Metallocenes cyclopolymerization

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. 

As for olefins different from propene, molecular modeling studies have also been able to rationalize the dependence on metallocene symmetry of E-Z selectivity for 2-butene copolymerization as well as the stereoselectivity of the cyclization step, which determines the cis or trans configuration of the rings, for cyclopolymerization of nonconjugated dienes. 

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... 

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. 

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. 

Asymmetric synthesis cyclopolymerization of 1,5-pentadiene (261) was performed with an optically active metallocene catalyst. The polymer (262) obtained by (S)-ethylenebis(tet-rahydroindenyl)zirconium (S)-binaphtholate ([a] 435+1848°) in the presence of methyl aluminoxane (MAO) showed molecular rotation [(p] 4os-49.3°, and NMR analysis showed that the polymer had -68% tram structure. An optically active copolymer consisting of cyclic 262 units and linear units formed by 1,2-insertion shows LC phases. ... 

An interesting type of polymerization—cyclopolymerization—occurs with certain bridged metallocenes exposed to short o ,( -dienes such as 1,5-hexadiene (205,206). The homopolymerization, shown by (equation 29, involves the initial 1,2-insertion of the first vinylic group followed by the 1,2-insertion of the second, leading to cyclic groups. 

Cyclopolymerization. As discussed earlier, nonconjugated dienes can be polymerized with metallocene-based catalysts to afford cyclopol5miers. In contrast to linear polyolefins which have only two microstructures of maximum order (isotactic and syndiotactic), cyclic polymers have four microstructures due to the possibility of configurational isomerism (cis vs trans) in the main chain (Fig. 16). Of these the frares-diisotactic structure contains no mirror planes of S5unmetry and is chiral by virtue of its main-chain stereochemistry (481). Two criteria for chirality of this microstructure are the presence of trans rings and isotacticity (the same... 

This chapter will discuss various homogeneous catalyst systems for the cyclopolymerization of nonconjugated diolefins, particularly 1,5-HD, which control the stereochemistry and microstructure and therefore the physical properties of the polymers synthesized. In addition, the effect of metallocene structure on ethylene/1,5-HD and propylene/l,5-HD copolymerizations will be described. 

FIGURE 19.1 Maximum order structures of the cyclopolymers produced by the cyclopolymerization of 1,5-HD (a), 1,6-heptadiene (b), and 1,7-OD (c) with metallocene catalysts. These structures are also referred to as cis-diisotactic (me o-diisotactic), cis-disyndiotactic (m 56>-disyndiotactic), trans-diisotactic (racemo-diisotactic), and trans-disyndiotactic (rac mo-disyndiotactic). 

Stereoregular Cyclopolymerization of 1, 5-Hexadiene with Metallocene Catalysts... 

Recently, Choo and Way mouth performed the copolymerization of ethylene with 1,5-HD using various metallocene catalysts (12, 13, 14, Figure 19.2). 1,5-HD cyclopolymerized exclusively to give MCP units in the copolymers, with only traces of uncyclized 1,2-inserted 1,5-HD. The diaste-reoselectivity of the cyclocopolymerization favored the formation of 1,3-cyclopentane rings for metallocenes (74% trans for 12, 81% trans for 13, and 66% trans for 14). For metallocenes 12 and 14, the ethylene/1,5-HD copolymerization yielded copolymers with similar comonomer compositions and sequence distributions to those observed for ethylene/1-hexene copolymerization with these catalysts. On the other hand, the copolymers derived from metallocene 13 showed very different compositions and sequence distributions. At comparable comonomer feed ratios, the poly(ethylene-c -l,5-HD)s were enriched in the 1,5-HD comonomer and deficient in ethylene as compared to the analogous polymers prepared from ethylene and 1-hexene. The copolymerization behavior of 13 provided support for a dual-site alternating mechanism for 1,5-HD incorporation, wherein one coordination site of the active catalyst center is highly selective for the initial 1,2-inserion of 1,5-HD and the other site is selective for cyclization. 

Polyolefins with cyclic units in the backbone show high TgS and high transparencies, and thus are suitable for optical and medical applications." " Even though they can be prepared by the copolymerization of ethylene and cyclic olefins (such as norbomene) using metallocene catalysts, the cyclopolymerization of nonconjugated dienes offers another access route into cyclopolymer materials. 

This unified volume explains the mechanistic basics of tactic polymerizations, beginning with an extensive survey of the most important classes of metallocene and post-metallocene catalysts used to make polypropylenes. It also focuses on tactic stereoblock and ethylene/propylene copolymers and catalyst active site models, followed by chapters discussing the structure of more stereochemically complex polymers and polymerizations that proceed via non-vinyl-addition mechanisms. Individual chapters thoroughly describe tactic polymerizations of a-olefins, styrene, dienes, acetylenes, lactides, epoxides, acrylates, and cyclic monomers, as well as cyclopolymerizations and ditactic structures, olefin/CO copolymers, and metathesis polyalkenamers. 

Figure 23 The metallocene-mediated cyclopolymerization of 1,5-hexadiene can be (a) trans selective or (b) cis selective, depending on the catalyst structure, (c) The cyclopolymerization of a,a>-dienes occurs efficiently for 1,5-hexadiene, 1,6-hepfadiene, and 1,7-ocfadiene.

S. a,(a-IHolefin Polymers (cychpofymerization) Cyclopolymerization of 1,5-hexadi-ene using metallocene catalyst (5,5)-l in the presence of MAO gives an optically active polymer 20 (Scheme 11.4) [58,59]. The polymerization proceeded exclusively via the cyclization mechanism, and the obtained polymer 20 was rich in rrans-units (73%) having no plane of symmetry and it showed molecular rotation of [ ]4os -51.2°. 

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