Syndiotactic poly[(S)‐4‐methyl‐1‐hexene] and poly[(R)(S)‐4‐methyl‐1‐hexene]

Abstract

As reported in the literature homogeneous catalytic systems based on isopropylidene(cyclopentadieny1) (9-fluorenyl)zirconium dichloride (Me,C(Cp)(9-Flu)ZrCI, and methylaluminoxane promote not only the syndiotactic-specific polymerization of propene but also the syndiotactic polymerization of linear (e. g. l-butene)'*') and /3branched a-olefins (e. g. 4-methyl-I -pentene) ' 9 ').In the presence of the quoted catalyst both (S)-4-methyI-l-hexene and racernic-4methyl-1 -hexene afford stereoregular crystalline polymers. The X-ray diffraction spectra and the I3C NMR spectra of poly[(S)-4-methyl-l-hexene] (sample a) and poly[(R) (S)-4-methyl-l-hexene] (sample b) are reported in Figs. 1 and 2, respectively. A comparison of the I3C NMR spectra with those of samples of the corresponding polymers obtained in the presence of isotactic-specific catalysts based on (dimethylsilylene)bis(l-indeny1)zirconium dichloride (Me,SiIn,ZrCl,) [isotactic poly[(S)-4-methyl-1 -hexenel, sample c] and TiCI, /MgCI, /A1 (i-C4H9)' [isotactic poly[(R) (S)-4-methyl-l-hexene], sample d] shows that samples a and b are not isotactic but reasonably syndiotactic as expected in view of the catalyst used for promoting the polymerization. The most interesting feature of the I3C NMR spectrum of sample a is the presence of two resonances of equal intensity at 40,4 and 40,6 ppm from HMDS (hexamethyldisiloxane) arising from methylene carbons. Each resonance has, approximately, one half of the intensity of the resonances of the other carbons of the monomer unit drawn in the following scheme: Scheme 1: 2 1 -CH-CH2-3 CH, 4 CH-CH, 5 CH, 6 CH,

Asymmetric Polymerization

“…The discrepancy may be attributed to a single -handed helical conformation [298] . Similar optical properties have been observed for syndiotactic poly -( S ) -215b prepared with a homogeneous zirconium catalyst [299] . Isotactic polymers obtained by anionic polymerization from optically active vinyl ketones also adopt a helical conformation in solutions [300 -302] .…”

Stereospecific and Stereoselective Polymerization of 4‐Methyl‐1‐hexene by Enantiomeric Binaphthyl‐Bridged Salen Dichlorozirconium (IV) Complexes

“…The measured M w values were lower than that of poly(1-hexene) samples produced by the same catalyst, but they were still significantly high (at least twice as large as the values reported in the literature [11,12] ).…”

“…The assignment of the resonances (see Figure 2), made on the basis of literature data, [11,13] revealed an essentially isotactic microstructure. The broadening of the signals at 18.02, 28.73, 40.28 and 41.41 ppm indicated the formation of an isotactic poly[(R)(S)-4MH] that is a copolymer of the two enantiomers of 4MH.…”

Molecular Mechanics and Stereospecificity in Ziegler−Natta 1,2 andCis-1,4 Polymerizations of Conjugated Dienes

“…More in detail, the models for the pre-insertion intermediates (Figure ) include an alkyl-substituted allyl (simulating the portion of a growing chain closest to the catalytic metal atom) and a diene molecule, anti -η 3 and cis -η 4 coordinated to a metal (titanium) atom, respectively. A cyclopentadienyl ligand (Cp in the following) has been also coordinated to the metal (Figure ), as proposed for the case of catalytic systems based on CpTiCl 3 , which are able to promote polymerization of butadiene and several methyl-substituted butadienes, − as well as styrene and substituted styrenes. , This choice is mainly due to our desire for a reasonable filling of the coordination sphere of the catalytic models and also presents the advantage of relating the model to a catalytic system whose stereospecific behavior is well established. − A further advantage of the catalytic models with a Cp coordinated to the metal is the possibility of assuming several geometrical parameters in analogy to the values observed for the crystalline structure of the (η 3 -allyl)(η 4 -butadiene)(η 5 -cyclopentadienyl)zirconium complex, as will be detailed in the next section. Without loss of generality our calculations refer only to models for which the anti -η 3 -allyl group (which is present in the pre-insertion intermediate) is re -coordinated to the metal, that is for the chirality of coordination of the allyl group shown in Figure .…”