Addition Polymers and Their Uses
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Questions and Answers

What is the purpose of a radical initiator in radical polymerization?

  • To convert the monomer into a radical
  • To break homolytically into radicals and start the growth of the polymer chain (correct)
  • To react with another monomer and add a new subunit to the chain
  • To terminate the growth of the polymer chain

    • What is the result of chain combination in termination steps?

  • One chain is oxidized to an alkene and the other reduced to an alkane
  • The monomer radical reacts with another monomer
  • Two chains combine at their propagating sites (correct)
  • The chain grows indefinitely

    • What is the name of the site where the unpaired electron is located in the propagation steps?

  • Radical site
  • Propagation site (correct)
  • Termination site
  • Initiation site

    • What happens to the concentration of monomer over time?

    <p>It decreases gradually until it is depleted</p> Signup and view all the answers

    What is the result of disproportionation in termination steps?

    <p>One chain is oxidized to an alkene and the other reduced to an alkane</p> Signup and view all the answers

    What type of polymerization is associated with step growth?

    <p>Condensation polymerization</p> Signup and view all the answers

    What is the maximum number of subunits that can be added to the chain in the propagation steps?

    <p>Thousands</p> Signup and view all the answers

    What is the term for regions of a polymer where the chains are highly ordered with respect to one another?

    <p>Crystallites</p> Signup and view all the answers

    What is the type of reaction that occurs when a radical reacts with a monomer?

    <p>Radical addition reaction</p> Signup and view all the answers

    What type of polymer has a continuous chain of repeating units?

    <p>Linear polymer</p> Signup and view all the answers

    What is the process by which the chain grows in the propagation steps?

    <p>The monomer radical reacts with another monomer, adding a new subunit to the chain</p> Signup and view all the answers

    What is the result of substituents or branches on polymer chains?

    <p>Decreased density of the polymer</p> Signup and view all the answers

    What is the role of the monomer radical in the propagation steps?

    <p>To react with another monomer, adding a new subunit to the chain</p> Signup and view all the answers

    What is the term for the process by which oligomers react to form longer chains?

    <p>Step growth</p> Signup and view all the answers

    What type of forces hold individual polymer chains together?

    <p>Van der Waals forces</p> Signup and view all the answers

    What is the result of a polymer being highly crystalline?

    <p>It becomes more resistant to heat and more dense</p> Signup and view all the answers

    What is the result of the propagating site removing a hydrogen atom from a different polymer chain?

    <p>Chain branching</p> Signup and view all the answers

    What type of initiators are used in cationic polymerization?

    <p>Electrophiles</p> Signup and view all the answers

    What is the difference between short and long branches in a polymer chain?

    <p>Short branches are formed near the end of the chain, while long branches are formed near the middle</p> Signup and view all the answers

    What is the effect of branching on the physical properties of a polymer?

    <p>Branching makes the polymer more flexible and soft</p> Signup and view all the answers

    What is the advantage of using Lewis acids as initiators in cationic polymerization?

    <p>They do not have accompanying nucleophile that could act as a chain terminator</p> Signup and view all the answers

    What is the result of the propagating site removing a hydrogen atom from the same polymer chain?

    <p>Chain branching</p> Signup and view all the answers

    Why are linear polyethylene chains able to pack together more closely than branched polyethylene chains?

    <p>Because linear chains have a more regular structure</p> Signup and view all the answers

    What is the type of reaction that governs cationic polymerization?

    <p>Electrophilic addition reaction</p> Signup and view all the answers

    Study Notes

    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.

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    Description

    This quiz covers addition polymers, their mechanisms, and uses. It explains radical, cationic, and anionic polymerization, including initiation, propagation, and termination steps.

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