Hexadienes cyclopolymerization

The second termination reaction is alkyl chain end transfer from the active species to aluminium [155]. This termination becomes major one at lower temperatures in the catalyst systems activated by MAO. XH and 13CNMR analysis of the polymer obtained by the cyclopolymerization of 1,5-hexadiene, catalyzed by Cp ZrCl2/MAO, afforded signals due to methylenecyclopentane, cyclopentane, and methylcyclopentane end groups upon acidic hydrolysis, indicating that chain transfer occurs both by /Miydrogen elimination and chain transfer to aluminium in the ratio of 2 8, and the latter process is predominant when the polymerization is carried out at — 25°C [156]. The values of rate constants for Cp2ZrCl2/MAO at 70°C are reported to be kp = 168-1670 (Ms) 1, kfr = 0.021 - 0.81 s 1, and kfr = 0.28 s-1 [155]. 

Recently, a metallocene/MAO system has been used for the polymerization of non-conjugated dienes [204, 205]. The cyclopolymerization of 1,5-hexadiene has been catalyzed by Zieger-Natta catalyst systems, but with low activity and incomplete cyclization in the formation 5-membered rings [206]. The cyclopolymerization of 1,5-hexadiene in the presence of ZrMe2Cp2/MAO afforded a polymer (Mw = 2.7 x 107, Mw/Mn = 2.2) whose NMR indicated that almost complete cyclization had taken place. One of the olefin units of 1,5-hexadiene is initially inserted into an M-C bond and then cyclization proceeds by further... 

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. 

Cyclopolymerization of Nonconjugated Dienes. Cyclopolymerization is an addition polymerization that leads to introduction of cyclic structures into the main chain of the polymer. Nonconjugated dienes are the most deeply studied monomers for cyclopolymerization and for cyclocopolymerizations with alkene monomers 66 In general, (substituted and unsubstituted) dienes with double bonds that are linked by less than two or more than four atoms cannot undergo efficient cyclization and result in crosslinked materials.12 In fact, efficient cyclopolymerization processes have been described, for instance, for a,oo-dienes like 1,5-hexadiene, 2-methyl-l,5-hexadiene, 1,6-heptadiene, and 1,7-octadiene,67 73 which lead to formation of homopolymers and copolymers containing methylene-1,3-cycloalkane units. 

Recent advances in the development of well-defined homogeneous metallocene-type catalysts have facilitated mechanistic studies of the processes involved in initiation, propagation, and chain transfer reactions occurring in olefins coordi-native polyaddition. As a result, end-functional polyolefin chains have been made available [103].For instance, Waymouth et al.have reported about the formation of hydroxy-terminated poly(methylene-l,3-cyclopentane) (PMCP-OH) via selective chain transfer to the aluminum atoms of methylaluminoxane (MAO) in the cyclopolymerization of 1,5-hexadiene catalyzed by di(pentameth-ylcyclopentadienyl) zirconium dichloride (Scheme 37). Subsequent equimolar reaction of the hydroxyl extremity with AlEt3 afforded an aluminum alkoxide macroinitiator for the coordinative ROP of sCL and consecutively a novel po-ly(MCP-b-CL) block copolymer [104]. The diblock structure of the copolymer... 

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. 

Optically active catalyst 1 can be obtained either by enantiomer-selective reaction of rac.-2 with optically active lithium (l,l -binaphthyl)-2,2 -diolate or by direct resolution by chiral HPLC. Optically active 21 and 22 in addition to 1 were successfully obtained by HPLC resolution and used for the polymerization of 1,5-hexadiene [60-62], Both catalysts gave an optically active polymer through cyclopolymerization. The optical activity and the content of tranj-structure in the main chain of the polymers obtained with 21 and 22 were comparable with those of the polymers synthesized with 1 [61,62],... 

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. 

L. Cavallo, G. Guerra, P. Corradini, L. Resconi, and R. M. Waymouth, Macromolecules, 26, 260 (1993).. Model Catalytic Sites for Olefin Polymerization and Diastereoselectivity in the Cyclopolymerization of 1,5-Hexadiene. 

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. 

With metallocene catalysts, not only homopolymers such as polyethylene or polypropylene can be synthesized but also many kinds of copolymers and elastomers, copolymers of cyclic olefins, polyolefin covered metal powders and inorganic fillers, oligomeric optically active hydrocarbons [20-25]. In addition, metallocene complexes represent a new class of catalysts for the cyclopolymerization of 1,5- and 1,6-dienes [26]. The enantio-selective cyclopolymerization of 1,5-hexadiene yields an optically active polymer whose chirality derives from its main chain stereochemistry. 

Cyclopolymerization of 1,5-hexadienes Oligomerization to Optically Active Hydrocarbons Polymerization in the Presence of Filling Materials... 

Cyclopolymerization of 2-methyl-l,5-hexadiene is catalyzed by a cationic zir-conocene complex [75]. Isolation of methylenecyclopentane derivatives 55-57 from the low molecular weight oligomeric products provides convincing evidence for chain transfer via P-methyl elimination. 

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. 

Waymouth and Coates employed the homogeneously catalyzed cyclopolymerization of 1,5-hexadiene giving poly (methylene-1,3-cyclopentane) as previously developed in his group in order to utilize the stereoselectivity of the monomer insertion for the construction of a polymer with main-chain chirality. The cyclopolymerization is a remarkable chain growth reaction during which a conventional... 

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