Trans-Hexadiene polymer

Figure 1. GLC data for the polymerization and isomerization of trans-1,4-hexa-diene at 25°C with a Et3Al/S-TiCls catalyst (Al/Ti atomic ratio is 2). n-Hexane was used as the internal standard. Key O, trans-7,4-hexadiene , polymer A, cis-2-tzans-4-hexadiene tTans-2-trans-4-hexadiene A, 1,3-hexadiene. Reproduced, with permission, from Ref. 14. Copyright 1980, John Wiley Sons, Incorporated...

In contrast to the spectrum of isotactic trans-l,4-hexadiene polymer (Figure 5), the 300 MHz -H-NMR spectra of the 5-methylhexadiene polymer in both CCI4 and CgDg solutions exhibit only one peak for its backbone methylene protons. As in the case of cis-l,4-hexadiene polymer (14), the backbone methylene protons were not resolvable. The absence of a doublet for the methylene protons in these polymers does not necessarily preclude the possibility that they are isotactic. 

To clarify the tacticity problem, trans-l,4-hexadiene and 5-methyl-l,4-hexadiene polymers were examined by X-ray diffraction. Fiber diagrams were obtained from samples stretched to four times their original lengths. Eight reflections from the poly(trans-1,4-hexadiene) fiber pattern may be interpreted on the0basis of a pseudo-orthorhombic unit cell with a = 20.81 + 0.05 A b =... 

Isomer of 2,4-hexadiene Polymer trans, trans cis, trans... 

We have reported earlier (14) that during the polymerization of trans-l,4-hexadiene with a Et3Al/6-TiCl3 catalyst (Al/Ti atomic ratio = 2) at 25°C, a major portion of the consumed monomer was converted to isomerized products, thereby accounting for the relatively low conversion to isotactic 1,2-polymer (Figure 1). The relative amounts of the hexadiene isomerization products were in the following order cis-2-trans-4-hexadiene> trans-2-trans-4-hexadiene> 1,3-hexadiene > 1,5-hexadiene >cis-2-cis-4-hexadiene. 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

Scheme 68 illustrates cyclopolymerization of 1,5-hexadiene catalyzed by a homogeneous chiral zirconocene complex to form optically active poly(methylenecyclopentane), whose chirality derives from configurational main-chain stereochemistry (757). This polymer is predominantly isotactic and contains predominantly trans cyclopentane rings. 

The symmetry properties of cycloaliphatic polymers are such that polymers with certain microstructures, e.g. tram-isotactic poly (methylene-1,3-cyclopen-tane), are chiral therefore, the cyclopolymerisation of a, trans selective catalysts of C2 symmetry, such as methylaluminoxane-activated resolved (li )-(Thind CH2)2Zr l,l -bi-2-naphtholate, yielded optically active tram-isotactic poly(-methylene-1,3-cyclopentane). The cyclopolymerisation with the (15) enantiomer of the catalyst gave an enantiomeric polymer [505], On the basis of analysis of 13C NMR spectra, the degree of enantioface selectivity for this cyclopolymerisation was estimated to be of 91% [503,505]. 

Stereoregular polymers that can be afforded by 2,4-hexadiene and other symmetric terminally disubstituted butadienes (of the CHR CH CH CHR type) exhibit still more complex stereoisomerism, since each monomeric unit in these polymers possesses three sites of isomerism. The formation of these polymers involves 1,2- and 1,4-polymerisation. The 1,2-polymers derived from the CHR=CH—CH=CHR monomers exhibit the same type of stereoisomerism as polymers with a 3,4 structure obtained from monomers of the CH2 CH CH=CHR type. However, owing to the presence of the R substituent at the double bond in the side group of the polymer derived from a monomer of the CHR=CH—CH=CHR type, two types of eryt/zro-diisotactic, t/zraz-diisotactic and disyndiotactic polymer are foreseeable, each type with either cis or trans configuration of the double bond, as in the 1,2-polymer derived from a monomer of the CH2 CH CH CHR type. Thus, six stereo-isomeric forms of 1,2-polymer are possible for the CHR CH CH CHR monomer. The 1,4 monomeric units in the polymers formed by the polymerisation of CHR CH CH CHR monomers contain one double bond (in either cis or trans configuration) and two tertiary carbon atoms and therefore can exist as two sets of enantiomers, erythro and threo ... 

An even more significant difference in microstructures was found for polymers obtained by the Nd catalyzed polymerization of BD, IP, E-l,3-pentadiene and E,E-2,4-hexadiene. The first three monomers yield polymers with high cis- 1,4-contents whereas the latter monomer yields a polymer with 100% trans-1,4-configuration [495,496]. 

