Polymerization Classifications

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Questions and Answers

What happens to the concentration of monomer over time?

  • It decreases gradually (correct)
  • It remains constant
  • It fluctuates randomly
  • It increases rapidly

What is the primary characteristic of step growth polymerization?

  • Oligomers react together to form longer chains (correct)
  • Monomers react in a single step
  • The reaction is reversible
  • Chain growth is rapid and continuous

What is the main difference between linear and branched polymers?

  • Branched polymers have a network structure
  • Branched polymers have a backbone with short branches (correct)
  • Linear polymers have a single chain structure
  • Linear polymers have cross-links

What is the main characteristic of cross-linked polymers?

<p>They have covalently bonded linking units between chains (C)</p>
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What is the primary factor that determines the physical properties of a polymer?

<p>The arrangement of individual chains (D)</p>
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What is the primary type of force that holds polymer chains together?

<p>Van der Waals forces (D)</p>
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What are the regions of a polymer where the chains are highly ordered called?

<p>Crystallites (C)</p>
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What happens to the density of a polymer when it has substituents or branches that prevent close packing?

<p>It decreases (D)</p>
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What type of polymers have both ordered crystalline regions and amorphous noncrystalline regions?

<p>Thermoplastic polymers (A)</p>
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What is the reason thermosetting polymers cannot be remelted by heating?

<p>Because the cross-links are covalent bonds (A)</p>
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What happens to the polymer chains when elastomers are stretched?

<p>They stretch out and crystallize (A)</p>
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What type of polymers are rubber and spandex examples of?

<p>Elastomers (C)</p>
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What type of polymerization is involved in the formation of addition polymers?

<p>Chain-growth polymerization (A)</p>
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What is the common monomer used to form vinyl polymers?

<p>Ethylene (D)</p>
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Why do elastomers return to their original state after the stretching force is removed?

<p>Because of weak van der Waals forces (A)</p>
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What is the characteristic of thermoplastic polymers that allows them to be molded when heated?

<p>Chain slippage (D)</p>
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What is the purpose of molecule XY in radical polymerization?

<p>To terminate the growing chain (C)</p>
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Why is head-to-tail addition favored in radical polymerization?

<p>Due to the radical stabilization by the substituent (A)</p>
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What type of bond is required for molecule XY to react with the growing chain?

<p>Bond that can readily be cleaved homolytically (D)</p>
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What is the role of the initiator in radical polymerization?

<p>To initiate a new chain (D)</p>
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Why does head-to-head addition occur when Z is small?

<p>Due to the lack of radical stabilization (D)</p>
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What is the effect of a phenyl substituent on the radical?

<p>It stabilizes the radical by electron delocalization (C)</p>
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What type of cleavage occurs in radical polymerization?

<p>Homolytic cleavage (C)</p>
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What is the result of chain transfer in radical polymerization?

<p>Termination of the growing chain (A)</p>
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What is the role of the initiator in anionic polymerization?

<p>It acts as a nucleophile to react with the monomer. (D)</p>
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What type of shift can occur in the propagating site during cationic polymerization?

<p>1,2-hydride shift (D)</p>
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What is the characteristic of the monomers that undergo cationic polymerization?

<p>They have substituents that stabilize the positive charge. (D)</p>
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What is the type of carbocation formed at the propagating site before rearrangement?

<p>Secondary carbocation (D)</p>
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What is the reason for the difficulty in nucleophilic attack on an alkene in anionic polymerization?

<p>Alkenes are electron-rich. (C)</p>
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What is the characteristic of the initiator in anionic polymerization?

<p>It is a very good nucleophile. (C)</p>
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What is the purpose of the electron-withdrawing substituent in anionic polymerization?

<p>To decrease the electron density of the double bond. (B)</p>
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What is the result of a 1,2-hydride shift in the propagating site during cationic polymerization?

