Star-structured polyethylene nanoparticles via Pd-catalyzed living polymerization : synthesis, characterization, and catalytic applications.

Abstract

The arm-first synthesis of large unimolecular star-structured polyethylene nanoparticles or SPE-NPs (MW > 1,000 kg/mol, PDI ≈ 1.1) joined by a cross-linked polynorbornadiene (PNBD) core is described in this thesis. SPE-NPs having high arm number (fn > 100) and tunable arm topologies (hyperbranched HBPE or linear-but-branched LBPE) are conveniently synthesized in a single reactor following four consecutive steps. In step 1, living ethylene polymerization is catalyzed by 0.1 mmol of Pd-diimine catalyst 1 to grow HBPE arms (1 atm C2H4/15 °C) or LBPE arms (27 atm C2H4/5 °C) of tunable lengths (tE = 1-5 h, Mn = 11-40 kg/mol). In step 2, the norbornadiene (NBD) cross-linker is added into the ethylene reactor for several hours (tNBD = 1-4 h) yielding PE-b-PNBD block copolymers with a short PNBD segment bearing cross-linkable pendant double bonds. SPEs are then formed in step 3 during precipitation in acidified methanol (H+/MeOH) and the final SPE-NPs are formed in step 4 after several hours of drying in vacuo at 120 °C. A thorough systematic investigation of the reaction parameters indicates that to produce increasingly larger SPE-NPs, it is essential to add a significant molar excess of NBD to 1 ([NBD]0/[1]0 > 50) and synthesize short LBPE arms but large HBPE arms. When synthesized with LBPE arms, the SPE-NPs have higher MW compared to those synthesized with HBPE arms due to the lower steric hindrance of the linear arms which enables a high number of arms to be joined at the PNBD core. Furthermore, the Pd-diimine catalyst used in the synthesis of the SPE-NPs was encapsulated within the cross-linked PNBD core. These encapsulated Pd(II) species were tested for their activity in hydrogenation reactions of terminal alkenes and alkynes (1-octene, 1-hexene, and 1-hexyne) and Heck coupling reactions of iodobenzene and n-butyl acrylate. Preliminary data suggests that these SPE-NPs may be used as models for the design of more advanced recyclable nanovessel for Pd(II) catalysts.