2. Polymer Synthesis

Question 1

Types of chain growth polymers

Answer 1

• free-radical polymerisation
• ionic polymerisation

Question 2

Common feature of chain growth polymerisation; rate of initiation vs rate of polymerisation

Answer 2

• The active centre of the growing chain is retained by one molecule throughout the course of its growth- you’ll find negligible amounts of chains of intermediate Mw at any given point.
• Polymers formed during early stage of polymerisation are comparable in Mw to those formed at the advance stage.
• This is because the rate of propagation is higher than that of the initiation of polymer chains.

Question 3

Production of initiating radical (chain growth)

Answer 3

Homolysis of weak or otherwise unstable covalent bonds
e.g. thermal decomposition of AIBN
Radical attacks less hindered end of alkene double bond in initiation step (sterics, more stable secondary radical)

Question 4

Termination mechanisms in chain growth polymers

Answer 4

• Combination (of two chain ends)
  e.g. polystyrene
• Disproportionation (one radial abstracts a hydrogen from the other
  -> alkene and alkane
  e.g. Perspex (poly(methyl methacrylate))

Question 5

Step growth polymerisation description

Answer 5

Stepwise reaction between bi-functional/multi-functional monomers in which a high-Mw polymer is formed only after a large no. of steps.
DP, whilst in theory is unlimited, in practice is typically low at n = 100-200

Question 6

Why is DP of step-growth polymers limited?

Answer 6

• Importance of achieving exact stoichiometry of reaction
• Step-wise nature

Question 7

Synthesis of nylon (types of reaction for polyamide formation)

Answer 7

• Ring opening polymerisation of cyclic lactam
• Direct amide formation

Question 8

Compare and contrast the two types of mechanism for polymerisation (6 points)

Answer 8

• Active site vs many increasing chains
• Monomer concentration changes that occur throughout reaction
• Is there a termination step?
• Reaction speed and its contributors
• Effect of longer reaction times on Molecular weight of product
• Composition of different chain lengths at a given point in reaction

Question 9

What is the nucleophile in ring opening polymerisation of a cyclic lactam? How is it produced?

Answer 9

• The nucleophile in ring opening polymerisation of a 7M cyclic lactam is another 7M cyclic lactam; this is deprotonated by attack by nucleophilic NaH
• Very small concentration of NaH is therefore required at the start which is subsequently removed

Question 10

What is the role of solvents such as NMP, DMAc?

Answer 10

• In amination of diacyl chlorides with diamines, these basic solvent are use to solvate the polymer and keep it in solution until high Mw achieved
• They also serve to absorb HCl as it is produced