The statistical anionic copolymerization
of isoprene (I) and styrene
(S) is commonly used to synthesize tapered block copolymers, enabling
control of the phase behavior by adjusting the order–disorder
transition temperature, T
ODT. Alkyllithium
initiation in hydrocarbons is known to afford tapered block copolymers
of I and S in one step. The effect of tetrahydrofuran (THF) on the
copolymerization kinetics and the resulting copolymers was systematically
investigated by increasing the [THF]/[Li] ratio from 0 to 2500 (0
to 29%vol THF). For this purpose, in situ near-infrared
(NIR) spectroscopy was employed as a versatile and fast method to
track the highly accelerated consumption of the individual monomers.
Changes in the I/S comonomer sequence and in the polyisoprene (PI)
regioisomers, caused by variation of the THF concentration, were independently
determined via NMR and in situ NIR spectroscopy. Reactivity ratios
were determined as a function of the [THF]/[Li] ratio. They revealed
a gradual reversal from r
I ≫ r
S over r
I ≈ r
S to r
I ≪ r
S. Corresponding changes in the copolymer composition
profile up to a complete inversion are evident in thermal properties
and morphologies. Although all copolymers possess the same comonomer
composition (50%mol = 57%vol polystyrene (PS)
units), small-angle X-ray scattering and transmission electron microscopy
give evidence of a wide variation in bulk morphologies depending on
the gradient profile. Overall, the phase diagram is symmetric, and
the succession of phases bears certain similarities to the PI-b-PS case. This is discussed in terms of the increasing
incompatibility of PS with 3,4-PI and the more symmetric polymer conformational
parameter. The degree of segregation, as well as the nanodomain structure,
was found to control the mechanical properties, showing a remarkably
different viscoelastic response leading to either hard/brittle or
ductile/soft materials. The accessibility of tailored gradient profiles,
as well as their in-depth understanding by simply using THF as a microstructural
modifier, opens a variety of possible applications. As an example,
the synthesis of a PI-selective hydrogenated tapered triblock, possessing
a THF-modified, phase-compatibilizing tapered block incorporated in
the well-established SIS block architecture, is presented.
Multi-functional polyethers with ene or diene moieties were prepared via the polymerisation of tailored functional glycidyl ether monomers to create a platform for click chemistry with triazolinediones (TADs).
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