Polyisoprene pyrolysis

From the time that isoprene was isolated from the pyrolysis products of natural mbber (1), scientific researchers have been attempting to reverse the process. In 1879, Bouchardat prepared a synthetic mbbery product by treating isoprene with hydrochloric acid (2). It was not until 1954—1955 that methods were found to prepare a high i i -polyisoprene which dupHcates the stmcture of natural mbber. In one method (3,4) a Ziegler-type catalyst of tri alkyl aluminum and titanium tetrachloride was used to polymerize isoprene in an air-free, moisture-free hydrocarbon solvent to an all i7j -l,4-polyisoprene. A polyisoprene with 90% 1,4-units was synthesized with lithium catalysts as early as 1949 (5). 

The usefulness of analytical pyrolysis in polymer characterization, identification, or quantitation has long been demonstrated. The first application of analytical pyrolysis can be considered the discovery in 1860 of the structure of natural rubber as being polyisoprene [10]. This was done by the identification of isoprene as the main pyrolysis product of rubber. Natural organic polymers and their composite materials such as wood, peat, soils, bacteria, animal cells, etc. are good candidates for analysis using a pyrolytic step. 

Another source of generating a variety of compounds during pyrolysis is the diversity of intramolecular transfer steps. This explains for example the formation of 1-methyl-4-isopropenylcyclohexene (limonene) during the pyrolysis of polyisoprene (see Section 6.1). 

Pyrolysis of polyisoprene and ion fragments formation from oligomers of isoprene. 

Pyrolysis of polyisoprene takes place by a free radical mechanism and generates mainly isoprene (MW=68), a dimer of isoprene (DL-limonene, MW=138), and several other unsaturated hydrocarbons (see Sections 2.6 and 6.1). The mass spectra fragmentation (generating fragments with specific mass/charge (m/z) ratios) of a model molecule simulating polyisoprene (isoprene oligomer) occurs by the mechanisms as indicated below ... 

The fragments from the pyrolysis and the mass spectra generated from a model isoprene oligomer are therefore very similar. This result is expected, due to the high number of it electrons in the polyisoprene molecule [22],... 

The compounds with the formula C6H7-(C5H8)k-C5H9 will have the molecular weight (68)k where k = 0,1,2. ... The formation of 1-methyl-4-isopropenylcyclohexene (DL-limonene) during the pyrolysis of polyisoprene also comes from an intramolecular transfer step ... 

Vulcanized rubber pyrograms frequently show, in addition to the compounds generated by the pyrolysis of polyisoprene, other compounds such as benzothiazole, which are generated by the pyrolysis of the additives. 

Polybutadiene, CAS 9003-17-2, is a common synthetic polymer with the formula (-CH2CH=CHCH2-)n- The cis form (CAS 40022-03-5) of the polymer can be obtained by coordination or anionic polymerization. It is used mainly in tires blended with natural rubber and synthetic copolymers. The trans form is less common. 1,4-Polyisoprene in cis form, CAS 9003-31-0, is commonly found in large quantities as natural rubber, but also can be obtained synthetically, for example, using the coordination or anionic polymerization of 2-methyl-1,3-butadiene. Stereoregular synthetic cis-polyisoprene has properties practically identical to natural rubber, but this material is not highly competitive in price with natural rubber, and its industrial production is lower than that of other unsaturated polyhydrocarbons. Synthetic frans-polyisoprene, CAS 104389-31-3, also is known. Pyrolysis and the thermal decomposition of these polymers has been studied frequently [1-18]. Some reports on thermal decomposition products of polybutadiene and polyisoprene reported in literature are summarized in Table 7.1.1 [19]. 

Figure 7.1.3. Pyrogram from a Py-GC/MS analysis of polyisoprene (cis), = 38,000. Pyrolysis done on 0.4 mg material at 600° C In He, with the separation on a Carbowax type column.

