Polystyrene Prepared by Ionic Chain-Growth Polymerization

Much research was devoted to both cationic and anionic polymerizations. An investigation of cationic polymerization of styrene with Al(C₂H₅)₂Cl/RCl (R = alkyl or aryl) catalyst/cocatalyst system was reported by Kennedy [161, 162]. The efficiency (polymerization initiation) is determined by the relative stability and/or concentration of the initiating carbocations that are provided by the cocatalyst RCl. N-butyl, isopropyl, and sec-butyl chlorides exhibit low cocatalytic efficiencies because of low tendency for ion formation. Triphenylmethyl chloride is also a poor cocatalyst because the triphenylmethyl ion that forms is more stable than the propagating styryl ion. Initiation of styrene polymerizations by carbocations is now well established [163].

Anionic polymerization of styrene with amyl sodium yields an isotactic polymer [164]. Polymerizations catalyzed by triphenylmethylpotassium also yield the same stereospecific polysty rene [165]. The same is true of organolithium compounds [166, 167]. In butyllithium initiated polymerizations of styrene in benzene termination were claimed to occur by association between the propagating anions and the lithium counterions with another butyllithium molecule [168]. This was contradicted by claims that terminations result from association of two propagating chains [169]. Alfin catalysts polymerize styrene to yield stereospecific products [170]. Coordination catalysts based on aluminum alkyls and titanium halides yield isotactic polystyrene [171–174]. The polymer matches isotactic polystyrene formed with amyl sodium. It is composed of head-to-tail sequences with the main chain fold being helical. There are three monomer units per each helical fold [171, 172]. The catalyst composition, however, has a strong bearing on the microstructure of the resultant polymer [175, 176]. Ishihara et al. reported in 1986 that syndiotactic polystyrene can be prepared with the aid of organic or inorganic titanium compounds activated with methylaluminoxane [177]. There is much greater incentive to commercialize syndiotactic polystyrene than the isotactic one. This is because isotactic polystyrene crystallizes at a slow rate. That makes it impractical for many industrial applications. Syndiotactic polystyrene, on the other hand, crystallizes at a fast rate, has a melting point of 275C, compared to 240C for the isotactic one, and is suitable for use as a strong structural material. Many catalysts were investigated for the preparation of syndiotactic polystyrene. High activity is claimed for half-sandwich titanocenes of the type CpTiCl3, IndTiCl3, and substituted IndTiCl₃ with methylaluminoxane as cocatalyst [178]. It was also reported that BMe(C₆F₅)₃ and other borates can be used as precursors instead of methyl-aluminoxane [178]. In addition, it was disclosed that fluorinated catalysts exhibit very high activities and produce polymers with higher molecular weight polymers [178]. The mechanism of polymerization was investigated by different groups. Grassi and Zambelli claim that the syndiotactic styrene polymerization proceeds through a secondary insertion of styrene into Ti–alkyl (or growing polymer chain) bond. In the half titanocene catalyst, the polymer chain appears to be Z6-coordinated to the metal of the active species [179]:

Maron and coworkers [180] reported that theoretical methods were used to investigate the syndiospecificity of the styrene polymerization catalyzed by single-site, single-component allyl ansa-lanthanidocenes:

Two limiting chain end stereo control mechanisms were studied by them, namely, migratory insertion through a site epimerization and site stereo configuration independent of backside insertion on a “stationary” polymer chain. Four consecutive insertions of styrene were computed to reveal that (i) backside insertions are more favorable than, or at least as favorable as, frontside insertions. The formation of a syndiotactic polymer is controlled by the thermodynamics. Moreover, the odd (first and third) insertions are of 2,1-down-si-type and are kinetically favored over the 2,1-up-re-ones. This control is the conjunction of two effects: minimization of styrene–styrene and styrene (phenyl ring)–fluorenyl repulsions. The steric hindrance of the polymer chain induces a fourth insertion by an exocyclic coordination of the fluorenyl ligand that is compensated by the n⁶ coordination of one of the phenyl ring in the growing chain. Syndiotactic polystyrene is available commercially under the trade name of Questra. This material is produced with the aid of a metallocene catalyst and is sold in several grades [181]. There is a small interest in forming isotactic polystyrenes with vary narrow molecular weight distributions, because of some very limited practical applications, and from purely academic interests. Several preparations of virtually monodisperse polystyrenes of Mw/Mn = 1.06 by anionic polymerizations were developed. The materials are available commercially [181–186], small quantities for use as standards for GPC.

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

Preparation of Polystyrene by Free-Radical Mechanism

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