Addition Polymers: Mechanisms and Types

Addition Polymers

• Addition or chain-growth polymerization proceeds by three mechanisms: radical polymerization, cationic polymerization, and anionic polymerization.
• Each mechanism has three distinct steps: initiation, propagation, and termination.

Radical Polymerization

• Initiation step: a radical initiator is added to the monomer to generate a radical that starts the growth of the polymer chain.
• Propagation steps:
  • A monomer radical reacts with another monomer, converting it into a radical.
  • The radical reacts with another monomer, adding a new subunit to the chain.
  • The unpaired electron is at the end of the last unit added to the chain (propagating site).
• Termination steps:
  • Chain combination: two chains combine at their propagating sites.
  • Disproportionation: one chain is oxidized to an alkene and the other reduced to an alkane.

Branching of the Polymer Chain

• Branching occurs when the propagating site removes a hydrogen atom from a chain.
• Branching can occur at different points on the polymer chain, leading to short or long branches.
• Short branches are more likely to be formed than long ones.
• Branching affects the physical properties of the polymer:
  • Unbranched chains can pack together more closely than branched chains.
  • Linear polyethylene (high-density polyethylene) is a hard plastic, used for artificial hip joints.
  • Branched polyethylene (low-density polyethylene) is more flexible, used for trash bags and dry-cleaning bags.

Cationic Polymerization

• In cationic polymerization, the initiator is an electrophile that adds to the monomer, causing it to become a carbocation.
• The initiator cannot be an acid such as HCl, but rather a Lewis acid, such as BF3 or AlCl3.
• The advantage of these initiators is that they do not have accompanying nucleophiles that could act as chain terminators.

Classification of Polymers

• Polymer growth:
  • Chain growth: associated with addition polymerization.
  • Step growth: associated with condensation polymerization, where chains have at least two growth points.
• Polymer structure:
  • Linear polymers: have a continuous chain of repeating units.
  • Branched polymers: have short branches connected to the main chain.
  • Cross-linked polymers: have covalently bonded linking units between chains.
• Physical properties:
  • Polymers acquire their physical properties as a result of the arrangement of their individual chains.
  • The more crystalline (ordered) the polymer is, the denser, harder, and more resistant it is to heat.
  • Substituents or branches on the polymer chains can prevent them from packing closely together, reducing the density of the polymer.