14.6 Diene Polymers: Natural and Synthetic Rubbers

Diene Polymers

Structure of diene polymers

Diene polymers form from monomers that contain two double bonds (dienes), such as 1,3-butadiene and isoprene. Because these monomers are conjugated dienes, they can undergo 1,4-addition polymerization, which is what gives the resulting polymer its distinctive structure.

Here's how 1,4-addition polymerization works:

1. An initiator (radical, cation, or anion) attacks carbon 1 of the diene, forming a new bond and generating a reactive center at carbon 4.
2. That reactive center attacks carbon 1 of the next monomer, forming another bond and shifting the reactive center to the new monomer's carbon 4.
3. The chain keeps growing until a termination event stops propagation.

The key structural result: the polymer backbone retains one double bond per monomer unit, sitting between carbons 2 and 3. These remaining double bonds can adopt either cis or trans geometry, and that distinction has a huge effect on properties.

• Cis configuration: polymer chains can't pack tightly, so the material stays flexible and elastic (this is natural rubber).
• Trans configuration: chains pack more efficiently into a crystalline arrangement, producing a harder, more rigid material (this is gutta-percha).

Natural vs synthetic rubbers

Natural rubber (cis-polyisoprene)

• Harvested as latex from the Hevea brasiliensis tree
• Almost entirely cis-1,4 configuration, which gives it high elasticity and flexibility
• Common uses: tires, rubber bands, gloves, elastic products

Gutta-percha (trans-polyisoprene)

• Harvested from the sap of certain tropical trees (genus Palaquium)
• Almost entirely trans-1,4 configuration, making it more rigid and less elastic than natural rubber
• Common uses: golf ball covers, dental fillings, historical undersea cable insulation

Both natural rubber and gutta-percha have the same molecular formula for their repeating unit ($\text{C}_5\text{H}_8$), yet their physical properties differ dramatically because of cis vs. trans geometry. This is a great example of how stereochemistry controls macroscopic behavior.

Synthetic rubbers were developed to address shortcomings of natural rubber, such as supply limitations and vulnerability to oils or chemicals.

• Neoprene (polychloroprene): Made by polymerizing 2-chloro-1,3-butadiene. The chlorine atom on the backbone gives neoprene strong resistance to oils, chemicals, and weathering. You'll find it in wetsuits, gaskets, and industrial hoses.
• Styrene-butadiene rubber (SBR): A copolymer of styrene and 1,3-butadiene. The styrene units add rigidity and abrasion resistance, while the butadiene units provide flexibility. SBR is cheaper than natural rubber and widely used in car tires, conveyor belts, and shoe soles.

Vulcanization process and effects

Unvulcanized rubber is sticky when warm and brittle when cold. Vulcanization solves this by creating covalent cross-links between polymer chains, locking them into a three-dimensional network.

The process works like this:

1. Raw rubber is mixed with sulfur (typically 1–8% by weight) and heated to around 140–160 °C.
2. Sulfur atoms react with the double bonds remaining in the polymer backbone.
3. Short chains of sulfur atoms ($\text{S}_x$ bridges, where $x$ is typically 1–8) form covalent cross-links between adjacent polymer chains.
4. The result is a network structure where chains are connected but can still stretch and recoil.

Effects of vulcanization on rubber properties:

1. Increases tensile strength and durability by preventing chains from sliding past each other permanently.
2. Reduces plasticity and permanent deformation so the rubber snaps back to its original shape.
3. Improves resistance to solvents, chemicals, and abrasion.
4. Decreases solubility and gas permeability because the cross-linked network is harder to penetrate.

The degree of cross-linking controls the final product. Low sulfur content (few cross-links) gives soft, elastic rubber like that in rubber bands. High sulfur content (many cross-links) produces hard, rigid material like ebonite, which was historically used for bowling balls and fountain pens.

Polymer Classification and Properties

Diene polymers fall into broader categories based on their thermal and mechanical behavior:

• Elastomers: Polymers that can stretch significantly and return to their original shape. Vulcanized natural rubber is the classic example. Cross-links are sparse enough to allow chain movement but dense enough to provide recovery.
• Thermoplastics: Polymers that soften when heated and harden when cooled, and this cycle is repeatable. Unvulcanized rubber and some lightly processed synthetic rubbers behave this way. No permanent cross-links hold the chains together.
• Thermosets: Polymers with extensive, irreversible cross-links formed during curing. Heavily vulcanized rubber (ebonite) is a thermoset. Once cured, it cannot be reshaped by heating.