Rubbers

Basics of production: diene polymerization

The main reagents for rubber synthesis are diene hydrocarbons — butadiene and isoprene. An important role in the reactivity of these compounds is played by the conjugation effect, which directly affects the formation of reaction products. This effect becomes significantly more pronounced at elevated ambient temperatures.

The polymerization process of these substances proceeds easily, and the growth of the polymer chain follows the 1,4-addition mechanism. For the successful reaction and formation of the required structure, specific catalysts are used:

• Alkali metals.
• Organic compounds of alkali metals.
• Ziegler–Natta catalysts.

Molecular structure and stereoisomerism

From a chemical point of view, rubbers are non-conjugated polyenes. Internal rotation is possible around single bonds within the polymer chain, which gives molecules flexibility. However, the presence of double bonds causes the phenomenon of stereoisomerism — hydrogen atoms and hydrocarbon radicals can be arranged in space in two ways:

1. cis-arrangement — substituents (entry and exit of the polymer chain) are on the same side of the double bond plane. Molecules are directed in the same direction.
2. trans-arrangement — substituents lie on opposite sides of the double bond, forming a zigzag structure.

Stereoregularity is critically important for operational characteristics. Polymers with a cis-structure possess the best properties (for example, cis-butadiene and cis-isoprene rubbers). If cis- and trans-configurations alternate randomly in the macromolecule, such rubber is called irregular, and it is significantly inferior in quality to stereoregular analogues.

Vulcanization: conversion of rubber into rubber

Rubber itself has a limited scope of application, but it serves as the basic raw material for obtaining rubber. This process is called vulcanization and consists in co-heating rubber with sulfur (S).

Mechanism of the vulcanization process:

1. During heating, some of the double bonds in the polymer chains of rubber break.
2. Sulfur atoms attach at the break sites.
3. So-called "sulfur bridges" (cross-links) arise between individual linear macromolecules.
4. The linear structure turns into a single spatial three-dimensional network. In essence, a piece of rubber is one giant cross-linked molecule.

The change in structure radically alters the physical properties of the material. The main difference manifests in solubility: while the initial rubber dissolves in hydrocarbons, rubber only swells in them (solvent molecules penetrate into the cells of the spatial network between hydrocarbon chains and sulfur bridges, but cannot break the network itself). The bulk of rubber produced today goes into the manufacture of car tires.

Briefly: answers to topical questions

What else to study on this topic

• Mechanisms of action of Ziegler–Natta catalysts in diene polymerization
• Features of sequential halogen addition to isoprene upon heating
• Properties of irregular rubbers and causes of their poor quality
• Chloroprene rubber: structural features and operational characteristics

Other topics in "Chapter 15. Hydrocarbons"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.