Polyisobutylene Decomposition

Lehrle and Pattenden [4] applied Py-GC-MS to a study of the thermal decomposition of polyisobutylene (PIB) which had been exposed to y-radiation in a Co source. 

Grimbley and Lehrle [8-10] studied the thermal degradation of PIB and in contrast to the previous proposals stating that backbiting was the predominant mechanism, they found that depropagation accompanied by random scission were the principal primary mechanisms. 

The comparison of thermal stabilities before and after irradiation was best achieved by Lehrle and Pattenden [4] by measuring the rate constant ( specific rate , for the evolution of volatiles from the samples at specified temperatures. The smaller the value of k, the more thermally stable was the sample. 

The mean values all exceed unity, indicating that irradiation has conferred some thermal stabilisation on PIB. (Vertical lines represent standard deviation). Reproduced with permission from R.S. Lehrle and C.S. Pattenden, Polymer Degradation and Stability, 1998, 62, 211. 1998, Elsevier [4] 

---

The low molecular weight materials produced by this process are used as lubricants, whereas the high molecular weight materials, the polyisobutylenes, are used as VI improvers and thickeners. Polybutenes that are used as lubricating oils have viscosity indexes of 70—110, fair lubricating properties, and can be manufactured to have excellent dielectric properties. Above their decomposition temperature (ca 288°C) the products decompose completely to gaseous materials. 

Polyisobutene or polyisobutylene, CAS 9003-27-4, with the model formula [-CH2C(CH3)2-]n is another common polymer, which is used in practice mainly for sealants and adhesives and in various copolymers. Polyisobutylene copolymer with a small amount of isoprene is used as a synthetic rubber, the added isoprene making the material vulcanizable. The polymer is typically obtained by cationic polymerization using, for example, BF3 or AICI3 as catalysts. The thermal decomposition of the polymer generates various fragments [76,119-123]. The heating between 288° C and 425° C... 

Figure 1.10 Dependence of the rate of thermal degradation on the degree of decomposition of polyisobutylene with different molecular masses 1 - 23,400, 2 - 40,000, 3 - 440,000, 4 - 100,000, 5 - 198,000 and 6 - 700,000. Source Author s own files...

The chains must be crosslinked to form a network (cf. Fig 7.16). In most elastomers containing double bonds, covalent bonds are introduced between chains. This can be done either with sulfur or polysulfide bonds (the well known sulfur vulcanisation of natural rubber is an example), or else by direct reactions between double bonds, initiated via decomposition of a peroxide additive into radicals. Double bonds already exist in the chemical structure of polyisoprene, polybutadiene and its copolymers. When this is not the case, as for silicones, ethylene-propylene copolymers and polyisobutylene, units are introduced by copolymerisation which have the property of conserving a double bond after incorporation into the chain. These double bonds can then be used for crosslinking. This is how Butyl rubber is made from polyisobutylene, by adding 2% isoprene. Butyl is a rubber with the remarkable property of being impermeable to air. It is used to line the interior of tyres with no inner tube. 

Synthetic polyolefins were first synthetisized by decomposition of diazomethane [2]. With the exception of polyisobutylene, these polymers were essentially laboratory curiosities. They could not be produced economically. The situation changed with the discovery of the high pressure process by Fawcett and Gibson (ICI) in 1930 ethene was polymerized by radical compounds [3]. To achieve a sufficient polymerization rate, a pressure of more than 100 MPa is necessary. First produced in 1931, the low density polyethene (LDPE) was used as isolation material in cables. 

---