<p>A more stable tertiary carbocation is formed. (D)</p>
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What is the functional group that reacts with the carboxylic acid group in the formation of nylon?

<p>Amino group (B)</p>
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What is the name of the polyamide formed from the polymerization of 6-aminohexanoic acid?

<p>Nylon 6 (A)</p>
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What is the common name of the polyester polymer formed by the transesterification of dimethyl terephthalate with ethylene glycol?

<p>Dacron (B)</p>
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What is the characteristic of epoxy resins that makes them resistant to solvents and high temperature?

<p>Extensive cross-linking (B)</p>
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What is the functional group that defines a polyurethane?

<p>Urethane group (carbamate) (D)</p>
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What is the starting material for the synthesis of nylon 6?

<p>ε-caprolactam (A)</p>
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What is the name of the polyamide formed from two different bifunctional monomers—adipoyl chloride and 1,6-hexanediamine?

<p>Nylon 66 (C)</p>
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What is the reaction that forms Kodel polyester?

<p>Transesterification (D)</p>
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Study Notes

Polymerization

  • The concentration of monomer decreases gradually until it is depleted during polymerization.

Classification of Polymers: Growth

  • Step growth: Chains have at least two growth points, forming oligomers that react to give longer chains, associated with condensation polymerization.
  • Chain growth: Associated with addition polymerization.

Classification of Polymers: Structure

  • Linear polymers: Continuous chain of repeating units.
  • Branched polymers: Short branches connected to the main chain.
  • Cross-linked polymers: Covalently bonded linking units between chains.

Classification of Polymers: Physical Properties

  • Physical properties of polymers are a result of individual chain arrangement.
  • Regions of highly ordered chains (crystallites) are interspersed with amorphous, noncrystalline regions.
  • Polymers classified according to properties:
    • Thermoplastic polymers: Have both ordered crystalline and amorphous noncrystalline regions, can be molded when heated.
    • Thermosetting polymers: Cross-linked, rigid, and cannot be remelted by heating.
    • Elastomers: Flexible, can be stretched and return to original state, have some cross-linking.

Addition Polymers

  • Formed from ethylene or substituted ethylenes (vinyl polymers).
  • Radical polymerization:
    • Termination steps: Chain transfer, may occur by reaction with a molecule XY.
    • Favored head-to-tail addition due to steric hindrance and stabilization by substituents.
    • May undergo head-to-head or tail-to-tail addition in some cases.
  • Cationic polymerization:
    • Mechanism involves chain-initiating, chain-propagating, and termination steps.
    • Carbocation intermediates can undergo rearrangement by 1,2-hydride or 1,2-methyl shift.
    • Best monomers have substituents that stabilize the positive charge.

Anionic Polymerization

  • Initiator is a nucleophile that reacts with the monomer to form an anion propagating site.
  • Nucleophilic attack on an alkene requires a very good nucleophile and electron-withdrawing substituents on the alkene.

Classes of Step-Growth Polymers

  • Polyamides
  • Polyesters
  • Epoxy resins
  • Polyurethanes

Polyamides

  • Nylon 6: Formed from 6-aminohexanoic acid, a polyamide.
  • Nylon 66: Formed from adipoyl chloride and 1,6-hexanediamine.
  • Starting material for nylon 6 synthesis is ε-caprolactam.

Polyesters

  • Containing many ester groups.
  • Dacron: Made by transesterification of dimethyl terephthalate with ethylene glycol.
  • Kodel polyester: Formed by transesterification of dimethyl terephthalate with 1,4-di(hydroxymethyl)cyclohexane.

Epoxy Resins

  • Strongest adhesives known, extensively cross-linked systems.
  • Adhere to almost any surface, resistant to solvents and high temperature.
  • Epoxy cement: Consisting of a low-molecular-weight prepolymer and a hardener that react to form a cross-linked polymer.

Polyurethanes

  • Containing urethane (carbamate) groups.
  • Formed by reaction of a prepolymer with a hardener.

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