Chemistry Reference
In-Depth Information
The redistribution process leads to production of polymers from low molecular weight radicals. It
appears that this process is unlikely to take place with high molecular weight radicals because there
are too many steps involved in the production of monomer radicals. Quinone ketals are the
intermediates in the rearrangement. The carbonyl oxygen of a ketal is within bonding distance of
the
position of the next succeeding benzene ring [
104
]. The rearrangement can, therefore, give
rise to a new ketal in which the second ring carries the carbonyl oxygen. The carbonyl oxygen finally
ends up on a terminal unit [
104
] and is reduced to OH:
para
+
O
O
O
O
O
O
O
O
rearranegem
ent
O
O
O
O
OH
H
The quinone rearranges to a phenol through enolization. The product is identical to one that is
obtained by direct head to tail coupling of two aryloxy radicals [
102
].
It is important to realize that both processes, redistribution and rearrangement reactions, can occur within
the same polymer molecule. At any point during the rearrangement, there may be dissociation into aryloxy
radicals. Also, redistribution does not have to occur by transfer of only a single unit. Rearrangement,
followed by dissociation, allows any number of monomer units to be transferred in an essentially single step.
2,6-Diphenylphenol and 2,6-dimethylphenol can copolymerize by oxidative coupling [
104
]. If the
diphenyl derivative is polymerized first and subsequently the dimethyl derivative is added to the
reaction mixture, block copolymers form. If, however, the order is reversed or both phenols are
polymerized together, a random copolymer results [
104
].
Poly(phenylene oxide)s can also be formed by oxidative displacement of bromides from 4-bromo-
2,6-dimethylphenol [
102
,
105
]. Compounds, like potassium ferricyanide, lead oxide, or silver oxide,
catalyze this reaction:
K
2
FrCNO
6
KOH
n
Br
OH
O
n
Poly(phenylene oxide)s also form through photodecomposition of benzene-1,4-diazooxides [
102
]:
hν
n
+
N
2
O
O
N
2
n
Oxidative coupling is the only process used commercially. Although poly(phenylene oxide) is an
important commercial material, there was initially a processing problem when the material was first
introduced. Currently, a large portion of the polymer is sold as a blend with polystyrene (probably
high-impact) to make it more attractive economically and easier to process. The ratios of poly(2,6-
dimethylphenylene oxide) to polystyrene range from approximately 1:1 to 1:2 and the materials are
sold under the trade name of Noryl. A fire retardant grade, containing about 5% of an additive,
believed to be triphenyl phosphate, is also on the market.
