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One-Step Block Copolymer Synthesis versus Sequential Monomer Addition: A Fundamental Study Reveals That One Methyl Group Makes a Difference
journal contribution
posted on 2018-04-27, 18:23 authored by Eduard Grune, Tobias Johann, Michael Appold, Christian Wahlen, Jan Blankenburg, Daniel Leibig, Axel H. E. Müller, Markus Gallei, Holger FreyBlock copolymers of polyisoprene
and polystyrene are key materials
for polymer nanostructures as well as for several commercially established
thermoplastic elastomers. In a combined experimental and kinetic Monte
Carlo simulation study, the direct (i.e., statistical) living anionic
copolymerization of a mixture of isoprene (I) and 4-methylstyrene
(4MS) in nonpolar media was investigated on a fundamental level. In situ 1H NMR spectroscopy enabled to directly
monitor gradient formation during the copolymerization and to determine
the nature of the gradient. In addition, a precise comparison with
the established copolymerization of isoprene and styrene (I/S) was
possible. Statistical copolymerization in both systems leads to tapered
block copolymers due to an extremely slow crossover from isoprene
to the styrenic monomer. For the system I/4MS the determination of
the reactivity ratios shows highly disparate values with rI = 25.4 and r4MS = 0.007,
resulting in a steep gradient of the comonomer composition. The rate
constants determined from online NMR studies were used for a kinetic
Monte Carlo simulation, revealing structural details, such as the
distribution of the homopolymer sequences for both blocks, which are
a consequence of the peculiar kinetics of the diene/styrene systems.
DFT calculations were used to compare the established copolymerization
of isoprene and styrene with the isoprene/4-methylstyrene system.
A variety of gradient copolymers differing in molecular weight and
monomer feed composition were synthesized, confirming strong microphase
segregation as a consequence of the blocklike structure. The one-pot
synthesis of such tapered block copolymers, avoiding high vacuum or
break-seal techniques, is a key advantage for the preparation of ultrahigh
molecular weight block copolymers (Mn >
1.2 × 106 g/mol) in one synthetic step. These materials
show microphase-segregated bulk structures like diblock copolymers
prepared by sequential block copolymer synthesis. Because of the living
nature of the tapered block copolymer structures, a vast variety of
complex structures are accessible by the addition of further monomers
or monomer mixtures in subsequent steps.
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Difference Block copolymersweight block copolymers1 H NMR spectroscopysequential block copolymer synthesismaterials show microphase-segregated bulk structuresblock copolymer structuresgradientOne-Step Block Copolymer Synthesisr 4 MScopolymerizationmonomer feed compositionMonte Carlo simulationDFTSequential Monomer AdditionMonte Carlo simulation studyisopreneblock copolymers
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