On the Polymerization of Acetylenic Derivatives. XXIV. Some Structural Peculiarities of Poly(α-ethynylnaphthalene)

Abstract

ABSTRACT:The present paper deals with the study of conjugated chains of a trans-cisoidal poly(a-ethynylnaphthalene), (a-EN), obtained by bulk polymerization of a-EN with (PPha)2· NiBr2. Based on the results obtained from electronic spectra and electrical conductivity measurements, using the fractions with molecular weight between 3732 and 528, a polymer structure built up from interrupted conjugated blocks is proposed. It is assumed that this interruption of the electronic conjugation system has structural reasons i.e., it is given by the rotation of parts of the polymer chain around some C-C single bonds, thus hindering the complete overlapping of certain rr orbitals. These conjugated blocks have 3-4 conjugated or partial conjugated monomeric units. This structure could explain the low values of electrical conductivity as well as the paramagnetic centers of polyacetylenes with aromatic substituents.KEY WORDS a-Ethynylnaphthalene / Trans-Cisoidal / Conjugated Block / Electrical Conductivity / Electronic Spectra / Paramagnetic Center / Partial Conjugation / Taking into account the effective interaction energy (for Little's four-carbon unit) calculated 1 for ethynylnaphthalene (-2.1 eV), and using a value included among the energy values required by Little for a macromolecular superconductor; we proposed to study the structures and properties of poly(a-ethynylnaphthalene) 3 " 4 and poly (,8-ethynylnaphthalene). 5 First of all, the question arises if it is possible for a head-to-head (or tail-to-tail) structure for poly(ethynylnaphthalene) to obey the structure conditions required by Little's theory. The reduced probability to get such a structure by the polymerization of aromatic acetylenes 6 does not justify the unsatisfactory electrical properties of the polymers, since the value of the effective interaction energy, calculated for the two-carbon unit, is 0.252 e V. 1 This value is also included within the limits required by Little's theory for an organic superconductor.However, the electrical properties of the semiconducting3·4 and superconducting polymers 7 • 8 synthesized did not fulfill our hopes. EXPERIMENTAL Materialsa-Ethynylnaphthalene (a-EN), was synthesised by Leroy's method 9 from a-acetylnaphthalene (a-AN) by chlorination and subsequent dehydrochlorination. Characteristic of the IR spectrum of a-EN are the 2190 cm-1 (llcaac) and 3380 cm-1 (liaac-H) bands (Figure 1). The NMR spectrum of a-EN exhibits at 3.35 ppm the resonance of ethynylic proton and at 7. 05-7.80 and 8.15-8.40 ppm the peaks of aromatic protons (Figure 2). NiBr2· (PPh3)2 was synthesized by Venanzi's method. 10 Poly(a-ethynylnaphthalene) was obtained by bulk polymerization of a-EN with 5-% NiBr2-(PPh3)s at 140°C and was purified by reprecipitation four times from benzene by the addition of methanol. 4 The IR spectrum of poly(a-ethynylnaphthalene) synthesized by this method is characteristic of a trans-configuration (Figure 1). The IR spectrum of a cisconfiguration presents the cis C-H out-of-plane deformation at 700 cm-1 • 4 According ...

Polyarylacetylenes. Structure and properties

“…Scheme 5 from interrupted conjugated blocks, and for a given conformation, the electrical and paramagnetic properties do not depend on the polymer molecular weight. This fact was already proved (Table IV) [48]. All polyarylacetylenes whose structure does not change with temperature (i.e., trans-polymers) obey the Curie law (Fig.…”

Polymerization of acetylenic derivatives. XXX. Isomers of polyphenylacetylene

“…3a), which was assigned to protons 2,4, and 6 of 1,3,5-triphenylbenzene. 11 According to these NMR spectra the structures C could have a trans-cisoidal configuration.…”

“…Two fractions were separated from polymers obtained with Ziegler-Natta catalysts: one insoluble in benzene (I) and one soluble in benzene but insoluble in methanol (11). Cationic and thermal polymerizations and polymerizations of PA with (PPh3)yPdClz yielded only to benzene-soluble, methanol-insoluble fractions.…”

Acetylenic Polymers: Syntheses, Structures, and Functions

“…However, the catalysts could only polymerize sterically unhindered monosubstituted acetylenes into insoluble polymers and/or soluble oligomers. Examples include the polymerizations of phenylacetylene ( 1 ) by Berlin, Percec, − Kern, and Ehrlich . Efficient polymerization of 1 was achieved by Masuda and Higashimura in 1974: WCl 6 and MoCl 5 catalyzed the polymerization of 1 to give poly(phenylacetylene) (PPA) with molecular weights over 10 000 .…”