Block Copolymers of Poly(a-Olefin) s

These block copolymers form readily when appropriate Ziegler-Natta catalysts are used [436]. This is discussed in Chaps. 4 and 6. In addition, there is a special technique for preparations of such block copolymers. At the outset of the reaction, only one monomer is used in the feed. A typical catalyst might be a-TiCl3/(C2H5)3Al. After the first monomer has been bubbled in for a short period, perhaps 5 min, the addition is stopped and the unreacted monomer removed by evacuation or by flushing. The second monomer is then introduced and may also be bubbled in for the same period. The addition of the second monomer may then be stopped, the second monomer evacuated, and the whole thing might be repeated. If equal length of each block is desired, the addition times of each monomer may be varied to adjust for different rates of polymerization [436]. Kuhlman and Klosin [436] reported forming multiblock copolymers of polyethylene and tuning block composition by catalyst selection. Chain shuttling, the fast exchange of growing polymer chains among catalyst centers mediated by a chain shuttling agent (CSA), has enabled the production of ethylene-based olefin block copolymers. Diethyl zinc was used as a chain shuffling agent. Thus, copolymerization of ethylene and a-olefin by certain catalyst pairs in the pretense of a CSA produces blocks that are alternately highly or lightly branched. Chain shutling polymerizations are bet conducted in continuous reactors. A catalyst system derived from titanocene complex and methyl-aluminoxane (see Chap. 5) has been used to polymerize propylene and, depending on polymerization conditions, produce a block copolymer of crystalline and amorphous, elastomeric polypropylene [440]:

In the above process, the formation of crystalline domains involves consecutive insertions from one of the lateral coordination sites of the catalyst so as to give rise to isotactic sequences, whereas consecutive insertions at the other site (2) give rise to atactic amorphous sequences. Interconversion between these two states must occur within the lifetime of a given polymer chain in order to generate a physically cross-linked network and is believed to occur via occasional isomerizations of the polymer chains (i.e., interconversion at the metal center). Preparation of an oscillating catalyst that yields an elastomeric polypropylene was also reported by others [441].

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تشهد تشييع جثمان المحقق الشيخ محمد رضا المامقاني

قسم الشؤون الفكرية يختتم برنامج (فتية سيد الماء) لرعاية الأيتام بنسخته الثانية

لتطوير المهارات الإدارية.. العتبة العباسية المقدسة تطلق الدورة التعليمية الأولى لمسؤولي وحداتها

قسم التطوير يقيم دورة تدريبية عن الظهور الإعلامي لملاكاته

المزيد

الآخبار الصحية

دراسة تكشف "السبب الخفي" وراء ترك التمارين الرياضية

كشف التوقيت "المثالي" لتغيير فرشاة الأسنان

بلاستيك في أنسجة بشرية.. دراسة تكشف مفاجأة طبية

دراسة تحذر من الإفراط في ممارسة الرياضة.. كيف يضر دماءك؟

المزيد

الآخبار التكنلوجية

"ديب سيك" تحجب أحدث نماذجها عن شركات الرقائق الأميركية

"سامسونغ" تطلق سلسلة هواتف "غالاكسي إس 26"

حملات سرية ورسائل تشويه.. كيف يتم استغلال "تشات جي بي تي"؟

أثر نفسي غير متوقع للرياضة.. مرتبط بالغضب

المزيد

مواضيع ذات صلة

Special Reactions for Preparation of Block Copolymers

Preparation of Block Copolymers by Homogeneous Ionic Copolymerization

Simultaneous Use of Free Radical and Ionic Chain-Growth Polymerizations

Polyurethane Ionomers

Polyamide-Polyester Block Copolymers

Block Copolyesters

Block Copolymers

Miscellaneous Graft Copolymerizations

Preparation of Graft Copolymers with Ionic Chain-Growth and Step-Growth Polymerization Reactions

Graft Copolymer Formation with the Aid of High-Energy Radiation

Photochemical Syntheses of Graft Copolymers

Initiations of Polymerizations from the Backbone of Polymers