Poly (phenylene oxide) s

These polymers are known for good thermal stability and good mechanical properties. Commercially, these aromatic polyethers are prepared by oxidative coupling of phenols [102]. To obtain linear polymers and achieve high molecular weights, 2 and 6 positions of the phenol must be protected by substituents. This causes the aromatic rings to couple in 1,4 positions. When 2,6-dimethylphenol is used, the reaction takes place at room temperature. Oxygen is bubbled through a solution of the phenol in the presence of an amine-cuprous salt catalyst:

Phenols with halogen substituents require higher temperatures. Large substituents can lead to carbon-to-carbon coupling instead of carbon to oxygen:

The active catalyst is believed to be a basic cupric salt that forms through oxidation of cuprous chloride followed by complexation with two molecules of the amine [102]:

This is a step-growth polymerization involving phenoxy radicals. The polymer formation can be illustrated as follows:

Dissociation leads to aryloxy radicals or to two new radicals that couple. Quinone ketals are formed initially. They dissociate to yield the original aryloxy radicals and then couple [102]:

Formation of aryloxy radicals as intermediates was established with ESR spectroscopy studies that showed presence of both monomeric and polymeric radicals in the reaction mixture [103]. Coupling occurs by two paths. One of them through rearrangements and the other one through redistribution. In the redistribution process, two aryloxy radicals couple to yield an unstable quinone ketal as shown above [102]. This ketal decomposes rapidly either back into the original aryloxy radicals or into two different aryloxy radicals as follows:

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 para 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:

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:

Poly(phenylene oxide)s also form through photodecomposition of benzene-1,4-diazooxides [102]:

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.

اخر الاخبار

اخبار العتبة العباسية المقدسة

ضمن فعاليات برنامج مخيّم (خيام الكفيل) مركز الثقافة الأسرية يقدم محاضرات عقائدية ونفسية لنحو 150 مشارِكةً من ذوي الشهداء

قسم السياحة الدينية يُطلق حملة الساقي لأداء مناسك العمرة

العتبة العباسية المقدسة تختتم مشاركتها في معرض بغداد الدولي بدورته الـ 49

بقدرة 72 كيلو واط.. قسم الاتصالات ينهي تنصيب منظومة الطاقة الشمسية الثانية في مجموعة العميد التربوية

المزيد

الآخبار الصحية

عادات يومية تدمر قلبك بصمت.. خبراء يطلقون التحذير

خبراء تغذية يكشفون "أفضل وقت لتناول الإفطار"

أطعمة تزيد فرص وفاة مرضى السرطان 60 بالمئة.. ما هي؟

اختبار ثوري جديد يكشف مبكرا عن سرطان البنكرياس

المزيد

الآخبار التكنلوجية

الذكور أم الإناث؟ دراسة "تنسف الشائع" بشأن التوحد

خبير يكشف عن اقتراب العلماء من فك شفرات "لغات الحيوانات" !

دراسة تدفعك لشراء ساعة ذكية "فورا".. هذا ما تقوله عن قلبك

علماء حاصلون على "نوبل": الذكاء الاصطناعي لن يستبدل الإنسان

المزيد

مواضيع ذات صلة

Higher-order structures

The convergence of energy levels

Absorption intensities

Butadiene and π-electron binding energy

Reversible expansion

Free expansion

The hydrogen cycle

Ring and cluster compounds

Resolutions can be carried out by chromatography on chiral materials

Axes and centres of symmetry

المرافق الإنزيمي حامض الليبويك Lipoic acid

التأثير التحريضي (الحاث) Inductive effect