Chemistry Reference
In-Depth Information
The overall objective of this phase of our efforts was geared toward creating a
polymer system that retained the low dielectric constant and the high glass transi-
tion temperature of the hydroxy-6F-PBO polymer while becoming solvent resis-
tant and mechanically robust via post-polymer chemistry. The reaction paradigm
to achieve this goal was examined with select copolymer compositions incorpo-
rating hydroxy-6F/12F benzoxazole structural units, as will be described later. A
further step in our investigation also sought to control crosslink density by the
partial allylation of the hydroxyl groups attached to the aromatic benzoxazoles.
To accomplish this objective, two random tri-block aromatic 6F/12F benzoxazole
copolymers were successfully synthesized using 5 and 10 mole percents of allyl
bromide to effect partial O-allylation of the previously synthesized hydroxyl pen-
dant-6F/12-F-PBO through a post-polymerization reaction. The properties of the
resulting partially-allylated tri-block copolymer systems were also studied and are
reported here.
2. EXPERIMENTAL
2.1. Materials
Monohydroxyterephthalic acid (MHT) was prepared by the procedure described
in the literature [8]. 2,5-dihydroxyterephthalic acid (DHT) was purchased from
Aldrich and recrystallized from ethanol (mp = 319-320°C). Polyphosphoric acid
(assay > 83%; Fluka) was used as received, as a polymerization solvent. Ethyl-
salicylate, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (BAHH) and
2,2-bis(4-carboxyphenyl)hexafluoropropane (BCPH) were obtained from com-
mercial sources or through custom synthesis and used as received.
2.2. Monomer synthesis
The required monomer, monohydroxyterephthalic acid [8], was prepared from the
displacement reaction of 2-bromoterephthalic acid and sodium hydroxide in the
presence of sodium acetate and copper powder. Recrystallization from ethanol
provided white crystals, m.p. 325-326
°
C.
2.3. Polymer synthesis
In an effort to establish what effect intramolecular hydrogen bonding might have
on the dielectric properties, 2-hydroxybenzobisoxazole rigid-chain polymer was
synthesized (Scheme 4). The polymer from monohydroxyterephthalic acid (MHT)
and 1,3-dihydroxy-4,6-diaminobenzene dihydrochloride (DHDAB) was prepared
in polyphosphoric acid (PPA) following the P
2
O
5
polymerization method [9]. The
polymerization was carried out at a polymer concentration of 14 wt% to promote
the formation of the anisotropic rigid-rod polymer composition. It was shown that
the intramolecular hydrogen bonding had a minimal effect, if any, on the dielec-
tric constant of polybenzobisoxazole (PBO) backbone (2.93 for pristine PBO)
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