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Polymer chemistry

Polymer chemistry. Part 3 Molecular Weight and MWD. Factors Influence on the Molecular Weight. Molecular Weight Control in Linear Polycondensation. Molecular Weight Distributions in Linear Polycondensation. The Factors Influence on the Molecular Weight of Polycondensation.

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Polymer chemistry

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  1. Polymer chemistry

  2. Part 3Molecular Weight and MWD Factors Influence on the Molecular Weight Molecular WeightControlin LinearPolycondensation Molecular Weight Distributionsin LinearPolycondensation

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

  4. The Catalyzed System Self-Catalyzed system： = 2k3 t + 1 External Catalysis system： = k2C0t + 1

  5. The Extent of Reaction With the samemole ratio of groups ： =

  6. Molecular WeightControlin LinearPolycondensation Key: Deactivation of the functional end groups, i.e., stabilization of molecular weight • One way is slightly over weight of one reactants (non-stoichiometric). Finally another reactant completely react and all the chain ends posses thesame functional group. • Another way is to add a spot of monofunctional monomer.

  7. Case 1 Non-stoichiometric of Functional Groups 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. and if the reaction tends to the end, B-B is present in excess. The stoichiometric imbalance r of the two functional groups is given by r=Na/Nb( r≤1).

  8. The non-stoichiometric reactants At time of 0 , 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

  9. The total of macromolecules At time of t , • The extent of reaction of group a is given by Pa • Reacted a ： NaPa b : NaPa • Unreacteda ：Na(1－ Pa) b : Nb － NaPa Then • The total of unreacted a andb is given by: Na＋Nb－2NaPa • The total of macromolecules is : (Na＋Nb－2NaPa ) /2

  10. Thus Substitute r=Na/Nb The group A is used up ( Pa=1) The moles of the two groups are equal（r=1） If r =1

  11. 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.

  12. Excessive mole-percent q • The Excessive percentage q ofb-R‘-b molecule is often used besides r. • The q is defined as: namely

  13. Case 2 Adding monofunctional monomerR"-b to capture the end group • 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.

  14. ① The systems of a-R-a + b-R‘-bwiththe 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.

  15. ② The system of a-R-b The number of the R"-b is NC

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

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

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