of mesoscopically ordered structures. The precise control, therefore, over structural parameters in polymacromonomers such as the backbone segment length and the number of graft segments in addition to the graft segment length becomes an important prerequisite for the design and eventual modulation of the supramolecular structures achieved by them. Polymacromonomers of high branch numbers have so far been prepared by a radical homopolymerization process under rigorously controlled conditions.11 While the control over the number of branch segments (the DP of the product) has been difficult to achieve. Very recently, a living radical polymerization process has been applied, and uniform size polymacromonomers have been synthesized using macromonomer molecular weights of a few thousand.12 A macromolecular coupling reaction between a polyfunctional prepolymer and complemental end-functional polymer precursors, on the other hand, is an alternative means to produce a polymacromonomer with controlled structures, since both polymer precursors are, in principle, prepared by a living polymerization technique and fully characterized prior to the coupling reaction.13 In practice, however, polymer coupling processes by conventional reaction mechanisms have scarcely been applied because of the inefficiency by the low concentration of reactive groups located only at the chain end of a polymer precursor, as well as by the repulsive interaction and eventual macroscopic phase separation of two immiscible polymer precursors. We have proposed an effective macromolecular coupling reaction process by addressing the above-mentioned drawbacks.14,15 Thus, a series of poly(THF) polymacromonomers were prepared in high yields by use of poly(THF) having an azetidinium salt,16 as well as a tetrahydrothiophenium salt,17 as a single end group. A highly efficient ion-exchange reaction of the poly(THF) precursor was observed to occur with poly(sodium acrylate), by precipitation of the former into aqueous solution of the latter. During this process an ionically self-assembled product is presumed to be formed, and this has been assumed a critical intermediate for the exceptionally efficient polymer coupling reaction. In this particular reaction, however, the ionically self-assembled precursor could not be isolated, since the azetidinium (four-membered ammonium) group and the tetrahydrothiophenium (five-membered sulfonium) group undergo a quantitative ring-opening reaction by carboxylate anions in poly(acrylate) at ambient condition to produce amino-ester and thio-ester (thus covalent bond) groups, respectively. The isolation of the ionically self-assembled polymacromonomer precursor has subsequently been achieved by use of poly(THF) having a less strained N-phenylpyrrolidinium salt as a single end group.18 The relevant ion-exchange process with poly(sodium acrylate) could produce the ionically self-assembled and isolatable polymacromonomer precursor, and the subsequent heat treatment could convert the ion pair into covalent bond quantitatively to produce a polymacromonomer in high yield. In this relevance, a variety of noncovalently selfassembled, comb-shaped polymers and copolymers either by hydrogen bonds or by ionic interactions have also been reported to exhibit unique phase behaviors in bulk.19-21 As a further extension of our “electrostatic selfassembly and covalent fixation” process, we report herein an efficient synthesis of a series of poly(THF) polymacromonomers with controlled backbone segment length as well as graft segment length. Thus, uniform size poly(THF) having an N-phenylpyrrolidinium salt group was subjected to the coupling reaction with poly(sodium acrylate)s of defined DP values, prepared through the living polymerization of tert-butyl acrylate and subsequent saponification treatment.