Pentadiene-1,3 cationic polymerization

Double-stage dehydration of isopentane is one of the major methods among industrial ways of isoprene production. Dehydration of isopentane and isopentene is carried out at high temperatures and is accompanied by formation of by-products. The most noticeable among them is pentadiene-1,3 (piperylene) [239, 240]. Piperylene yield is 10-15% from diene monomers sum. 

One of the directions of piperylene application is cationic oligomerization for production of liquid rubber - the base of synthetic drying oil of SKOP brand. Use of piperylene hydrocarbon fraction as feed stock for synthetic drying oil production allows solving the problem of isoprene production by-product utilization and reducing consumption of plant oils in paintwork and other industry branches [206,241, 242]. 

It is accepted that piperylene oligomerization in the presence of electrophilic catalysts (TiCU, AICI3 OR2, SnCU and so on) proceeds according to traditional for cationic polymerization scheme. However, it was revealed in [240], that piperylene oligomers received in toluene medium contain 3-5 phenyl groups on 100 main piperylene units, i.e. toluene taken as a solvent take part in chain transfer reaction. This leads to molecular mass reduction under piperylene polymerization in benzene or toluene in comparison with paraffin hydrocarbons as solvents (900-1200 against 1260-1430). Mechanism of chain transfer (TiCU-HaO catalyst) can be presented by following scheme 

Ability of internal double bonds of forming macromolecule to further transformations initiated by cationic initiators is the characteristic particularity of cationic polymerization. This character leads to reduction of polydienes unsaturation due to the formation of brunched and cyclic structures even at low monomer conversions [175]. Total unsaturation of resulting polypiperylene usually is about 60-90% from theoretical [243]. One supposes [244] that these chain units consist of monocycle units of type I-III  

Structures I and III correspond to the most probable cycle units formation. Moreover, decreasing of total oligopiperylene unsaturation may occur at the expense of macromolecules bonding [243] that should be taken into account when selecting catalytic systems for polymerization. 

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Cationic polymerization provides, independent of the isomer of the 1,3-pentadiene, high trans-, A and trans-1,2 microstructures (Table 17). Studies on the insertion mechanism and the various side reactions have been carried out [373,374],... 

In comparison, the cationic polymerization of 3-methyl-1,3-pentadiene yields up to 70% cyclized poly(3-methyl-l,3-pentdiene) with 1,2- and 1,4-microstructure of the remaining double-bond portion [378]. 

These hydrocarbon resins are produced from the cationic polymerization of mixed olefin and diolefin feedstocks. These raw materials consist of C5 feedstocks such as pentenes, pentadiene, isoprene, and amylene, and C6 streams of hexenes and hexa-dienes. 

Controlled cationic polymerization of linear dienes, such as 1,3-pentadiene, has never been achieved, because frequent side reactions occur such as cross-linking, isomerization, cyclization, and chain transfer reaction (Figure 8). Bennevault-Celton et conducted a series of kinetic studies on the polymerization of 1,3-pentadiene initiated by AICI3 in a... 

Ever since 1962, when Williams, Okamura, and their associates started to publish propagation rate-constants k+p for the cationic bulk polymerization of cyclo-pentadiene, isobutene, styrene, a-methylstyrene and isopropylvinyl ether by ionizing radiations, these constants have been accepted as the best, most likely, values for the k+p of unpaired cations in a medium of low-polarity, and those obtained subsequently by Stannett and his collaborators, using similar methods, enjoyed the same status, (The loci classici are Bates et al. (1962), Bonin et al. (1964), Taylor Williams (1969) and the three papers by Ueno et al. (1967), Hayashi et al. (1967) and Williams et al. (1967).)... 

Natta, Porri and co-workers have studied the stereospecific polymerization of trans 1.3-pentadiene (51,52,53,54,55). This monomer acts as a propylene vinylogue in 1.2-polymerization. It requires a cationic catalyst such as alkylaluminum dichloride to produce the 1.2 polymer. 

Fig. 7. Anionic catalysts and their effectiveness for the steric control in the polymerization of butadiene and isoprene. Cationic catalysts in the polymerization of trans- 1.3-pentadiene...

Another observed difference between the polymerization of butadiene and pentadiene concerns the influence of the solvent on the type of polymer obtained. In polymerizing pentadiene, cis-1,4 polymers have been obtained only in aromatic solvents, while in aliphatic solvents, at least with catalysts prepared from A1(C2H5)2C1, polypentadienes having a 1,2 syndiotactic structure have been obtained (9). (The catalyst systems prepared from Al(C2H5)Cl2 complexes with pyridine or thiophene are practically insoluble in aliphatic solvents and yield only small amounts of low molecular weight polymer, probably of cationic origin.)... 

Methyl-l,3-pentadiene and 4-methyl-l,3-pentadiene are easily polymerized via a cationic route [376,377]. Even weak acids at low temperatures give high-molecular-weight polymers that are not cyclized and contain a large trans-1,4 portion. However, 4-methyl-1,3-pentadiene can also give mostly 1,2 structures at low yields. The same compounds that are used for the polymerization of 1,3-pentadiene are employed as acids (Table 18). 

Table 17 Polymerization of 1,3-pentadiene with cationic catalysts in benzene at 20°C. 

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