Gareth

Question 1

What is the definition of a polymer?

Answer 1

A molecule of relatively high relative molecular mass, the structure of which essentially comprises the multiple repitition of units derived actually or conceptually from molecules of low molecular mass

Question 2

When are polymers most useful?

Answer 2

When Mn is > 10000

Question 3

Why is the value of Mw higher than Mn?

Answer 3

Mw is more influenced by heavier chains

Question 4

How are the different molecular weight averages distributed?

Answer 4

Mn

Question 5

What is step growth polymerisation?

Answer 5

Two compounds interact and eliminate a small molecule like water
Also called condensation

Question 6

What is chain growth polymerisation?

Answer 6

A molecule with some degree of unsaturation adds to others

Also called addition polymerisation

Question 7

What is assumed in the Carothers equation?

Answer 7

The reactivity of the end group is independent of chain length so we use a single rate constant and equilibrium constant

Question 8

Why can we only quote an average molecular weight for a polymer?

Answer 8

Polymerisation are statistical reactions so you get a distribution of chain lengths and we can only measure the average values over all the chains in the sample

Question 9

Why can we only quote an average molecular weight for a polymer

Answer 9

Polymerisations are statistical reactions
You get a distribution of chain lengths in a sample
We can only measure the average values over all the chains in the sample

Question 10

What are the assumptions underlying the Carothers equation?

Answer 10

All the reacting groups have equal reactivity and this reactivity does not depend on the chain length
So we use a single rate and equilibrium constant

Question 11

Why is it difficult to make long chain polymers using step growth polymerisation

Answer 11

As the reaction proceeds most of the reactants will be combined into short polymer chains
These grow by reaction between their chain ends but as the polymer gets longer the viscosity of the polymer chains rises and the reactivity of the end groups goes down which means there is less chance of a reaction occurring

Question 12

What is the attraction of using ring opening polymerisation techniques?

Answer 12

You don’t have to control the stoichiometry in order to get high molecular weight chains
Also no small molecules are formed so there are no issues with removing condensation products to drive the equilibrium forward

Question 13

How might you follow the concentrations of acid groups as the reaction proceeds?

Answer 13

the tradition method is via acid-base titrations. More usually these days IR spectroscopy is used. However it is not this straightforward as you might think as there can be problems with calibration and quantification

Question 14

Why might you add acetic acid to a polyesterification?

Answer 14

To control and keep the molecular weight of a polymer from a reaction that is very efficient

Question 15

How can you make a polycarbonate without using phosgene?

Answer 15

The usual way is to take advantage of an ester interchange (transesterification) reaction
Diphenyl carbonate is often used since the phenol can be recycled and used to make disphenol A monomer. The reaction conditions are challenging but safer than using phosgene.

Question 16

Esterifications are equilibrium reactions. What are the implications of this for step growth polymerisation?

Answer 16

The equilibrium may be established with large amounts of water present which will limit the yield of the polymer and also mean that conversions are low so only low molecular weights can be achieved

Question 17

Why is there no organic equivalent of the siloxanes?

Answer 17

There are these are called polyether

The C-C bond is much stronger than the C-O bond so that all the carbon chains are more stable

Question 18

Why are polystannanes less stable than polysilanes?

Answer 18

Mainly due to the Sn-Sn bonds being much weaker than the Si-Si bonds since the atoms are much bigger and the electrons are less tightly held
Sn is more reactive to oxidation, hydrolysis than Si

Question 19

Why are the molecular weight distributions arising from I’m step growth and chain growth reactions different?

Answer 19

The kinetic schemes are different. In step growth all the molecules are potentially active and react.
The random statistical nature means that high molecular weights are only formed at high conversions
In chain growth the statistics applies to which chains are growing at any one time - only a few are active
Long chains form early on in the reaction after which not much change to the chain length occurs
Since the propagation in ionic reactions is relatively slow, steady growth occurs throughout the reaction.

Question 20

Explain why free radical polymerisations are normally carried out either under nitrogen or under vacuum. What difficulty might be encountered in polymerising 4-nitrostyrene?

Answer 20

This is since oxygen (a triplet in its ground state) reacts with radicals and so would prevent chain growth and lead to side reactions. The nitro group added to styrene means that there are more resonance forms for a nitrostyrene radical and so it will lead to many more side products as well as the main addition reaction to form polymers. It will act as an inhibitor.

Question 21

Why do most radical reactions give atactic polymers but ISO- or syndiotactic polymers commonly are produced in ionic polymerisations

Answer 21

Radical propagation reactions are rapid and non-specific. Hence the addition happens in whichever orientation the molecule happens to adopt when it encounters the radical.
Ionic reactions are slower and so the monomer has time to adopt the more thermodynamically/sterically preferred conformation as the addition takes place.

Question 22

What do you think are some of the factors which will influence the kinetics of Ziegler-Natta polymerisations? Explain which you think may be the rate limiting steps

Answer 22

The reactants are gases and the catalyst is heterogenous so the mixing and mass transfer will be important. Getting the polymer from the reaction and removing the catalyst will probably be rate limiting.

Question 23

In vinyl polymerisation initiated by radicals the initiation is the rate limiting step. In most ionic processes, the propagation is the rate limiting step. Summarise the main effects this has on the reaction.

Answer 23

Radical propagation reactions are rapid and non-specific. Hence the addition happens in whichever orientation the molecule adopts when it encounters the radical.
The main effect is the chain growth is distributed more evenly between the chain so that a much narrower distribution of molecular weights results from ionic processes

Question 24

In an attempt to increase the rate of polymerisation, the temperature of the reaction was raised. However no polymerisation occurred. Suggest a possible explanation.

Answer 24

It is likely the temperature has been raised above the ceiling temperature. This is the temperature at which the entropic contribution to the polymerisation reaction TdeltaS becomes dominant over the exothermic enthalpy change which favours polymerisation at lower temperatures