Chemistry Reference
In-Depth Information
p
c
¼
2
=f
ave
where
f
ave
is the functionality. This equation, however, was written for reaction mixtures that contain
two different functional groups in stoichiometric proportions to each other. All the functional groups,
however, for various reasons, might not participate in photocross-linking reactions. Even though
these reactions may involve identical groups, this equation probably would not apply.
A statistical approach was developed by Flory [
107
,
108
] and by Stockmayer [
109
] to derive an
expression for predicting the extent of reaction at the gel point. It is expressed as
0
:
5
p
c
¼
1
=½rðf
wA
1
Þðf
wB
1
Þ
where
f
wB
are weight average functionalities of A and B functional groups. Here the
functionalities are defined as,
f
wA
and
.
.
X
A
i
N
A
i
X
X
B
i
N
B
i
X
2
2
f
wA
¼
f
f
A
i
N
A
i
and
f
wB
¼
f
f
B
i
N
B
i
When the cross-linking, however, takes place by cyclization through dimerization of pendant
groups, it appears that the gelation should probably be treated instead as a case of multiple
dimerizations.
10.5.1 Polymers That Photocross-link by Formation of Cyclobutane Rings
Many of the photocross-linkable polymers for photo-imaging in use today react by a 2
type
dimerization with the accompanying formation of the cyclobutane rings [
108
,
109
]. The formation of
the cyclobutane ring can be simply shown on the photocross-linking reaction of poly(naphthyl vinyl
acrylate), a polymer that also undergoes this type of dimerization [
110
]:
p
+2
p
O
O
O
2
UV ligh
y
O
O
O
The naphthalenes become bonded to cyclobutane ring in 1,2 and 1,3 positions. Many polymers
with other functional groups can also photocross-link by 2
p
p
+2
addition. Following is an illustra-
tion of some of these groups [
111
,
112
]:
R
R
N
S
R
O
O
