Chapter 18 — Case Study 2: Free-Radical Polymerization and the Plastics Industry

"Most plastics in your home — water bottles, plastic wrap, polystyrene foam, PVC pipes — are made by free-radical polymerization. Initiator radicals attack alkene monomers; chains grow; chains terminate. The chemistry of Chapter 18 is the chemistry of the plastic industry." — paraphrase from a polymer textbook

This case study traces free-radical polymerization — the chemistry that transformed the 20th century. From the discovery of LDPE in 1933 to modern controlled radical polymerization, the chemistry has evolved but the basic mechanism (radical chain) is the same.

The discovery of LDPE

In 1933, Eric Fawcett and Reginald Gibson at ICI (Imperial Chemical Industries, UK) accidentally discovered that ethylene gas, under high pressure (~1500 atm) and high temperature (~200 °C), polymerizes to a waxy white solid. This was low-density polyethylene (LDPE) — the first polyolefin.

The conditions were extreme: very high pressure was needed because ethylene polymerization is slow at ambient pressure. ICI scaled up production; LDPE became commercially available in 1939. World War II accelerated production: LDPE was used as cable insulation in radar systems.

After the war, LDPE went mainstream. By the 1950s, it was used in: - Plastic bags (groceries, garbage). - Plastic film and wrap (cling wrap; food packaging). - Squeeze bottles. - Cable insulation. - Many household items.

Today, LDPE production is ~30 million tons/year worldwide.

The radical chain mechanism

LDPE is made by the standard radical chain:

Initiation

A peroxide initiator (or oxygen at high T) generates radicals: $$ROOR \to 2 RO^{\bullet}$$

Common initiators: di-tert-butyl peroxide, benzoyl peroxide, AIBN (azobisisobutyronitrile).

Propagation

The radical adds to ethylene; the resulting radical adds to another ethylene; chain extends: $$RO^{\bullet} + CH_2{=}CH_2 \to RO{-}CH_2{-}CH_2^{\bullet}$$ $$RO{-}CH_2{-}CH_2^{\bullet} + CH_2{=}CH_2 \to RO{-}CH_2{-}CH_2{-}CH_2{-}CH_2^{\bullet}$$

The chain extends by one ethylene unit per propagation step. Typical chain length: 1000-100,000 ethylene units.

Chain transfer (gives branching)

A growing radical can abstract H from its own chain or another chain: $$\text{primary radical} + \text{C-H} \to \text{primary chain end} + \text{secondary radical (along the chain)}$$

The new radical (a secondary on a chain) can keep growing — but now from a different position. This creates branches.

LDPE has many branches (~1 branch per 100 carbons). The branched structure prevents tight crystalline packing → low density.

Termination

Two growing radicals combine (or one disproportionates): $$2 R^{\bullet} \to R{-}R$$ (combination) $$2 R^{\bullet} \to R{-}H + R{=}\text{ (alkene)}$$ (disproportionation)

LDPE vs HDPE

The branched, low-density polyethylene (LDPE, made by radical) is different from linear, high-density polyethylene (HDPE, made by Ziegler-Natta).

The same monomer, two different processes, two different polymer properties. Modern industrial chemistry uses both.

Other radical-polymerized monomers

Many vinyl monomers undergo radical polymerization to give important polymers:

Polystyrene (PS)

Styrene (C₆H₅-CH=CH₂) + radical initiator → polystyrene. The benzyl-stabilized radical intermediate makes polymerization efficient.

Uses: foam insulation, packaging, disposable cutlery, jewel cases.

Polyvinyl chloride (PVC)

Vinyl chloride (CH₂=CHCl) + radical initiator → PVC. ~50 million tons/year.

Uses: pipes, vinyl flooring, window frames, cables, sweet wrappers.

Polymethyl methacrylate (PMMA, Plexiglas)

Methyl methacrylate (CH₂=C(CH₃)CO₂CH₃) + radical → PMMA. Acrylic glass; impact-resistant transparent plastic.

Uses: aircraft windows, signage, medical devices, contact lenses.

Polyacrylonitrile (PAN)

Acrylonitrile (CH₂=CHCN) + radical → PAN. Used for: - Acrylic fibers (carpets, sweaters). - Carbon fiber precursor. - ABS plastic (with butadiene + styrene).

Polyvinyl acetate (PVA)

Vinyl acetate (CH₂=CH-OCOCH₃) + radical → PVA. Used in: - Wood glue (Elmer's glue). - Water-based paints. - Adhesives.

Other polymers

• Polychloroprene (Neoprene; from chloroprene).
• Polybutadiene (from butadiene; for tire rubber).
• Many specialty polymers.

Modern controlled radical polymerization

Classical free-radical polymerization gives broad molecular weight distribution (MWD; the polymer chains have a wide range of sizes). For some applications, narrow MWD is needed.

Controlled radical polymerization (CRP) gives narrow MWD by: - Reducing the number of growing radicals at any given time. - Reversibly capping the growing radical, slowing termination.

ATRP (atom transfer radical polymerization)

Cu catalyst transfers a halogen between dormant alkyl halide species and growing radicals. The alkyl halide is the dormant form; only a small fraction is "active" at any time.

• Krzysztof Matyjaszewski's group developed this in the 1990s.
• Gives narrow MWD and end-group control.

RAFT (reversible addition-fragmentation chain transfer)

A dithio compound reversibly caps growing radicals. Similar effect: narrow MWD, end-group control.

NMP (nitroxide-mediated polymerization)

A nitroxide radical reversibly caps growing radicals.

These methods produce polymers with controlled architecture (block copolymers, gradient copolymers) for specialty applications: - Block copolymers for self-assembly into nanostructures. - End-functionalized polymers for bioconjugation. - Polymers for drug delivery.

Industrial scale and environmental impact

Radical polymerization produces: - LDPE: ~30 million tons/year. - PVC: ~50 million tons/year. - PS: ~25 million tons/year. - PMMA: ~5 million tons/year.

Total radical-polymerized plastics: ~150 million tons/year.

Environmental concerns: - Plastic waste: most LDPE/PVC end up in landfills or oceans. - Microplastics: mechanical degradation of plastics gives small particles. - Recycling: limited by the variety of plastic types. - Bioaccumulation: PVC and other plastics may release plasticizers (e.g., phthalates) into food.

Modern green chemistry addresses some issues: - Bioplastics: from renewable biomass (PLA, PHA). - Recyclable polymers: designed for chemical recycling. - Biodegradable polymers: break down in environmentally-relevant timescales.

Take-home

• Free-radical polymerization is the chemistry of much of the plastic industry.
• Mechanism: radical initiator + alkene monomer → propagation (chain extension) → termination.
• LDPE (low-density polyethylene): radical-polymerized at high T and pressure. Branched, soft, used for films and bags.
• HDPE (high-density polyethylene): Ziegler-Natta-polymerized (Ch 37). Linear, rigid, used for pipes.
• Other radical-polymerized polymers: PVC, PS, PMMA, PAN, PVA.
• Chain transfer in radical polymerization gives branching (and thus LDPE's lower density).
• Modern controlled radical polymerization (ATRP, RAFT, NMP) gives narrow molecular weight distribution for specialty applications.
• Plastic waste, microplastics, and recycling are major environmental concerns; bioplastics and chemical recycling are emerging solutions.
• Mastery of Chapter 18 radical chemistry is the foundation for understanding the polymer industry.