Chemistry Reference
In-Depth Information
carried out on these chain ends can convert them into other reactive func-
tional groups. These functionalized polymers can undergo a variety of
subsequent reactions, some of which are listed in table 3.1.
97
For example,
hydroxyl-terminated chains, can undergo condensation reactions with
alkoxysilanes (orthosilicates).
95, 98
A difunctional alkoxysilane leads to
chain extension, and a tri- or tetrafunctional one to network formation.
Corresponding addition reactions with di- or triisocyanates provide other
possibilities. Hydrogen atoms on silicon atoms are sufficiently reactive to
react with unsaturated groups at elevated temperatures in the presence of
platinum catalysts Thus, hydrogen-terminated chains can react with mol-
ecules having unsaturated groups, or vinyl chain ends or side groups can
react with the active hydrogen atoms on silanes.
95, 98-99
There are reactive
hydrogen atoms in the repeat units of poly(hydrogenmethylsiloxane)
[-SiH(CH
3
)-O-].
25, 100
Other possibilities can be found elsewhere.
101
A pair of vinyl or other unsaturated groups can also be linked by their
direct reactions with free radicals. Similar end groups can be placed on
siloxane chains by the use of an end blocker during polymerization.
102-103
Reactive groups such as vinyl units can be introduced as side chains by
random copolymerization involving, for example, methylvinylsiloxane
trimers or tetramers.
95
One of the most important uses of end-functionalized polymers is the
preparation of block copolymers.
102-103
The reactions are identical to the
chain extensions already mentioned, except that the sequences being
joined are chemically different. In the case of the—OSiR
2
R'Y chain end R'
is typically (CH
2
)
3-5
and Y can be NH
2
, OH, COOH, CH = CH
2
, and so on.
The siloxane sequences containing these ends have been joined to other
polymeric sequences such as carbonates, ureas, urethanes, amides, and
imides. Other functional groups include amines and sulfonic acids,
104-105
ammonium groups,
106
epoxides,
105
and chloroalkyl groups.
107
It is also possible to prepare polysiloxane ionomers. For example, PDMS
with carboxyl side groups has been prepared with a controlled number of
Table 3.1.
REACTIVE POLYMERS OF THE TYPE
XSi(CH
3
)
2
O[-Si(CH
3
)
2
O-]
n
S i(CH
3
)]
2
X
X
Reactant
Hydroxyls,—OH
Alkoxysilanes such as [Si(OC
2
H
5
)
4
]
Active hydrogens,—H
Unsaturated groups
Unsaturated groups such as—CH = CH
2
Active H atoms
"
Free radicals
