Pentadiene-1,3 Polymerisation

Among commercial methods of isoprene production, one of the main methods is the process of two-stage isopentane dehydrogenation. Isopentane and isopentene dehydrogenation is carried out at elevated temperatures followed by the formation of a number of by-products among which a great part is piperylene (pentadiene-1,3) [9, 10]. Piperylene yield is 10-15% of the sum of the diene monomers during isoprene production. 

The use of different catalytic systems of both ionic [10] and ionic-coordination [13, 14] types in piperylene polymerisation has been proposed. Because of a high reaction rate of chain transfer to monomer (which grows as catalyst acidity increases) and to solvent (which drops as solvent polarity increases) in cationic polymerisation, a polymer with low MW is obtained. Some halides of metals of groups III-V were tested as catalysts of cationic piperylene polymerisation the most suitable were TiCl4 and SnC. The application of SbCl5 and InClj does not ensure an acceptable polymerisation rate and in the case of using AICI3, the insoluble polymer is obtained. 

Electrophilic (cationic) piperylene polymerisation may be schematised as follows  

The elementary steps of the cationic piperylene polymerisation process are similar to those for isobutylene. 

The electrophilic catalysts used for cationic piperylene oligomerisation display different activity which stipulates in principle, the ability of upgrading the commercial production of synthetic drying oil at the stage of cationic piperylene oligomerisation by selecting a catalytic system with corresponding activity and selectivity. 

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The coordination polymerisation of 1,3-pentadiene is of particular interest owing to the complex phenomena of stereoisomerism displayed by its polymers. This monomer and other monomers of the CH2=CH—CH=CHR type exist in two isomeric forms - entgegen (E) and zusammen (Z) ... 

Both isomers are polymerised by some coordination catalysts, whereas with others only the (E) isomer is polymerised. Of the various possible stereoregular poly(l,3-pentadiene)s, only five have been prepared so far cis- 1,4-isotactic [11,12], trans- 1,4-isotactic [13], cA-l,4-syndiotactic [14,15], 1,2-cA-syndiotactic [16] and 1,2-traro-syndiotactic [17] polymers. In addition to these stereoregular polymers, amorphous polymers of various types, consisting of different monomeric units in variable proportions, have been obtained [7]. 

Table 5.2 Polymerisation of isoprene, 1,3-pentadiene and 2,4-hexadiene with aluminium alkyl-activated Ziegler-Natta catalysts...

A particularly high reactivity is exhibited by 4-methyl-1,3-pentadiene in polymerisation with the homogeneous catalysts discussed this monomer is 50-100 times more reactive than butadiene [38,39,43,117],... 

The validity of the proposed mechanism is borne out by the results of the polymerisation of (E,E)-1 -(2H)-1,3-pentadiene with Nd(OCOR)3—AlEt2 Cl A1 (z-Bu)3 and Co(Acac)2 AIEt2Cl—H20 catalysts [20-22,195], The Nd-based catalyst gave a czj-1,4-eryt/zro-diisotactic polymer, while the Co-based catalyst yielded a cA-l,4-t/zra>-disyndiotactic polymer. The formation of such polymers is shown by the schemes in Figure 5.5 [7,41],... 

Syndiotactic l,2-poly(4-methyl-l,3-pentadiene) has been formed by polymerisation with homogeneous catalysts, e.g. TiBz4—[Al(Me)0]x and CpTiCl3—[Al (Me)0]x [41,43]. The coordination of the monomer as an s-trans-t/2 ligand rather than an s-cis-r A ligand at the Ti atom has been postulated to be involved in the polymerisation. The s-cis-r A monomer coordination is less favoured for steric reasons in the case of 4-methyl-1,3-pentadiene. A possible scheme for the formation of the 1,2-syndiotactic polymer from this monomer is presented in Figure 5.7 [41,43],... 

It is interesting that the polymerisation of 4-methyl-1,3-pentadiene proceeds faster at —20 °C than at 20 °C. It has been suggested [43] that at a temperature below 0°C this monomer coordinates as an s-trans-t]2 ligand. At a higher temperature it can probably coordinate either as an s-trans-tf ligand or as an s-cis-tf ligand, but monomeric units in the polymer are derived mainly from the... 

The other components are C5+ olefins and dienes and in particular cycto-pentadiene which easily dimerises to a Cio compound di-cyclo-pentadiene). As well as a strong odour, these materials readily polymerise to form gum in the gasoline and the raw pyrolysis gasoline is usually hydro-treated prior to use. In some cases, these C5 dienes are extracted and used to form low melting resins. One approach to upgrading the pyrolysis gasoline stream is shown in Figure 5.5 . 

One of the issues that concern liquid feedstock cracking operations is a higher rate of fouling. This is not only a consequence of heavier coke forming precursors, but also as a consequence of long lived free radicals which act as agents for the formation of a polymer (often referred to as pop-corn polymer) in the primary fractionator and downstream units. For instance, free radicals based on styrene or indene have sufficiently long half-lives to pass from the pyrolysis section into the primary fractionator. These can concentrate in this unit and produce polymer (free radical polymerisation) when sufficient amounts of suitable olefins are present, in particular styrene itself and di-olefins such as cyclo-pentadiene or butadiene. 

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