Polymers In Solution

Question 1

Polymer solubility, what its determined by, what can water soluble polymers do, why are they useful?

Answer 1

• Solubility of polymers is determined by the
  same factors that govern the solubility of
  lower molecular weight molecules
  – Polymer sufficiently polar → likely to dissolve in water
• Water-soluble polymers are potentially able to:
  – Increase the viscosity of solvents
  – Swell / change shape (in solution)
  – Adsorb at surfaces and interfaces
• These properties make water-soluble polymers useful as
  suspending agents, e.g. xanthan gum1
• Insoluble polymers are often capable of forming thin films
  and are used in tablet coating, wound dressings,
  membranes etc

Question 2

Describe the process of polymer dissolution (water soluble)

Answer 2

• Polymer dissolution is a slower process
• Swollen (gel) layer appears at the surface
  of the polymer and outside this is a diffusion layer
• Polymer molecules may leave the bulk polymer from the
  diffusion layer
• Slow polymer dissolution rates + existence of these layers creates opportunities for developing modified release
  dosage forms

Question 3

Water soluble polymers: what two other factors have impact on solubility of polymers?

Answer 3

• Two other factors have a big impact on the
  solubility of polymers
• Polymer molecular weight
  – Partially determines the rate of dissolution
  – ↑ molecular weight causes ↓ in rate of dissolution
• Polymer crystallinity
  – Polymers can be crystalline or amorphous
  The more crystalline a polymer is then the lower the dissolution rate

Question 4

Water soluble polymers and pharmaceutical applications.. what’s the first application?

Answer 4

• Cellulose is insoluble in water, but many similar polymers derived from cellulose
  exhibit improved aqueous solubility and have applications
  in pharmacy

Question 5

Water soluble polymers and pharmaceutical applications.. what’s the next couple of applications?

Answer 5

Hydroxypropyl methylcellulose (HPMC) (cellulose derivative)
*HPMC is also known as
“Hypromellose” – used in eye
drops for dry eyes
*HPMC commonly used in the
polymer film coating of tablets

Question 6

Water soluble polymers and pharmaceutical applications.. what’s the final application?

Answer 6

• Dextran is a branched polymer of α-d-glucose (dextrose)
• Active ingredient uses: artificial tears,
  emergency blood plasma expander and anticoagulant
• Used as a protectant/ bulking agent in lyophilised (freeze
  dried) formulations

Question 7

Basis of pH sensitive polymers

Answer 7

• Ionisation of weak acids and bases is
  dependent on pH (Henderson-Hasselbalch)
• The same applies if acidic / basic groups on a polymer
• Charge of (substituent groups on) a polymer can also
  impact upon its molecular shape

Question 8

ionised vs unionised polymer in high pH?

Answer 8

Unionised → polymer contracted; low viscosity
Ionised (charged) → groups repel; expanded structure; increased viscosity

Question 9

pH sensitive polymers + enteric tablet coating

Answer 9

• pH-sensitive polymers are very important in
  enteric tablet coating
• Many reasons why tablets are coated – we will consider
  this in Year 2
• Polymethacrylates contain an
  acid functional group which only
  ionises at higher pH values