We present investigations of the intricate crystallization and melting behavior of an alternating copolymer synthesized by photopolymerization of 2,2′-dimercaptodiethyl sulfide with di(ethylene glycol)divinyl ether. Upon increasing temperature, we observed the succession of two distinctly separated melting processes, which we related to the sequential formation and disappearance of two crystalline polymorphs. Due to their well-separated melting temperatures T m1 and T m2 , we labeled these polymorphs as LOW-T m -form and HIGH-T m -form, respectively. X-ray diffraction results confirmed differences in the parameters of the crystal unit cells. However, upon cooling from the isotropic melt, we never obtained the HIGH-T m -form and could only generate the LOW-T m -form characterized by spherulitic crystals that melted completely at T m1 . Surprisingly, simultaneously everywhere within these molten spherulites, a large number of needle-like crystals were growing as a function of the time the sample was kept (well) above T m1 . All crystals exhibited an orientation largely following the radial direction of the initial spherulites. This observation suggests that molten polymer chains remembered for some time their previous alignment within the crystalline LOW-T m -form. This memory assisted the nucleation and thus exclusively enabled the formation of the HIGH-T m -form. Only when partially crystallizing a sample aboveT m1 in the HIGH-T m -form and subsequently cooling it below T m1 allowed to achieve coexistence of both polymorphs. When heating such a sample of coexisting crystalline structures above T m1 , only the LOW-T m -form melted and the HIGH-T m -form remained. The generality of our findings has been demonstrated by similar results obtained for complementary alternating copolymers. Our study suggests that the otherwise impossible nucleation of polymorphs with a high melting temperature can be enabled by prior orientation of chains within another easily established polymorph characterized by a lower melting temperature.
Boron-catalyzed
polymerization offers unique C3-polymer structures
because, in contrast to 1,4-polydienes, each double bond is separated
by only one methylene group. However, the geometrical regularity of
such unique C3-structures was less discussed, and their properties
have not been reported. In this work, well-defined poly(2-methyl-propenylene)s
with different molecular weights are prepared in the gram scale by
boron-catalyzed polymerization of 2-methylallyl arsonium ylide. 1H NMR, 13H NMR, and two-dimensional 1H–13C heteronuclear single quantum coherence NMR
spectra confirm the high selectivity toward trans-configuration (>99%) and C3 monomeric insertion (>98%). Density
functional theory (DFT) calculations at the wb97xd/tzvp level (solvent
= tetrahydrofuran) explain the high trans/C3. Furthermore,
the thermal parameters, T
c, T
m, T
g, ΔH
c, ΔH
m, and crystallinity
degree, of poly(2-methyl-propenylene)s are determined by differential
scanning calorimetry (DSC), fast scanning chip calorimetry (Flash
DSC), and Wide Angle X-ray Scattering (WAXS) for the first time and
are compared with those of trans-polyisoprene.
We report the first examples of carborane-containing non-fullerene acceptors (NFAs), and their use in organic photovoltaic (OPV) devices. NFAs employing an A-D-A’-D-A type design centred around a central electron withdrawing...
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