Hexadienes polymerizations

EPM and EPDM can be produced by solution polymerization, while suspension and slurry polymerization are viable options. EPDM can be gas-phase 1,-4 hexadiene polymerized using Ziegler-Natta catalysts. Union Carbide produces ethylene propylene rubber (EPR) using modified Unipol low-pressure gas-phase technology. 

Results of 1,5-Hexadiene Polymerization with Metallocene Catalysts 2b, 4b, and 7... 

Hexadiene was found to polymerize in DCA and apoCA canals on heating over 100°C for 10 to 20 days after Y-ray irradiation. Particullary trans, trans-2,4-hexadiene polymerized in an inclusion state via radical mechanism for the first time. The polymers from the monomer prefer erythro structure to threo structure in a DCA canal, while they do slightly threo to erythro in an apoCA canal. It is considered that the polymerization proceeded preferentially in the canals through trans opening to yield erythro diisotactic structure in a DCA canal [ll]. 

In addition to living propylene polymerization, vanadium acetylacetonate complexes have also been shown to be living for 1,5-hexadiene polymerization and 1,5-hexadiene/ propylene copolymerization (Doi et al, 1989). At —78°C, l/Et2AlCl polymerized 1,5-hexadiene to produce a low molecular weight polymer (M = 6600 g/mol, M IM = 1.4) that contained a mixture of MCP and VTM units in a 54 46 ratio. The distribution of these two units varied in 1,5-hexadiene-propylene random copolymers as a function of 1,5-hexadiene incorporation. 

Finally, Coates and coworkers found that bis(phenoxyimine) titanium complex 31 was also capable of living 1,5-hexadiene polymerization and 1,5-hexadiene/propylene copolymerization (Hustad and Coates, 2002). Homopolymerization of 1,5-hexadiene with 31/MAO at 0°C produced a high molecular weight polymer with a narrow PDI (M = 268 000 g/mol, Mw/Mn = 1.27). The polymer showed the presence of two distinct units - the expected MCP units as well as 3-vinyl tetramethylene (3-VTM) units. As shown in Scheme 9.4, the MCP units are proposed to arise from 1,2-insertion of 1,5-hexadiene followed by a 1,2-cyclization. However, an initial 2,1-insertion of 1,5-hexadiene followed by a 1,2-cyclization forms a... 

Figure 4 Polymers derived from 1,5-hexadiene polymerization.

In addition to cyclic olefins, Hustad and Coates found that bis(phenox3dmine) titanium complex 50 (Figure 15) was also capable of living 1,5-hexadiene polymerization and... 

Kinetic studies using 1,9-decadiene and 1,5-hexadiene in comparison widi catalyst 14 and catalyst 12 demonstrate an order-of-magnitude difference in their rates of polymerization, widi 14 being the faster of the two.12 Furdier, this study shows diat different products are produced when die two catalysts are reacted widi 1,5-hexadiene. Catalyst 14 generates principally lineal" polymer with the small amount of cyclics normally observed in step condensation chemistry, while 12 produces only small amounts of linear oligomers widi die major product being cyclics such as 1,5-cyclooctadiene.12 Catalyst 12, a late transition metal benzylidene (carbene), has vastly different steric and electronic factors compared to catalyst 14, an early transition metal alkylidene. Since die results were observed after extended reaction time periods and no catalyst quenching or kinetic product isolation was performed, this anomaly is attributed to mechanistic differences between diese two catalysts under identical reaction conditions. 

Polymerization/lsomerization. The polymerization of 5-methyl-1,4-hexadiene (>99% pure) was carried out in n-pentane with a (5-TiCl3/Et2AlCl catalyst at 0°C according to the procedure described previously (14). To assess monomer disappearance and identify isomerization products, samples were withdrawn at specified intervals from the reaction mixture for GLC analysis (14). The final polymer conversion was determined by precipitation in excess methanol. 

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. 

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...

The 1,2-polymerization of 5-methyl-l,4-hexadiene was further confirmed by ozonolysis of the polymer. The resulting solution, after triphenylphosphine treatment, contained only acetone and no detectable formaldehyde by GLC. As shown below in Scheme 1, the volatile products expected by the above chemical treatment of poly(5-methyl-l,4-hexadiene) are acetone and formaldehyde if the polymer was formed by 1,2- and 4,5-polymerization, respectively. 

The poly(5-fnethyl-l, 4-hexadiene) fiber pattern (Figure 6) gave an identity period of 6.3 A, indicating a 3 isotactic helix structure. The X-ray diffraction pattern was not very sharp, which may be due to the difficulty of the side chain with a double bond to fit in a crystalline lattice. The crystallinity was determined to be 15% using the Hermans and Weidinger method (27). A Chloroform-soluble fraction free from catalyst residues showed no improvement in the sharpness of the X-ray diffraction pattern. These data show that the configuration of the 1,2-polymerization units in the homopolymer of 5-methyl-1,4-hexadiene is isotactic. 

The reduced reactivity of 5-methy1-1-hexene is consistent with the expected steric effect due to methyl substitution at the 5-carbon position. Apparently, the internal double bond in 5-methyl-l,4-hexadiene assists in its complexation at the active site(s) of the catalyst prior to its polymerization and thereby the "local concentration" of this monomer is higher than the feed concentration during copolymerization with 1-hexene. This view is consistent with the observation that the overall rates of polymerization, under the same conditions, are much lower for the system containing 5-methyl-1,4-hexadiene. 

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. 

The trans/cis ratio of the product must, therefore, be determined at an earlier reaction stage and most probably by the ratio of species 27a and 27b. Steric or electronic factors affecting this ratio will influence the trans/cis ratio of the resulting 1,4-hexadiene. The phosphine and the cocatalyst effect on the stereoselectivity can thus be interpreted in terms of their influence on the mode of butadiene coordination. Some earlier work on the stereospecific synthesis of polybutadiene by Ni catalyst can be adopted to explain the effect observed here, because the intermediates that control the stereospecificity of the polymerization should be essen-... 

In the polymerization of butadiene, Teyssie (52-54) has shown that certain electron donors, such as alcohols or phosphines, can convert tt-allylnickel chloride from a catalyst which forms c/j-polybutadiene to one which produces frans-polybutadiene. These ligands presumably block a site on the nickel atom, forcing the butadiene to coordinate by only one double bond. While alcohols cannot be added directly to the hexadiene catalyst (as they deactivate the alkylaluminum cocatalysts), incorporation of the oxygen atom on the cocatalyst places it in an ideal position to coordinate with the nickel. The observed rate reduction (52) when the cri-polybutadiene catalyst is converted into a fra/w-polybutadiene catalyst is also consistent with the observed results in the 1,4-hexadiene synthesis. 

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