Figure 7.1.4. Time window between 44.0 min. and 70.0 min. for the pyrogram given in Figure 7.1.3. This window shows the trimer type compounds generated during pyrolysis of polyisoprene (cis).

Pyrolysis of polyisoprene takes place very similarly to that of polybutadiene. The p-chain scission (to the double bond) is the dominant initiation process, and the bond dissociation energy was estimated to be about 61.5-63 kcal mol V The cleavage of the carbon chain backbone has as the final result the elimination of some monomer, linear and cyclic dimers, trimers, etc. The yield of monomer depends on the heating conditions, degree of polymerization, etc. Table 2.1.1 indicates the formation of up to 58% monomer from natural rubber pyrolysis, while Table 7.1.4 shows only about 18% conversion. Some aspects regarding the thermal decomposition mechanism of polyisoprene were discussed in Section 2.1. 

Synthetic polyisoprene displays during pyrolysis an identical behavior with natural rubber. Pyrolysis of natural rubber, which is pure c/s-polyisoprene, is reported frequently in literature [29-35]. A review of the subject can be found in [36]. 

Pyrolysis of synthetic polyisoprene in the presence of oxygen also is expected to be identical to that of natural rubber. Thermal oxidation of natural rubber is assumed always to be associated with scission, although photo-oxidation at low temperature may involve peroxide formation without scission. The effect of oxygen is to increase the reaction rate of scission and therefore to decrease the temperature where the scission starts. The oxidation may take place after the initial formation of a free radical that reacts with oxygen ... 

The pyrolysis products of synthetic c/s-polyisoprene in an oxidative pyrolysis are very likely identical with those shown in Table 7.1.5. 

The results for the pyrolysis of a polyisoprene-gra/f-maleic anhydride sample with = 25,000, CAS 139948-75-7, is shown in Figure 7.1.6. The pyrolysis was done at 600° C in He with separation of a Carbowax column and MS detection, similarly to other polymers discussed in this book (see also Table 4.2.2). The peak identification for the chromatogram shown in Figure 7.1.6 was done using MS spectral library searches only and is given in Table 7.1.7. 

Except for the elimination of HCI, pyrolysis products of polychloroprene correspond rather well with those of isoprene. Besides the monomer and 3,7-dichloroocta-1,4,6-triene (which can be considered as a dimer of chloroprene), another compound found in appreciable levels in polychloroprene pyrolysate is 1-chloro-5-(1-chloroethenyl)-cyclohexene. This compound corresponds to diprene or 1-methyl-5-(1-methyivinyl)-cyclohex-1 -ene in the pyrolysate of polyisoprene. 

The dependence of the pyrolysis on the sample structure may be used in applying Py-GC to solve various physico-chemical problems. In particular, Py-GC has been used to study the kinetics of cyclization of polyisoprene rubber the results were later corroborated by other methods [268,269]. 

Infrared spectra of products obtained by heating the polymer in a test tube or in special heating equipment are also used to identify polymers in vulcaniz-ates. The pyrolysis temperature is a critical parameter since the same polymer can give different spectra when pyrolyzed at different temperatures. Comparison of pyrolysis products obtained by heating carbon black containing vulcanized compounds can be used to distinguish Hevea from synthetic polyisoprene. 

Kiran, E. and Gillham, J. (1976) Pyrolysis-molecular weight chromatography A new on-line system for analysis of polymers. II. Thermal decomposition of polyolefins Polyethylene, polypropylene, polyisoprene J. Appl. Polym. Sci. 20, 2045-2068. 