A study on the homo- and copolymerization of a variety of dienes such as 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, E-l,3-pentadiene, E-l,3-hexadiene, E-l,3-heptadiene, E-l,3-octadiene, E,E-2,4-hexadiene, E-2-methyl-l,3-pentadiene, 1,3-cyclohexadiene mainly focused on mechanistic aspects [139]. It was shown that 1,4-disubstituted butadienes yield frans-1,4-polymers, whereas 2,3-disubstituted butadienes mainly resulted in cis- 1,4-polymers. Polymers obtained by the polymerization of 1,3-disubstituted butadienes showed a mixed trans-1,4/cis-1,4 structure (60/40). The microstructures of the investigated polymers are summarized in Table 26. 

The polymerization tests with ethylene and 1-olefines as well as with dienes showed a good ability of the metallocene catalyst for copolymerization. Interesting results from practical and theoretical point of view could be gained in the copolymerization of ethylene and 1,5-hexadiene. During polymerization first a complexation of one of the double bonds of 1,5-hexadiene takes place at the vacant coordination side of the transition metal. After insertion into the polymer chain the complexation of the second double bond occurs followed by intramolecular cyclisation of the 5-membered ring. Analysis of the 13C-NMR spectra reveals an incorporation of 4.2 mole% 1,5-hexadiene and a predominance of trans rings caused by the diastereoselectivity of the cyclisation step. 

The microstructure and the properties depend on the cis/trans ratio of the ring bonding and on the stereochemistry between the rings. Poly(methylene-l,3-cyclopentane) obtained by cyclopolymerization of 1,5-hexadiene shows four different structures from which the tram isotactic structure is predominant, when using simple biscyclopentadienyl compounds. Higher substituted (pen-tamethyl) zirconocenes yield mainly as-connected polymers which are highly crystalline and have melting points up to 190 °C. 

Polymers with ring structures, interspaced with CH2 groups, can be obtained by polymerization of 1,5-dienes. 1,2-Insertion of the terminal double bond into the zireonium-carbon bond is followed by an intramolecular cyclization forming a ring. Waymouth describes the cyclopolymerization of 1,5-hexadiene to poly (methylene-1,3-cyclopentane) [67]. Of the four possible microstructures, the optically active trans-, isotactic structure (Figure 4) is predominant (68%) when using a chiral pure enantiomer of [En(IndH4)2Zr](BINAP)2 and MAO. 

The stereochemistry of the C=C bond in the polymer chains that result from ADMET of dienes of the type H2C=CH-(CH2) -CH=CH2 tends to be mostly trans in contrast to the result from ROMP of simple cycloalkenes, where trans C=C bond content may not be the predominant stereochemistry. For example, ADMET polymerization of 1,5-hexadiene gave a linear polymer with a trans C=C bond content of over 70% (catalyzed by Schrock catalyst 35), which is close to the value expected on the basis of thermodynamics.59 Earlier (equation 11.21), we saw that a similar polyalkenamer results from ROMP of methylcyclobutene (catalyzed by (CO)5W=CPh2) this time the stereochemistry of the C=C bond was 93% cis.60... 

The polymers with trans-fused five-membered rings linked with a diisotactic head-to-tail sequence have chirality, although the polymers composed of the cis-fused ring are achiral. Scheme 10 summarizes the structures of the stereoisomeric polymers. The optically active zirconocene complex with a C2 symmetric structure catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Eq. 20) [98, 99]. Although the polymer contains not only trans-fused ring but also cis-fused ring units (ca. 68 32), it shows optical rotation due to the main chain chirality. 

Zirconocene complexes with ferrocenyl groups promote selective cyclopolymerization of 1,5-hexadiene to give a polymer with high content of the trans-unit (up to 98% trans selectivity) (Eq. 21) [100]. Sita reported living... 

A new type of enantioselective diene polymerization is found with cyclopolymerization of 1,5-hexadiene which leads to polymers with a saturated chiral main chain28,58>109. As catalyst, (—)-(7 )-[l,T-ethylenebis(4,5,6,7-tetrahydro-l-indenyl)]zirconium (/ )-binaphtholate is used in the presence of methylalumoxane to give optically active poly(methylene-1,3-cyclopentane) (3) with 68% trans configuration in the five-membered ring (diisotacticity). If the (S)-enantiomer of the ansa-metallocene with (ft)-binaphthol is used as catalyst then the opposite rotation of the polymer is observed58. 

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