“Arm-First” Synthesis of Core-Cross-Linked Multiarm Star Polyethylenes by Coupling Palladium-Catalyzed Ethylene “Living” Polymerization with Atom-Transfer Radical Polymerization

Abstract

We report in this article the “arm-first” synthesis of core–shell-structured multiarm polyethylene (PE) star polymers having multiple linear-but-branched PE arms joined at a cross-linked polydivinylbenzene (polyDVB) core. This synthesis is achieved in two steps by coupling two mechanistically incompatible polymerization techniques, Pd-catalyzed coordinative ethylene “living” polymerization and atom-transfer radical polymerization (ATRP). Ethylene “living” polymerization was first carried out using a functionalized Pd–diimine catalyst, [(ArN═C(Me)–(Me)C═NAr)Pd(CH2)3C(O)O(CH2)2OC(O)C(CH3)2Br]+SbF6– (Ar = 2,6-(iPr)2C6H3) (1), to directly synthesize narrow-distributed PE macroinitiators (MIs) containing an end-capping 2-bromoisobutyryl group active for initiating ATRP. Featured with controllable molecular weights at low polydispersity, the PE MIs are employed subsequently in the second step to initiate cross-linking polymerization of divinylbenzene (DVB) via ATRP to obtain the core-cross-linked star polymers. As a demonstration, three PE MIs at different lengths (number-average molecular weight of 7.3, 10.3, and 13.7 kg/mol, respectively) were specifically synthesized and employed for star construction. In the ATRP step, the effects of the various polymerization parameters, including MI concentration, the molar ratio of DVB to MI, and MI length, on the polymerization kinetics, star yield, average arm number, and average molecular weights of the star polymers were investigated systematically. By controlling the polymerization parameters, a range of PE star polymers having narrow-distributed arm lengths (7.3–13.7 kg/mol) and controllable average arm numbers (ca. 5–43 per star) have thus been successfully synthesized. A study on dilution solution properties of these star polymers having various structural parameters reveals their spherical chain conformation and resemblance of rigid spheres and high-generation dendrimers, with their intrinsic viscosity depending primarily on arm length while not on arm number or polymer molecular weight.