Polyethylene: Production, Types, and History, Exams of Organic Chemistry

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Polyethylene

Monomer: Ethylene

 High Density Polyethylene (HDPE)  Low Density Polyethylene (LDPE)

 Linear Low Density Polyethylene (LLDPE)

 Low Density Polyethylene (LDPE)

High degree of short and long chain branching Density - 0.910–0.940 g/cm^3 Lower tensile strength and increased ductility  Free radical polymerization Plastic bags and film wrap

 High Density Polyethylene (HDPE) Low degrees of branching (essentially linear) Density > 0.940 g/cm^3  High tensile strength  Various catalysts (ZN, Metallocene)  Milk jugs, detergent bottles, garbage containers and water pipes

Long chain branching (HDPE)

Short chain branching (LLDPE)

Hyperbranched (LDPE)

Free Radical Polymerization

1. INITIATION (You need Initiators)

AIBN

Benzoyl Peroxide

Free Radical Polymerization

2. PROPAGATION

High pressure is needed during the propagation step in order to bring the ethylene monomer closer to the free radicals

Free Radical Polymerization

3. TERMINATION (Many ways)

b. DISPROPORTIONATION

Free Radical Polymerization

3. TERMINATION (Many ways)

c. CHAIN TRANSFER (Hydrogen Abstraction)

Disadvantages

1. Uncontrolled Process
   • Structure, Molecular weight
2. Requires high pressure
3. Reactions are highly exothermic
4. Inefficient process (20% ethylene polymerized)

Timeline of Polyethylene

Synthesized by accident while heating diazomethane (Called Poly-“methylene” due to repeating –CH 2 group)

 1930-  First polymerization of ethylene at Imperial Chemical Industries.  Advent of the free radical process to produce LDPE

 1950’s  Ziegler-Natta Catalyst (Inorganic Catalyst for HDPE)

Zieglar-Natta Catalyst

Mechanism

Timeline of Polyethylene

Synthesized by accident while heating diazomethane (Called Poly-“methylene” due to repeating –CH 2 group)

 1930-  First polymerization of ethylene at Imperial Chemical Industries.  Advent of the free radical process to produce LDPE  1950’s  Ziegler-Natta Catalyst (Inorganic Catalyst for HDPE)  1970’s  Metallocene Catalyst (Organic-Inorganic Hybrid Catalyst for HDPE)

Timeline of Polyethylene

Synthesized by accident while heating diazomethane (Called Poly-“methylene” due to repeating –CH 2 group)  1930-  First polymerization of ethylene at Imperial Chemical Industries.  Advent of the free radical process to produce LDPE  1950’s  Ziegler-Natta Catalyst (Inorganic Catalyst for HDPE)  1970’s  Metallocene Catalyst (Organic-Inorganic Hybrid Catalyst for HDPE)

 What is next?  Organic Routes (biological enzymatic reactions)^16

Zieglar-Natta Catalyst

Mechanism