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SolutionsDimerization and Polymerization

1. Types of polymerization

Which type of polymerization (cationic, anionic or radical) is best for each monomer? a) isobutylene, (CH3)2C=CH2; b) methyl 2-cyanoacrylate, CH2=C(CN)COOCH3; c) ethylene at high pressure.

  1. Cationic: the two methyl groups stabilize the tertiary carbocation formed.
  2. Anionic: the CN and COOCH3 groups stabilize the carbanion. Moisture (HO−) is enough to initiate it; this is how superglue works.
  3. Radical, initiated by peroxides; it gives highly branched low-density polyethylene.

2. The repeating unit

Write the repeating unit of polypropylene, poly(vinyl chloride) (PVC) and polystyrene.

  1. –[CH2–CH(CH3)]n–
  2. –[CH2–CHCl]n–
  3. –[CH2–CH(C6H5)]n–

Addition is head-to-tail: the substituent appears on alternate carbons, because each new bond gives the more stable intermediate.

3. Ziegler-Natta

What advantage do Ziegler-Natta catalysts (TiCl4/Al(CH2CH3)3) have in the polymerization of propylene?

They allow polymerization at low pressure and control the stereochemistry: they give isotactic polypropylene, with all the methyl groups on the same side of the chain, which is crystalline, strong and has a high melting point.

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