L1 - 2, Polymer Basics

Question 1

Features of polymers (transitions):

Answer 1

• Solid glassy -> (Glass transition) solid rubbery -> (melt) highly viscous liquid
• 2 solid phases
• Reversible transitions

Question 2

Mn:

Answer 2

• Number average
• Mn = sum of ni*Mi/ sum of ni
• Where ni = number of chains with a given molecular weight
• Mi = molecular weight

Question 3

Mw:

Answer 3

• Weight average
• Mw = sum of ni(Mi)^2/ sum of niMi
• Where ni = number of chains with a given molecular weight
• Mi = molecular weight

Question 4

Dispersity:

Answer 4

• Mw / Mn
• Important measure of how controlled a polymerisation is
• Influences most polymer properties (e.g. broader distribution of MW negatively affects melting point etc)
• Historically called polydispersity index

Question 5

What is a controlled / living polymerisation?

Answer 5

• Proceeds with no chain transfer reactions or terminations; all chain ends grow at same rate
• As a result, the polymer product should have a very narrow MW distribution
• Polymerisation are rarely truly living, but we can often achieve high levels of control by judicious design of catalysts and initiators

Question 6

Key features of a controlled polymerisation:

Answer 6

• A linear increase in Mn with percentage conversion of monomer
• A linear increase in Mn with 1/[initiator]0, at a given conversion
• A narrow dispersity (1.0-1.2)
• The ability to form a block copolymer
• ki»>kp (rates of propagation and initiation)
• kp»>kct (rate of chain transfer)

Question 7

Potential polymer tacticities:

Answer 7

• Isotactic (same side)
• Syndiotactic (perfectly alternating)
• Heterotactic (alternating pairs)
• Atactic (lack of order; random)
• Hemiisotactic: Alternating between one tacticity and random
• Measured by relationship between linkages
• Tactic materials typically have higher degree of crystallinity -> desirability and functionality

Question 8

Types of linkages in polymers:

Answer 8

• Meso: Alternating
• Racemic: Same
• Taken as probabilities
• Pm + Pr = 1
• There will always be some degree of stereoerror

Question 9

How might linkages be measured? Effects on certainty of results?

Answer 9

• NMR
• Level of certainty over linkages depends on sensitivity of NMR…
• Triad: 2 different linkages, tetras: 3 etc -> more accurate to consider more stereocentres

Question 10

Why should catalysts be discrete single-site catalysts?

Answer 10

• Variation between centres in the cluster would create a lot of inconsistency
• Ideally, to achieve controlled reactions the metal centres should be as identical as possible
• Ligands are used to create this effect

Question 11

Key polymer properties for commercial applications:

Answer 11

• Melting point
• Glass transition temperature
• Strength
• Elasticity
• Economics (scaleable, robust, react at lower temperatures and pressures)