5.3 Coordination polymerization and stereoregular polymers

Coordination polymerization uses special catalysts to make plastics with precise structures. These catalysts, like Ziegler-Natta and metallocenes, control how monomers link up, creating polymers with specific properties.

Stereoregular polymers have a neat arrangement of atoms along their backbone. This orderly structure gives them improved strength, heat resistance, and other useful qualities. Different types of stereoregular polymers are used in everyday items like food containers and car parts.

Coordination Polymerization

Coordination polymerization catalysts

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• Coordination polymerization utilizes transition metal catalysts (Ziegler-Natta and metallocene catalysts)
• Ziegler-Natta catalysts composed of a transition metal compound (TiCl4) and an organometallic co-catalyst (triethylaluminum)
• Metallocene catalysts are organometallic compounds with a metal atom (Zr, Ti) sandwiched between two cyclopentadienyl rings
• Metallocene catalysts offer better control over polymer structure and properties compared to Ziegler-Natta catalysts

Mechanism of coordination polymerization using Ziegler-Natta and metallocene catalysts

• Ziegler-Natta catalysis mechanism:

1. Activation: Co-catalyst (alkylaluminum) reduces the transition metal compound, creating active sites
2. Initiation: Monomer (propylene) coordinates to the active site, forming a metal-carbon bond
3. Propagation: Successive monomer units insert between the metal and the growing polymer chain
4. Termination: Polymerization ends by chain transfer or catalyst deactivation

• Metallocene catalysis mechanism:
  • Activation: Co-catalyst (methylaluminoxane) activates the metallocene, creating a cationic metal center
  • Initiation, propagation, and termination steps are similar to Ziegler-Natta catalysis

Stereoregular Polymers

Stereoregularity in polymers

• Stereoregularity refers to the spatial arrangement of substituents along the polymer backbone determined by the orientation of monomer units during polymerization
• Stereoregular polymers have a regular and ordered arrangement of substituents leading to improved physical and mechanical properties (higher crystallinity, melting point, and tensile strength)
• Stereoregularity enables closer packing of polymer chains, increasing intermolecular forces and facilitating the formation of crystalline regions, which contribute to strength, stiffness, and resistance to heat, chemicals, and stress cracking

Types of polymer tacticity

• Isotactic polymers have all substituents located on the same side of the polymer backbone (isotactic polypropylene)
• Syndiotactic polymers have substituents alternating regularly between opposite sides of the polymer backbone (syndiotactic polystyrene)
• Atactic polymers have substituents randomly arranged along the polymer backbone (atactic polystyrene)
• Isotactic and syndiotactic polymers are more crystalline, leading to higher melting points and better mechanical properties, while atactic polymers are amorphous, resulting in lower melting points and reduced mechanical strength

Applications of stereoregular polymers

• Isotactic polypropylene (iPP) used in packaging materials (food containers, bottles), automotive components (bumpers, interior trim), textiles (carpets, ropes, upholstery), and medical applications (syringes, disposable gowns)
• High-density polyethylene (HDPE) used in packaging materials (milk jugs, shampoo bottles), pipes and fittings for water and gas distribution, geomembranes for landfill and pond liners, and household goods (storage containers, cutting boards)
• Linear low-density polyethylene (LLDPE) used in flexible packaging (plastic bags, stretch wrap), agricultural films (greenhouses, mulch), wire and cable insulation, and dispensing containers (squeeze bottles)