Jackson and Walker [7] studied the applicability of pyrolysis combined with capillary column GC to the examination of phenyl polymers (e.g., styrene-isoprene copolymer) and phenyl ethers e.g., bis[w-(w-phenoxy phenoxy)phenyl]ether. In the procedure the polymer sample is dissolved in benzene. The pyrolysis Curie point temperature wire is dipped 6 mm into the polymer solution. The polymer-coated wires are then placed in a vacuum oven at 75-80 °C for 30 minutes to remove the solvent. Figure 6.2 shows a characteristic pyrogram of the copolymer (isoprene-styrene) resulting from a 10-second pyrolysis at 601 °C. When the polyisoprene is pyrolysed, C2, C3, C4, isoprene, and CjoHig dimers are produced. When PS is pyrolysed, styrene and aromatic hydrocarbons are the products. Figure 6.2 shows that the copolymer product distribution and relative area basis resemble the two individual polymer product distributions. 

A pyrogram of the copolymer (isoprene-styrene) resulting from a 10 s pyrolysis at 601°C yields product distributions similar to the sum of the two constituent product distributions. For example, when the polymer polyisoprene is pyrolyzed, C2, C3, C4, isoprene and Cjo dimers are produced. When polystyrene is pyrolyzed, styrene and aromatic hydrocarbons are the products. The copolymer product distribution and relative area basis resemble the two individual polymer product distributions. 

Figure 16.9 TEM image for cylindrical nanoceramic replica derived from Pl32o- -PFSs3 shell-crosslinked micelles through a pyrolysis process under N2 with a temperature ramp of 1 Kmin up to 600 °C. (Reprinted with permission from X.S. Wang, A.C. Arsenault, G.A Ozin et al, Shell cross-linked cylinders of polyisoprene-b-ferrocenyldimethylsilane Eormation of magnetic ceramic replicas and microfluidic channel alignment and patterning, Journal of the American Chemical Society, 125, 12686, 2003. 2003 American Chemical Society.)...

In one study, the pyrolysis of ds-l,4-polyisoprene exhibited mainly three exotherms and two endotherms in the DTA curve [a.233]. It was further found that major reactions during the pyrolysis are not affected seriously by temperature changes from room temperature to 430 °C. The major products were identified via Py-GC as dipentene, isoprene, trimeric isoprene, toluene, benzene and xylene (Figure 24) 594526). 

Analysis of the products showed that c/s-l,4-polyisoprene, when pyrolysed under an inert atmosphere, experienced a radical mechanism. The chain scission mainly occurred at the p-position of carbon-carbon single bonds, which are adjacent to double bonds. The pyrolysis process of ds-l,4-polyisoprene was accompanied by dehydrogenation and aromatisation. However, dipentene was obtained as a major pyrolysis product below 430 °C and its yield decreased strongly with increasing temperature [a.234]. The obtained Py-GG results are displayed in Figure 25. 

Figure 25. Py-GC chromatograms of c/s-l,4-polyisoprene at different temperature ranges (A) Pyrolysis of c/5-l,4-polyisoprene from room temperature to 330 °C. (B) Pyrolysis of ds-l,4-polyisoprene from 331 to 390 °C. (C) Pyrolysis of c/s-1,4-polyisoprene from 391 to 430 °C. (D) Pyrolysis of cis-1,4 polyisoprene from 431 to 600 °C. Peaks 1, isoprene 2, benzene 3, toluene 4, xylene 5, dipentene 6, trimeric isoprene...

Pyrolysis of NR yielded a volatile substance, called isoprene (chemical name 2-methyl-butadiene-1,3, as defined by lUPAC, the International Union of Pure and Applied Chemistry). Subsequently, isoprene was polymerized to yield synthetic polyisoprene, but it was no match for natural rubber. 

Protonation reactions, 370 [Pt(dddt)2]FeCl4, 191 P-type conductors, 648 PVK-bonded dinitrophthalocyanine, 566 PVK-graft-polyisoprene, 566 Pyrazino-fused TCNQs, 246 Pyrazinotetrathiafulvalene, 170 Pyrazoline, 541 Pyridine-TCNQ, 238 Pyrolysis of polyacrylonitrile, 624 Pyromellitic dianhydride (PMDA), 92, 93 Pyropolymers, 762... 

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