Polycarbonates

A special group of polyesters of carbonic acid are known as polycarbonates. The first polycarbonates were prepared as early as 1898 by Einhorn by reacting phosgene with hydroquinone and with resorcinol [33]. These materials lack desirable properties and remain laboratory curiosities. During the fifties, however, new polymers were developed from 4,40-dihydroxydiphenyl alkanes. These polycarbonates have high melting points and good thermal and hydrolytic stability. Nevertheless, to date only one polycarbonate has achieved significant commercial importance. It is based on 2,20 bis(4-hydroxyphenyl) propane. There are two main methods for preparing polycarbonates, one is by a direct reaction of phosgene with the diphenol and the other one by an ester interchange. The direct phosgenation is a form of a Schotten–Baumann reaction that is carried out in the presence of a base:

The reaction may be carried out in the presence of pyridine that acts as a catalyst and as an HCl scavenger. Often a chlorinated solvent is used as a diluent for the pyridine. Phosgene is bubbled through a solution of the diphenol at 25–35C. The pyridine hydrochloride precipitates out and after washing the pyridine solution with dilute HCl and water, the polymer is precipitated with a nonsolvent. An interfacial polymerization procedure is also employed in direct phosgenation. A caustic solution of the diphenol is dispersed in an organic chlorinated solvent containing small quantities of a tertiary amine. Phosgene is bubbled through the reaction mixture at 25C. When the reaction is complete, the organic phase contains the polymer. It is separated and the product isolated as above. The ester interchange method is carried out between the diphenol and diphenyl carbonate:

To obtain high molecular weights by this method, almost complete removal of the phenol is required. The reaction is carried out with typical basic catalysts, like lithium hydride, zinc oxide, or antimony oxide under an inert atmosphere. Initial reaction temperature is 150C. It is raised over 1 h period to 210C, while the pressure is reduced to 20 mmHg. Reaction mixture is then heated to about 300C for 5–6 h at 1 mmHg. Heating is stopped when desired viscosity is reached. A synthetic route to polycarbonates was reported that uses crown ethers. Crown ethers generally form stable complexes with metal cations, and by increasing the dissociation of ion pairs, provide highly reactive, unsolvated anions. This led to direct preparations of new polycarbonates from α,w dibromo compounds, carbon dioxide, and potassium carbonate or salts of the diols [36] in the presence of 18-crown-6-ether:

When a potassium salt of a diol is present in the reaction mixture, mixed polyesters form [36]:

In addition to crown ethers, cryptates and polyglyme exhibit similar behavior [37]. Another direct route to polycarbonate was reported by Okuyama et al. [36]. The process consists of an oxidative carbonylation procedure. It is catalyzed with a Pd complex system. {1,10-Di-tert-butyl 3,30-methylenediiimidazolin-2,20, dylidene} palladium dibromide and produces a highest molecular weight polymer, (Mn = 9,600, Mw = 24,000) in 80% yields. Table 7.2 presents some Tg and Tm values of some polycarbonates picked from the literature.

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الهيأة العليا لإحياء التراث تنهي استعداداتها لإقامة المؤتمر التمهيدي الأول لإحياء ألفية حوزة النجف الأشرف

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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

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الآخبار التكنلوجية

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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

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مواضيع ذات صلة

Aromatic Polyamides

Special Reactions for Formation of Polyamides

Fatty Polyamides

Nylons

Polyesters from Lactones

Network Polyesters for Surface Coatings

Linear Unsaturated Polyesters

Copolyesters

Synthetic Methods

Linear Saturated Polyesters

Techniques of Polymer Preparation

Ring Formation in Step-Growth Polymerizations