Page 385 - Elements of Chemical Reaction Engineering Ebook

356 Noneiementary Reaction Kinetics Chap. 7

7.3.1 Step Polymerization

Step polymerization requires that there is at least a reactive functional
group on each end of the monomer that will react with functional groups with
other monomers. For example, amino-caproic acid
NHz-( CHZ),-COOH

has an amine group at one end and a carboxyl group at the other. Some com-
mon functional groups are -OH, -COOH , -COC1 , -NH,.
In step polymerization the molecular weight usually builds up slowly
Structural Unit Repeating Unit

Dimer 2H ‘(NH-R-CO)’ OH --+ H (NH-R-CO); OH + HzO
For the case shown above the structural unit and the repeating unit are the same.
Letting A=H, R=NH-R,-CO, and B =OH, AB =H20. We can
write the above reaction as

Dimer ARB + ARB ___) A-Rz-B+AB
+ A-Rz-B -
Trimer ARB + A-R,-B A-R,-B + AB
Tetramer ARB + A-R,-B __j A-R4-B + AB
A-Rz-B A-R4-B + AB
Pentamer ARB + A-lX4-B - A-R,-B + AB

A-Rz-B + A-R,-B + A-R,-B + AB
Hexamer ARB + A-R,-B A-&-B +AB
A-Rz-B + A-Rd-B __j A-Rb-B + AB
A-R,-B + A-R,-B __j A-&-B +AB

etc.

overall: n NH,RCOOH __j H(NHRCO), OH + (n - l)HzO
n ARB + A-R,-B + (n - 1)AB‘

We see that from tetramers on, the -mer can be formed by a number of differ-
ent pathways.
The A and B functional groups can also be on different monomers such as
the reaction for the formation of polyester (shirts) from diols and dibasic acids.
Unit 1 Unit 2 Unit1 Unit2
- A-
nHOKOH +nHOOCR,COOH __j HO (R,OOCR,COO), H + (2n - 1) HzO
Repeating Unit
n AR, A + n BR, B - A (R, - R2), B + (2n - 1) AB