As imple and high-yielding route to tough polyarylenes of the type poly(meta,meta,para-phenylene) (PmmpP) is developed. PmmpP is tough even in its as-synthesized state which has an intermediate molar mass of M w % 60 kg mol À1 and exhibits outstanding mechanical properties at further optimized molecular weight of M w = 96 kg mol À1 ,E= 0.9 GPa, e = 300 %. Statistical copolymers with para,para-spiropyran (SP) are mechanochromic, and the toughness allows mechanochromism to be investigated. Strained samples instantaneously lose color upon force release.D FT calculations showt his phenomenon to be caused by the PmmpP matrix that allows build-up of sufficiently large forces to be transduced to SP,and the relatively unstable corresponding merocyanine (MC) form arising from the aromatic co-monomer.M Cu nits covalently incorporated into PmmpP show adrastically reduced half life time of 3.1 scompared to 4.5 hobtained for SP derivatives with common 6-nitro substitution.Polyarylenes have intrigued material scientists since the invention of transition-metal-catalyzed cross-coupling reactions. [1,2] In 2007, Schlüter et al. introduced ah igh molecular weight (MW) poly(meta,para-phenylene) (PmpP) by careful selection of catalyst, solvent mixture,c oncentration, and temperature. [3] In their pioneering work ac rude polymer of M w = 83 kg mol À1 was synthesized and the molecular weight further increased to M w = 255 kg mol À1 by fractionation. After fractionation, this material showed at oughness close to that of aromatic polycarbonates (E = 1GPa, e = 122 %). This was aremarkable feat both from asynthetic as well as amaterials science point of view,b ecause polyarylenes are inherently chemically more stable than polycarbonates as aresult of the exclusive presence of aryl-aryl bonds in the backbone. Subsequently,S chlüter et al. investigated various kinked polyarylenes,w ith some of them exhibiting toughness at high molecular weight after fractionation. [3][4][5][6][7] Our interest in tough polyarylenes stems from their potentially ideal use as stable,tough yet amorphous matrices for covalent incorporation of mechanochromic dyes,s uch as spiropyrans (SPs). [8][9][10] SPs isomerize to their colored merocyanine (MC) form under an umber of external stimuli, including force,and can therefore be used as mechanical force sensors. [11][12][13][14][15] Compared to matrix polymers used so far in combination with covalently linked SPs,t ough polyarylenes have several distinct advantages.T heir toughness allows drawing samples and thus to transduce mechanical force to SP.A lso,t hey are typically amorphous,h ence mechanochromism can be investigated within an isotropic matrix which is not possible with semi-crystalline polymers.H erein we show that the nature and high strength of polyarylenes is of striking additional advantage in that high forces can be transduced to SP co-monomers.We found existing methods for kinked polyarylene synthesis to be cumbersome,expensive,and inefficient owing to significant loss of polymer during fractionation. ...
Starting from ortho-bromostyrenes, 12 6,7-benzotropolones were synthesized in 3 or 4 steps. Br/Li exchanges provided the respective ortho-lithiostyrenes. The latter were acylated by a Weinreb amide, if unhindered, or hydroxyalkylated by an aldehyde, if hindered (an oxidation ensuing thereafter). Acetonide-containing benzannulated nonatrienones resulted. Ring-closing olefin metatheses converted them into acetonide-containing benzocycloheptadienones. Treatment with aq. HCl caused hydrolyses followed by β-elimination reactions giving 6,7-benzotropolones.
A catalytic conjunctive cross-coupling reaction is developed that allows the construction of chiral organoboronic esters from alkylboron ate complexes and alkyl iodide electrophiles. The process occurs most efficiently with a Ni/Pybox-comprised catalyst and with an acenapthoquinone-derived boron ligand. Because of the broad functional group tolerance of this reaction, it can be a versatile tool for organic synthesis. Applications to the construction of (R)-coniine and (−)-indolizidine 209D are described.
A simple and high‐yielding route to tough polyarylenes of the type poly(meta,meta,para‐phenylene) (PmmpP) is developed. PmmpP is tough even in its as‐synthesized state which has an intermediate molar mass of Mw≈60 kg mol−1 and exhibits outstanding mechanical properties at further optimized molecular weight of Mw=96 kg mol−1, E=0.9 GPa, ϵ=300 %. Statistical copolymers with para,para‐spiropyran (SP) are mechanochromic, and the toughness allows mechanochromism to be investigated. Strained samples instantaneously lose color upon force release. DFT calculations show this phenomenon to be caused by the PmmpP matrix that allows build‐up of sufficiently large forces to be transduced to SP, and the relatively unstable corresponding merocyanine (MC) form arising from the aromatic co‐monomer. MC units covalently incorporated into PmmpP show a drastically reduced half life time of 3.1 s compared to 4.5 h obtained for SP derivatives with common 6‐nitro substitution.
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