Chemistry Reference
In-Depth Information
is stereospecific and preserves the alkene geometry of the parent polybutadiene. Also, the addition
is random, showing that
the reactivity of the double bonds is independent of the sequence
environment [
57
].
Dichlorocarbene, generated in situ from an organomercury precursor, phenyl(bromo,
dichloromethyl)mercury (Seyferth reagent), adds to polybutadiene in a similar manner [
56
]. The
reactions take place under homogeneous conditions. They can be carried out on 1% solutions of the
polymer in benzene, using 10-20% mol excess of the reagent.
9.2.4 Electrophilic Additions of Aldehydes
These are additions to double bonds, like the
, and they can be carried out on natural
and synthetic rubbers [
59
,
60
]. They take place rapidly in the presence of acid catalysts. Aqueous
formaldehyde [
61
], or paraform in CCl
4
[
62
], can be used. The catalysts are inorganic acids or
anhydrous Lewis acids, like boron trifluoride in acetic acid solution [
63
]:
Prins reaction
OH
OH
Cl
+
OH
Cl
The reaction takes a different path in the absence of a catalyst [
62
]:
O
O
+
OH
H
O
The products of the Prins reaction with rubbers are thermoplastic polymers that possess fair
resistance to acids and bases. Free hydroxyl groups in the products are available for cross-linking
with diisocyanates [
64
] or by other means. The Prins reaction can be carried out directly on rubber
latexes [
65
]. It is also possible to just mill the rubber together with formaldehyde and then heat the
resultant mixture in the presence of anhydrous metal chlorides [
64
] to get similar results [
66
].
Higher aldehydes also react with natural rubber [
67
]. The reaction works best with purified rubber.
Additions take place without a catalyst at 180
C or in the presence of AlCl
3
-NaCl at 120
C. These
reactions can be carried out in the solid phase by milling the rubber with an aldehyde, like glyoxal
[
68
]. Heating in a pressure vessel at above 175
C is required to complete the reaction. Infra-red
spectra of the products from reactions in solution show presence of ether, carbonyl, and hydroxyl
groups [
69
]. Two types of additions appear to take place [
69
]:
