Page 209 - Engineering Plastics Handbook

182 Engineering Plastics

blend provided the basis for the family of PPE/PS alloys, which were
commercialized in 1966 under the Noryl* trademark [3]. Modified PPE
resins combined the best features of PPE resins and styrene polymers.
The Noryl family of resins has become the world’s most successful and
best-known polymer blends or alloys.

Polymerization and Polymer Structure
The unique catalytic oxidative coupling polymerization to high-molec-
ular-weight, linear aromatic ethers is typically carried out at room tem-
perature by bubbling oxygen through a solution of 2,6-xylenol monomer
in the presence of a copper-based catalyst [1, 2, 4].
An interesting feature of the oxidative coupling reaction is that it is
a step-growth condensation polymerization. The rate of 2,6-xylenol oxi-
dation is first-order in catalyst, first-order in oxygen pressure, and zero-
order in 2,6-xylenol [5]. When the targeted molecular weight is reached,
the polymerization is stopped by eliminating the oxygen.
The commercial synthesis of PPE is carried out at 77 to 122°F (25 to 50°C)
[4, 6]. However, these polymerizations are exothermic and generally require
cooling to attain high-molecular-weight products and selectivity. A typical
catalyst is composed of a cuprous halide salt and one or more amines such
as pyridine, dibutyl amine, or certain hindered diamines [5–8].
The accepted mechanism involves oxidation, radical coupling, dissocia-
tion, and enolization and is shown in Fig. 9.1 [9–11]. Initially 2,6-xylenol,
1, is oxidized by the catalyst to generate aryloxy radicals [12]. Then two
aryloxy radicals couple to form a quinone ether, 2, which undergoes eno-
lization to form dimer, 3. The enolization gives a more stable phenol-ter-
minated oligomer, which can be oxidized to generate a new aryloxy radical
and continue the polymerization. In like manner dimers are converted to
trimers and eventually oligomers to higher-molecular-weight oligomers
until high-molecular-weight polymer, 6, is obtained.
In addition, coupling can occur between various oligomers. For exam-
ple, two dimer radicals, 4, can couple to form a quinone ketal, 5. The
unstable quinone ketal then dissociates to trimer and 2,6-dimethylphenoxy
radical. The coproduct of the dissociation, 2,6-dimethylphenoxy radical, can
undergo further coupling reactions with other oligomers or another 2,6-
dimethylphenoxy radical. By this mechanism oligomers would undergo
oxidation, coupling, and dissociation to form higher-molecular-weight
oligomers and eventually to polymer, 6.
The principal repeat unit in high-molecular-weight polymer, 6, is the 2,6-
dimethyl-1,4-phenylene unit [13, 14]. In general, a 2,6-dimethylphenoxy
tail end group and a 3,5-dimethyl-4-hydroxyphenyl head end group are
present at opposite ends of the polymer chain [15].