Zhang et al. report in Polymer Chemistry the synthesis of well-defined polyethylene-b-polycaprolactone and polyethylene-b-polymethylmethacrylate star copolymers.
Star homo- and copolymers have always been the centre of attention thanks to their interesting solution and self-assembly properties in comparison to their linear counterparts. Among the available strategies of synthesizing star polymers, the “core first” approach is of significant importance although polyethylene-based (PE) stars via this method have been rarely reported. Towards this end, Hadjichristidis and co-workers have prepared a 4-arm OH-terminated polyethylene macroinitiator in three steps via cyclic hydroboration/polyhomologation/oxidation reactions.
The successful synthesis has been confirmed by both nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) analysis. The hydroxyl groups of the star macroinitiator were subsequently utilized to initiate the ring-opening polymerization (ROP) of ε-caprolactone with the GPC confirming a clear shift to higher molecular weights while retaining narrow molecular weight distributions.
In order to demonstrate the potential of this approach, the hydroxyl groups have also been esterified to yield eight initiating sites capable of enabling the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). Pleasingly, a monomodal distribution and a quantitative shift to higher molecular weights were observed by GPC, suggesting the efficiency of the reaction.
As such, this strategy revealed the compatibility of polyhomologation with other controlled/”living” polymerization techniques and thus allowing access to the synthesis of novel and well-defined materials.
Tips/comments directly from the authors:
- An excess of thexylborane should be used to ensure all divinyl groups are reacted (cyclic hydroboration), otherwise the tetrafunctional initiator will be contaminated with difunctional.
- Unreacted thexylborane should be quenched with methanol before the addition of ylide since it will initiate polyhomologation too towards linear PE.
- Temperature higher than 80 oC should be used for ROP and ATRP to ensure good solubility of polyethylene.
Read this exciting research for free until 30/10/2016 through a registered RSC account:
Well-defined 4-arm stars with hydroxy-terminated polyethylene, polyethylene-b-polycaprolactone and polyethylene-b-(polymethyl methacrylate)2 arms
Z. Zhang, Y. Gnanou and N. Hadjichristidis
Polym. Chem., 2016, 7, 5507-5511
DOI: 10.1039/C6PY01090C
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