GR-S Rubber

Copolymerization of butadiene with styrene by free-radical mechanism has been explored very thoroughly [146]. The original efforts started during World War I in Germany. Subsequent work

during the 1930s was followed by a particularly strong impetus in the United States during World War II. This led to a development of GR-S rubber in the United States and Buna-S rubber in Germany. After World War II further refinements were introduced into the preparatory procedures and "cold" rubber was developed. Industrially, the copolymer is prepared by emulsion copolymeriza- tion of butadiene and styrene at low temperatures in a continuous process. A typical product is a random distribution copolymer, with the butadiene content ranging from 70 to 75%. The diene monomer placement is roughly 18% cis-1,4; 65% trans-1,4; and 17% 1,2. M1 of these copolymers is about 100,000. A "redox" initiator is used in the cold process, but not in the "hot" one. Also, the "hot" process is carried out at about 50°C for 12 h to approximately 72% conversion. The "cold" process is also carried for 12 h, but at about 5°C to a 60% conversion. The two recipes for preparation of GR-S rubbers are shown in Table 6.10 for comparison of the "hot" and "cold" processes. In both polymerizations, the unreacted monomer has to be removed. In the "hot" one the reaction is often quenched by addition of hydroquinone, and in the "cold" one by addition of N, N-diethyldithiocarbamate. After the monomers are steam stripped in both processes, an antioxidant like N-phenyl-2-naphthylamine is added. The latex is usually coagulated by addition of a sodium chloride-sulfuric acid solution. The "cold" process yields polymers with less branching than the "hot" one, slightly higher trans to cis ratios.

During the middle 1960s a series of butadiene-styrene and isoprene-styrene block-copolymer- elastomers were developed. These materials possess typical rubber-like properties at ambient temperatures, but act like thermoplastic resins at elevated ones. The copolymers vary from diblock structures of styrene and butadiene

Fig. 6.4 Illustration of polystyrene and polybutadiene domains to triblock ones, like styrene–butadiene–styrene:

Atypical triblock copolymer may consist of about 150 styrene units at each end of the macromol ecule and some 1,000 butadiene units in the center. The special physical properties of these block copolymers are due to inherent incompatibility of polystyrene with polybutadiene or polyisoprene blocks. Within the bulk material, there are separations and aggregations of the domains. The polystyrene domains are dispersed in continuous matrixes of the polydienes that are the major components. At ambient temperature, below the Tg of the polystyrene, these domains are rigid and immobilize the ends of the polydiene segments. In effect they serve both as filler particles and as cross-links. Above Tg of polystyrene, however, the domains are easily disrupted and the material can be processed as a thermoplastic polymer. The separation into domains is illustrated in Fig. 6.4. These thermoplastic elastomers are prepared by anionic solution polymerization with organometallic catalysts. A typical example of such preparation is polymerization of a 75/25 mixture of butadiene/styrene in the presence of sec-butyllithium in a hydrocarbon–ether solvent blend. At these reaction conditions butadiene blocks form first and when all the butadiene is consumed, styrene blocks form. In other preparations, monomers are added sequentially, taking advantage of the “living” nature of these anionic polymerizations. These block copolymers have very narrow molecular weight distributions. Also, the sizes of the blocks are restricted to narrow ranges to maintain optimum elastomeric properties.

اخر الاخبار

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

Preparation of Polystyrene by Free-Radical Mechanism

GR-N 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