Preparation of Polystyrene by Free-Radical Mechanism

Styrene is one of those monomers that lends itself to polymerization by free-radical, cationic, anionic and coordination mechanisms. This is due to several reasons. One is resonance stabilization of the reactive polystyryl species in the transition state that lowers the activation energy of the propagation reaction. Another is the low polarity of the monomer. This facilitates attack by free-radicals, differently charged ions, and metal complexes. In addition, no side reactions that occur in ionic polymerizations of monomers with functional groups are possible. Styrene polymerizes in the dark by free-radical mechanism more slowly than it does in the presence of light [149]. Also, styrene formed in the dark is reported to have greater amount of syndiotactic placement [150]. The amount of branching in the polymer prepared by free-radical mechanism increases with temperature [136]. This also depends upon the initiator used [151].

The following information has evolved about the free-radical polymerization of styrene:

1. Styrene polymerizes thermally . 

 2. Oxygen retards polymerizations of styrene. At higher temperatures, however, the rate is accelerated due to peroxide formation.

 3. The rate of styrene polymerizations in bulk is initially, at low conversions, first order with respect to monomer concentrations. In solution, however, it is a second order with respect to monomer .

Polystyrene that is manufactured by free-radical polymerization is atactic. Isotactic polystyrene formed with Ziegler–Natta catalysts was introduced commercially in the 1960s, but failed to gain acceptance. Syndiotactic polystyrene is now being produced commercially. Industrially, free-radical styrene polymerizations are carried out in bulk, in emulsion, in solution, and in suspension. The clear plastic is generally prepared by mass polymerization. Because polystyrene is soluble in the monomer, mass polymerization, when carried out to completion, results in a tremendous increase in melt viscosity. To avoid this, when styrene is polymerized in bulk in an agitated kettle, the reaction is only carried out to 30–40% conversion. After that, the viscous syrup is transferred to another type of reactor for the completion of the reaction. According to one early

Fig. 6.5 Adiabatic tower for mass polymerization of styrene German patent, polymerization is completed in a plate and frame filter press [157]. Water circulating through the press removes the heat of the reaction, and the solid polymer is formed inside the frames. This process is still used in some places [158]. Another approach is to use adiabatic towers. Styrene is first partially polymerized in two agitated reaction kettles at 80–100C. The syrup solution of the polymer in the monomer is then fed continually into the towers from the top. The temperatures in the towers are gradually increased from 100–110C at the top to 180–200C at the bottom. By the time the material reaches the bottom, in about 3 h, the polymerization is 92–98% complete [159]. The unreacted monomer is removed and recycled. A modification of the process is to remove the monomer vapor at the top of the tower for reuse. An improvement in the above procedure is the use of agitated towers [160]. To avoid channeling inside the towers and for better heat transfer, three towers are arranged in series. They are equipped with slow agitators and with grids of pipes for cooling and heating [160]. Polymeric melt is heated from 95 to 225C to reduce viscosity and help heat transfer. A solvent like ethyl benzene may be added. A vacuum devolatalizer removes both monomer and solvent from the product .

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اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة: نهج البلاغة مصدر صافٍ للفكر والمعرفة ومحصِّنٌ من الشبهات

العتبة العباسية المقدسة تطلق محاضرات محفل نهج البلاغة الأسبوعي في ذي قار

العتبتان المقدستان الحسينية والعباسية تدعوان للمشاركة في مهرجان الشموع بنسخته السادسة والعشرين

بالتعاون مع مستشفى الكفيل.. قسم الشؤون الطبّية يواصل إقامة البرنامج التدريبي لملاكاته النسوية

المزيد

الآخبار الصحية

الصحة العالمية تكشف حقيقة انتشار فيروس نيباه خارج الهند

متى يصبح محيط الخصر "مؤشر خطر"؟

دراسة تكشف دور "العوامل الوراثية" في تحديد عمر الإنسان

رائحة كريهة في الجسم.. "علامة خفية" أخطر مما تتصور

المزيد

الآخبار التكنلوجية

مايكروسوفت تكشف عن الجيل الثاني من شرائحها للذكاء الاصطناعي

خرافة "ثمانية أكواب يوميا".. كم من الماء يحتاج جسمك فعليا؟

هل توجد حياة خارج كوكبنا؟.. اكتشاف جديد يثير اهتمام العلماء

اكتشاف الخلايا المسؤولة عن فقدان ذاكرة الطفولة

المزيد

مواضيع ذات صلة

Polystyrene Prepared by Ionic Chain-Growth Polymerization

GR-N Rubber

GR-S Rubber

Cyclopolymerization of Conjugated Dienes

Special Polymers from Dienes

Chloroprene Rubber, Poly(2-chloro-1,3-butadiene)

Synthetic Polyisoprenes

High Molecular Weight Polybutadiene

Liquid Polybutadiene

Polybutadiene

Ionomers

Copolymers of Ethylene with a-Olefins and Ethylene with Carbon Monoxide