Polymerization by Coordination Mechanism

The coordination catalysts for these reactions are diverse. They can be compounds of alkaline earth metals, like calcium amide, or calcium amide-alkoxide. They can also be Ziegler-Natta type catalysts. These can be alkoxides of aluminum, magnesium, or zinc combined with ferric chloride. Others are reaction products of dialkylzinc with water or alcohol. They can also be bimetallic μ-oxoalkoxides, such as [(RO)₂AIO₂]Zn. Other catalysts are aluminum or zinc metalloporphyrin derivatives (see Fig. 5.1). From propylene oxide these catalysts yield crystalline, isotactic polymers [16]. Living polymerizations with metalloporphyrin derivatives are difficult to terminate and are, therefore, called by some immortal [18]. Catalysts like, (C₆H₅)₃–SbBг₂-(C₂H₅)₃N in combination with Lewis acids also yield crystalline poly(propylene oxide). Others, like pentavalent organoantimony halides are useful in polymerizations of ethylene oxide [19]. Polymerizations of epoxides by coordination mechanism result in high molecular weight products. The details of the reaction mechanism have not been fully resolved yet, but it is commonly believed to involve coordination of the monomers to electrophilic centers of the catalyst. This is followed by activation for an attack by the anion [2]. Such mechanism [1] can be illustrated by the following reactions:

where, Me represents the metal catalyst. Ferric chloride polymerizes propylene oxide, a monomer with an asymmetric carbon atom, with retention of asymmetry in the backbone [3]. The products of polymerization contain either optically active polymers or racemic mixtures, depending upon the monomers used. When only a pure optical isomer monomer is used the products are crystalline polymers composed of the same optically active units:

The polymers are fairly high in molecular weight, approximately 100 times greater than the products from KOH initiations. Propylene oxide initially reacts with ferric chloride to form an oligomer, a chloropolyalkoxide. The material contains approximately four or five propylene oxide repeat units. This forms two different halogen sites. It can be illustrated as follows:

The above compound may be the catalyst or one closely related to it for forming stereoregular polymers. Water appears to play a role, because the proportion of crystallinity increases with addition of water. When water is added in a molar ratio of 1.8:1.0 of water to iron, the proportion of crystalline to amorphous fraction increases from 0.13 to 0.86. Price and Organ [17] suggested that the polymerization proceeds in a step-growth mechanism as follows:

The solid surface of the catalyst causes the transition state to be more compressed. Steric repulsions between the incoming monomer and the ultimate unit are minimized if the incoming monomer molecule is forced to be trans to the methyl group of the previous unit. Such a conforma- tional approach also results in minimum repulsion between the incoming monomer and the bulky growing polymer chain [18–20]. Also, ferric alkoxides are associated in nonpolar solvents. A dimer may have the following structure:

By comparison, intramolecular chelation can be expected to reduce the degree of association of the catalyst. Addition of water results in increased association after hydrolysis of the ferric alkoxide. This may explain the effect of promoting stereoregularity by addition of water [20]. The ferric alkoxide catalyst can also be made highly stereospecific by partial hydrolysis and still remain soluble in ether, the polymerization medium [21]. This led to a suggestion [22] that the catalyst may contain active Fe- O-Fe bonds. Such bonds would be formed from condensation of partially hydrolyzed alkoxide derivative. The monomer insertion between the iron-oxygen bonds can be illustrated as follows:

The forces of interaction between the iron atoms and the various oxygen atoms as shown above assure a cis opening of the epoxide ring. The mechanism of the reaction of the ferric alkoxide is an SN2 type. There is, therefore, increased restriction on the conformation of the monomer unit as it approaches the reaction center [22]. Many other coordinated anionic catalysts that are metal alkoxides or metal alkyls are also much more reactive in the presence of water or alcohols. The function of these co reactants is to modify the catalyst itself. For instance, diethylzinc combined with water in a ratio of 1:1 yields a very reactive species. The exact nature of the catalyst is still not fully established, however, the reaction product is pictured as follows [23, 24]:

C₂H₅ - Zn-O-Zn - C₂H₅ Several reaction mechanisms were proposed. One suggested pathway for propylene oxide poly- merization pictures an initial coordination of the monomer with a cationically active center [25]:

The propagation is preceded by an intramolecular rearrangement:

Another mechanism is derived from the structure of the diethylzinc-water catalyst [25] that is visualized as a dimer:

A similar structure pictured can be shown for diethylzinc-alcohol. The asymmetric induction is suggested to takes place during coordination of the monomer to the catalyst site. This is a result of indirect regulation that results form interactions between the monomer and the penultimate unit [25]. In yet another mechanism the initial coordination and subsequent propagation steps are pictured as follows [26]

While the detailed structures of most catalyst sites are still unknown, it was established that stereoselectivity does not come from the chirality of the growing chain end. Rather it is built into the catalyst site itself [27, 28]. Normal preparations of the catalysts give equal numbers of (R) and (S) chiral catalyst sites. These coordinate selectively with (R) and (S) monomers respectively in the process of catalytic-site control [23].

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تختتم فعاليات يوم الخدمة بذكرى ولادة أبي الفضل العباس (عليه السلام)

العتبة العباسية المقدسة تكرم منتسبيها المتميزين خلال عام 2025

العتبة العباسية المقدسة تكرم أقسامها المتميزة في تقديم أفضل تقرير سنوي ضمن فعاليات يوم الخدمة

ضمن فعاليات يوم الخدمة.. العتبة العباسية المقدسة تكرِّم عوائل المنتسبين المتوفين خلال عام 2025

المزيد

الآخبار الصحية

دراسة: محاولة متعمدة لنشر الإنفلونزا تنتهي بـ"مفاجأة"

من خبير في هارفارد.. 6 نصائح يومية "فعالة" لإبطاء الشيخوخة

7 مشروبات طبيعية لتقوية الذاكرة وزيادة التركيز

عرض شائع على الأصابع.. قد يكشف سرطان الرئة مبكرا

المزيد

الآخبار التكنلوجية

وسائل التواصل بين الإفراط والامتناع.. ما هو الحد الآمن للمراهقين؟

أعمق خندق في الكوكب الارض !

روساتوم: روسيا تقترب من تطوير ساعات ذرية فائقة الدقة

مفاجأة علمية عن الإنجاب.. كم طفلا يطيل عمر المرأة؟

المزيد

مواضيع ذات صلة

Steric Control in Polymerizations of Oxiranes

Cationic Polymerization

Kinetics of Ring-Opening Polymerization

Chemistry of Ring-Opening Polymerizations

Configurational Statistics and the Propagation Mechanism in Chain-Growth Polymerization

Group Transfer Polymerization

Copolymerizations by Ionic Mechanism

Polymerization of Ketones and Isocyanates

Polymerizations of Di Aldehydes

Polymerization of Unsaturated Aldehydes

Anionic Polymerization of Aldehydes

Polymerization of Aldehydes