Chemistry Reference
In-Depth Information
p
M
n
σ
n
5
Consider the following step-growth polymerization in which A can
react with both B and C but B and C do not react with each other.
Monomer
Molecular weight
Mass (g)
A
350
875
AA
BB
1000
2000
CC
150
15
C
225
225
CC
(b) What are the functionalities of the above monomers?
(c) What are the equivalents of the monomers?
(d) Calculate f
avg
of the formulation.
(e) At what level of the extent of reaction does gelation occur?
(f) What is the polydispersity index of the final product?
(g) GPC analysis shows that M
w
of the final product is 12,000. What is
the standard deviation of the number distribution of the molecular
weight of the polymer made?
7-14
Polyurethane is a step-growth polymer that is formed by reacting di- or
poly-isocyanates (NCO) with di- or polyols (OH). Linear polymers are
formed when both reaction partners are di-functional while three-dimen-
sional, highly cross-linked networks are formed if one or more of the reac-
tion partners has three or more reactive groups per molecule. When
preparing two-component coating formulations, the amounts of isocyanate-
and hydroxyl-containing materials have to be controlled precisely to obtain
the desired properties. And this is usually accomplished through the calcula-
tions of the equivalent weights of all the reactants used. Here, equivalent
weight is defined as the weight of material (in the unit of g) that gives one
mole of reactive functional groups. With such a definition, equivalent
weight is essentially equal to the ratio of molecular weight to the functional-
ity of the reactant. The following expressions are commonly used to calcu-
late the equivalent weights for both NCO- and OH-containing materials:
4200
NCO%
Equivalent weight of NCO-containing materials
5
100
OH number
56
;
Equivalent weight of OH-containing materials
5
Consider the following two-component polyurethane coating formulation.
