Termination in Anionic Polymerization

Termination reactions in anionic polymerization, particularly with non-polar monomers and in non-polar solvent are not common. If carbanion quenching impurities are absent, many polymerization reactions may not terminate after a complete disappearance of the monomer. Styryl anion, (one of the most stable ones) for instance, can persist for a long time, such as weeks, after the monomer is consumed. Addition of more monomer results in a continuation of the reaction and a further increase in the molecular weight. The anionic “living” polymers retain their activities considerably longer periods of time than do the cationic “living” ones [180]. The termination steps in anionic polymerizations can result from deliberate introductions of carbanion quenchers, such as water or acids, or from impurities. Terminations, however, can take place in some instances through chain transferring a proton from another molecule like a solvent or a monomer or even from a molecule of another polymer. In some solvents, like liquid ammonia, transfer to solvent is extensive, as in styrene polymerization by amide ions [219]. In addition, in some polymerizations termination might occur from the following reactions:

 1. Elimination of a hydride-ion to form an unsaturated end.

2. Isomerization to an inactive anion.

 3. Some irreversible reaction of the active center with a molecule of a monomer or a solvent. It was observed, for instance that hydrogen transfer from monomer to the growing chain can be a way of termination in polymerizations of polar monomers, like acrylonitrile [220]:

where Me represents the metal cation. As mentioned above, the polystyryl carbanions are particularly stable and persist for weeks in nonpolar solvents. Yet, even in the absence of terminating agents, the concentration of the carbanion active centers decreases with time [159]. The mechanism of decay is not fully understood. Based on spectral evidence, it is believed to consist of a hydride-ion elimination [219]:

The above reaction can be followed by an abstraction of an allylic hydrogen from the product of elimination by another active center:

Polystyryl carbanions are much less stable in polar solvents. They decay within a few days at room temperature. At lower temperatures, however, the stability is considerably better. The termination in polar solvent occurs by a mechanism of abstracting a-hydrogens and/or by a nucleophilic attack on the carbon–oxygen bonds. Polar monomers, like methyl methacrylate, acrylonitrile, or methyl vinyl ketone contain substituents that react with nucleophiles. This can lead to terminations and side reactions that compete with both initiation and propagation [219]. An example is a nucleophilic substitution by an intramolecular backbiting attack of a propagating carbanion:

Side reactions like the one shown above can be minimized by using less nucleophilic initiators and low temperatures. This can yield “living” polymerizations of acrylic and methacrylic monomers. In addition, it is possible to add common ions like LiCl to alkyl lithium to tighten the ion pairs of the propagating anion-counterion species. That also increases the tendency to form “living” polymers [221]. This approach, however, offers only limited success. In addition. it was found that Lewis acid assisted polymerizations of methyl methacrylate with aluminum porphyrin initiators yield “living” polymers [222]. The polymerizations of methacrylate esters with alkylaluminum porphyrin initiators occur through formations of enolate aluminum porphyrin intermediate as the growing species [222]. For the sake of illustration, the methylaluminum porphyrin molecules (see Fig. 4.3) can be designated as:

Fig. 4.3 Methylaluminum porphyrin molecule The reaction can then be illustrated as follows [222]:

The preparatory procedure was improved [222] further by addition of sterically crowded organoaluminum phenolates to the reaction mixture:

where, R0 is H and R00 is butyl, or R0 is butyl and R00 is methyl. This yields “living,” very high molecular weight, monodisperse polymers of methyl methacrylate [222]. Monomer insertion might be taking place at the Al–C bond. The mechanism, however, is not yet fully understood [222]. Ishizone and coworkers [223] studied living anionic polymerization of a series of N,N-dialkyl methacrylamides carried out with diphenyl methyllithium or with triphenylmethyl potassium in the presence of LiCl or diethyl zinc in tetrahydrofuran. Polymers of methacroyl azetidine possessed predicted molecular weights and very narrow molecular weight distributions of 1.1. They were obtained quantitatively with both catalysts at 40 to 0C within 24 h. The acryloyl counterpart, N-acrytoyilazetidine, also underwent the anionic polymerization to afford the well-defined polymer quantitatively. By contrast, no polymer was obtained from the anionic polymerization system of N-methacroyl piperidine, nor with N,N-dimethyl methacrylamides. From the experimental results, it was demonstrated that the polymerizability of a series of N,N-dialkylmethacrytamides with cyclic substituents decreased drastically with increasing the ring size from 3 to 6.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تكرم المشاركين في مشروع مقرأة العميد الرقمية

تقديم محاضرة عن عالم الأرقام في القرآن الكريم ضمن حفل إطلاق مشروع مقرأة العميد

ضمن فعاليات إطلاق مقرأة العميد الرقمية.. عرض فيديو توثيقي عن فكرة المشروع

جامعة تكنولوجيا المعلومات: مشروع مقرأة العميد منصة رقمية متكاملة لتعليم القرآن الكريم وتلاوته وحفظه

المزيد

الآخبار الصحية

عرض شائع على الأصابع.. قد يكشف سرطان الرئة مبكرا

7 مشروبات طبيعية لتقليل التوتر والقلق

دراسة: "خطر مبكر" يهدد صحة الأطفال

الجوز أو الفول السوداني.. أيهما يتفوّق في حماية القلب؟

المزيد

الآخبار التكنلوجية

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

استطلاع يكشف "الأكثر قلقا" من تأثر الوظائف بالذكاء الاصطناعي

هل يمنحك تناول الجزر يوميا سمرة طبيعية؟

إنجاز طبي تاريخي.. جراحة قلب بلا شق في الصدر

المزيد

مواضيع ذات صلة

Polymerization of Aldehydes

Isomerization Polymerizations with Coordination Catalysts

Reduced Transition Metal Catalysts on Support

Post Ziegler and Natta Coordination Polymerization of Olefins

Steric Control in Polymerization of Conjugated Dienes

Steric Control with Homogeneous Catalysts

Homogeneous Ziegler–Natta Catalysts

Steric Control with Heterogeneous Catalysts

Heterogeneous Ziegler–Natta Catalysts

Coordination Polymerization of Olefins

Hydrogen Transfer Polymerization

Steric Control in Anionic Polymerization