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Well-Defined Bilayered Molecular Cobrushes with Internal Polyethylene Blocks and ω‑Hydroxyl-Functionalized Polyethylene Homobrushes
journal contribution
posted on 2016-02-15, 00:00 authored by Hefeng Zhang, Nikos HadjichristidisNovel well-defined bilayered molecular
cobrushes with internal
polyethylene blocks, P(PEcore-b-PScorona) (PE: polyethylene; PS: polystyrene), and ω-hydroxyl-functionalized
polyethylene homobrushes, P(PE-OH), were synthesized through the macromonomer
strategy. Two main steps were involved in the synthesis of the P(PEcore-b-PScorona) bilayered cobrushes:
(i) formation of norbornyl-terminated macromonomer (Nor-PE-b-PS) by esterification of PS-b-PE-OH (combination
of anionic polymerization, hydroboration, and polyhomologation) with
5-norbornene-2-carboxylic acid and (ii) ring-opening metathesis polymerization
(ROMP) of Nor-PE-b-PS. The synthesis of P(PE-OH)
was achieved by (i) hydroboration of tert-butyldimethylsilyl-protected
allyl alcohol, followed by polyhomologation of dimethylsulfoxoniun
methylide with the formed tri[3-(tert-butyldimethylsilyloxyl)propyl]borane
initiator, oxidation/hydrolysis, and esterification of the TBDMS-O-PE-OH
with 5-norbornene-2-carboxylic acid to afford the macromonomer TBDMS-O-PE-Nor,
and (ii) ROMP of TBDMS-O-PE-Nor, followed by deprotection. Nuclear
magnetic resonance spectroscopy (1H and 13C
NMR) and high temperature gel permeation chromatography (HT-GPC) were
used to characterize all macromonomers/molecular brushes and differential
scanning calorimetry (DSC) to study the thermal properties. The molecular
brush P(PE-b-PS) showed lower melting point (Tm) and better solubility in toluene than the
corresponding macromonomer PS-b-PE-Nor. In the case
of homobrushes, the thermal properties were strongly affected by the
presence of the PE end-groups.