Natural rubber: cis-1,4-polyisoprene

One of the most well-known and used natural polymers by man, since its discovery in the 18th century, is natural rubber or cis-1,4-polyisoprene. This polymer has long been obtained through filtration and coagulation, by means of heat, of whitish latex obtained from the incision of the bark of the rubber tree (Hevea brasiliensis), a native plant of the Amazon rainforest. This latex, in fact, was collected in containers, with a stabilizer inside (ammonia) to prevent coagulation, and then processed in order to obtain the polymer that was then marketed in the form of loaves.

Chemical composition of latex

The latex obtained from this plant represents an emulsion of different compounds, kept stable by emulsifying agents, naturally present inside it. The composition of latex is not always the same, but depends on various agronomic and climatic factors, such as the habitat in which the plant grows, climatic conditions, the age of the plant stick, etc.. Generally, latex contains about 30% of natural rubber, while the remaining part is the so-called serum, which consists of water, sugars, proteins, enzymes.

Chemical structure, properties and first applications of natural rubber

As already mentioned above, natural rubber consists of cis-1,4-polyisoprene. The monomeric unit, therefore, is the isoprene, bound in position 1.4 and with cis stereochemistry of the double bond (the most relevant substituents are found on the same side with respect to the double bond).

Monomer of cis-1,4-polyisoprene
Repeating unit (monomer) of cis-1,4-polyisoprene

This material, of course, shows stickiness, crumbling and is sensitive to heat and cold. These properties are conferred by the structure of the polymer macromolecule. In fact, despite having identical monomer units that repeat themselves, it is also cumbersome, due to the cis stereoisomery that places all the larger groups on the same side of the macromolecule. This steric encumbrance leads to an increase of the volume occupied by each macromolecule, with a consequent impossibility to pack it. This does not allow to reach the crystallinity and the poor properties of this polymer, if used as such, derive from it.

Portion of a macromolecule of cis-1,4-polyisoprene
Illustrative portion of a macromolecule of cis-1,4-polyisoprene in which it is possible to observe the considerable steric encumbrance

For this reason, since its discovery this material was used by the Maya and Aztecs, mostly as a waterproofing material (for example for huts and boots) and as an adhesive agent for the realization of balls used for a game similar to basketball.

New applications thanks to the invention of the vulcanization process

In 1839, Charles Goodyear and Thomas Hancock, after several experiments, found that heating the natural rubber in the presence of a few units of sulfur and basic lead carbonate (accelerator), you get a more elastic material and more resistant to solvents, thus going to suppress the original negative properties, such as abrasiveness and tackiness. This process was called the vulcanization process.

These new properties of natural rubber have made new applications possible. This material, in fact, is now widely used for the construction of the radial carcass of tires, thanks to its ability to dissipate heat very well (which is generated by the small slippages to which the macromolecules are subjected during the rolling of the tire). It should be noted that about 70% of all polyisoprene production is used for this purpose. The remaining 30%, on the other hand, is used for the production of technical articles.

The cis-1,4-polyisoprene today

Nowadays, only a small part of the entire global production of polyisoprene is still made from the latex of the rubber tree. This fraction can be estimated at around 30%. The remaining part, on the other hand, is now produced industrially, using processes that use Ziegler-Natta or lithium-alkyl catalysts.

These catalysts allow to obtain a very high stereospecificity, towards the product 1,4-cis, such as to justify their use. In fact, it is possible to obtain a product with excellent properties and even higher than that obtained by natural means. However, it is necessary to take into account that, at industrial level, cis-1,4-polyisoprene is obtained mainly from fossil sources. It is, therefore, an ungreen and unsustainable process.