Polymer Polymer chemistrychemistry

Polymer Polymer chemistrychemistry

Factors Influence on the Molecular WeightFactors Influence on the Molecular Weight

Molecular Weight Control in Linear Polycondensation

Molecular Weight Control in Linear Polycondensation

Molecular Weight Distributions in Linear Polycondensation

Molecular Weight Distributions in Linear Polycondensation

Part 3 Molecular Weight and MWD

Closed system ： = + 1 nXK

Unclossed system ： = nX

wPn

K

The Factors Influence on the Molecular Weight of PolycondensationThe Factors Influence on the Molecular Weight of Polycondensation

The Balanced CharacteristicThe Balanced Characteristic

Self-Catalyzed system ： = 2k3 t + 12

nX

External Catalysis system ： = k2C0t + 1nX

2

0C

The Catalyzed SystemThe Catalyzed System

With the same mole ratio of groups ：

nX = P1

1

The Extent of ReactionThe Extent of Reaction

Key: Deactivation of the functional end groups, i.e., stabilization of molecular weight

Molecular Weight Control in Linear PolycondensationMolecular Weight Control in Linear Polycondensation

One way is slightly over weight of one reactants (non-stoichiometric). Finally another reactant completely react and all the chain ends posses the same functional group.

Another way is to add a spot of monofunctional monomer.

For the systems of a-R-a + b-R‘-b, not a-R-b

The numbers of A and B functional groups are given by Na and Nb, respectively.

The stoichiometric imbalance r of the two functional groups is given by r=Na/Nb ( r≤1).

B-B is present in excess.

Case 1 Non-stoichiometric of Functional GroupsCase 1 Non-stoichiometric of Functional Groups

and if the reaction tends to the end,

a-R-a b-R'-b The number of group A is Na The number of group B is Nb

The number of a-R-a is given by Na/2

The number of b-R-b is given by Nb/2

The total of monomer molecules (namely the total of construction units) is given by （ Na ＋ Nb ） / 2

The non-stoichiometric reactants

At time of 0 ,

The total of macromolecules

The extent of reaction of group a is given by Pa Reacted a ： NaPa b : NaPa Unreacted a ： Na(1 － Pa) b : Nb － NaPa

The total of unreacted a and b is given by:

Na ＋ Nb － 2NaPa The total of macromolecules is :

(Na ＋ Nb － 2NaPa ) /2

At time of t ,

Then

aaba

bn 2-2

2

12

1

PNNN

NN

PNNN

NNX a

aaba

ba

Thus

Substitute r=Na/Nb

aPX

1

1n

The moles of the two groups are

equal （ r=1 ）

The group A is used up ( Pa=1)

If r =1

Theoretically, if the mole ratio of the two functi

onal groups in the systems of a-R-a + b-R‘-b can be

kept equal ， the average degree of the condensati

on polymer will reach the maximum to the end of re

action.

• The Excessive percentage q of b-R‘-b molecule is often used besides r.

• The q is defined as:

namely

Excessive mole-percent qExcessive mole-percent q

R"-b can react with group a in the polymer.

By this method, the end groups are captured, giving the stabilized molecular weight.

The molecular weight of polymer can be adjusted by R”-b.

This method can be used for both the system a-R-a / b-R’-b and the system of a-R-b.

Case 2 Adding monofunctional monomer R"-b to capture the end group

Case 2 Adding monofunctional monomer R"-b to capture the end group

① The systems of a-R-a + b-R‘-b with the same mole ratio

The number of the R"-b is NC

The coefficient 2 of Nc is required since quantitatively one molecule R”-b has the same effect as one excess molecule b-R’-b on restricting the polymer chain growing.

② The system of a-R-b

The number of the R"-b is NC

The high purity of monomer

Precise measurement

Proper temperature control

Protected by inert gas, along with using catalytic agent and vacuumizing the system

SummarySummary

The molecular weight distribution (also called Polydispersity Index) is then given by:

Polydispersity IndexPolydispersity Index

As P→1 ， D approaches to two. The value of D increases with increase of he extent of

reaction.

The broader the molecular weight distribution is, the more uniform the molecular weight is.