Polymer chemistry Polymer chemistry

Factors Influence on the Molecular Weight Molecular Weight Control in Linear Polycondensation Molecular Weight Distributions in Linear Polycondensation Part 3 Molecular Weight and MWD

Closed system ： = + 1 Unclossed system ： = The Factors Influence on the Molecular Weight of Polycondensation The Balanced Characteristic

Self-Catalyzed system ： = 2k 3 t + 1 External Catalysis system ： = k 2 C 0 t + 1 The Catalyzed System

With the same mole ratio of groups ： = The Extent of Reaction

Key: Deactivation of the functional end groups, i.e., stabilization of molecular weight Molecular 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 N a and N b, respectively. The stoichiometric imbalance r of the two functional groups is given by r=N a /N b ( r≤1). B-B is present in excess. Case 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 N a The number of group B is N b The number of a-R-a is given by Na/2 The number of b-R-b is given by N b /2 The total of monomer molecules (namely the total of construction units) is given by （ N a ＋ N b ） / 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 ＋ N b － 2NaPa  The total of macromolecules is : (Na ＋ Nb － 2NaPa ) /2 At time of t, Then

Thus Substitute r=Na/Nb 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 functional groups in the systems of a-R-a + b-R ‘ -b can be kept equal ， the average degree of the condensation polymer will reach the maximum to the end of reaction.

The Excessive percentage q of b-R ‘ -b molecule is often used besides r. The q is defined as: namely Excessive 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

① The systems of a-R-a + b-R ‘ -b with the same mole ratio The number of the R"-b is N C 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 N C

 The high purity of monomer  Precise measurement  Proper temperature control  Protected by inert gas, along with using catalytic agent and vacuumizing the system Summary

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