Group Transfer Polymerization

This technology offers considerable promise for commercial preparations of living polymers of methyl methacrylate without resorting to low temperature anionic polymerizations. Although the mechanism or polymerization is not completely explained, the propagation is generally believed to be covalent in character. A silyl ketene acetal is the initiator. It forms from an ester enolate [391]:

The initiation, that is catalyzed by either a nucleophilic or by a Lewis acid catalyst, was explained as consisting of a concerted attack by the ketene acetal on the monomer [392]:

This results in a transfer of the silyl ketene acetal center to the monomer. The process is repeated in each step of the propagation. The ketone double bond acts as the propagating center [391].

The above mechanism shows each step of chain-growth involving transfer of the trialkylsilyl group from the silyl ketene acetal at the chain end to the carbonyl group of the incoming monomer. This is disputed because it excludes silyl exchange between growing chains [393]. Such an exchange, however, was observed [392] and led to a suggestion that the mechanism can involve ester enolate anion intermediates. These are reversibly complexed with silyl ketene acetal chain ends, as the propagating species [394]. It should be noted, however, that the results do not exclude the possibility of transfer reactions occurring as well. Difunctional initiators cause chain growth to proceed from each end [395]. Because group-transfer polymerizations are “living” polymerization, once all the monomer has been consumed, a different monomer can be added and block copolymers can be formed. The most effective nucleophilic catalysts for this reaction are bifluoride (HF2 ) and fluoride ions. They can be generated from soluble reagents like tris(dimethylamino)sulfonium bifluoride. Other nucleophiles, like CN and nitrophenolate have also been used. These nucleophilic catalysts function by assisting the displacement of the trialkylsilyl group. They are effective in concentrations below 0.1 mol% of the initiator. Among electrophilic catalysts are Lewis acids, like zinc chloride, zinc bromide, zinc iodide, and dialkylaluminum chloride. Such catalysts probably function by coordinating with the carbonyl oxygens of the monomers and increasing the electrophilicity of the double bonds. This makes them more reactive with nucleophilic reagents. They must be used, however, in much higher concentrations. Water and compounds with active hydrogen must be excluded from the reaction medium. Oxygen, on the other hand, does not interfere with the reaction. Tetrahydrofuran, acetonitrile, and aromatic solvents are commonly used in polymerizations catalyzed by nucleophiles. Chlorinated solvents and dimethylformamide are utilized in many reactions catalyzed by electrophiles. Living polymerizations of methacrylate esters can be carried out at 0–50C. The acrylate esters, however, require temperatures below 0C for living, group-transfer polymerizations, because they are more reactive and can undergo side reactions. Weakly acidic compounds, such as methyl a-phenylacetate or a-phenylpropionitrile, are added to terminate the reaction. They are effective with anionic catalysts [396]. The trialkylsilyl group is transferred from the chain end to the transfer agent:

Group-transfer polymerizations yield very narrow molecular weight distribution polymers. When mixtures of monomers are used, random copolymers form. The polymerization reaction is very tolerant of other functional groups in the monomer. Thus, for instance, p-vinyl benzyl methacrylate is converted to poly(p-vinyl benzyl methacrylate) without the polymerization of the vinyl group [397]. In addition, it is possible to form polymers with high syndiotactic content. An example of copolymerization is that of methyl methacrylate with methyl a-phenylacrylate [398]. While no homopolymer formed from methyl a-phenylacrylate, alternating copolymers were obtained from mixtures of the two. Equimolar incorporation of monomers was reported even with methyl methacrylate in excess, as long as methyl a-phenylacrylate is present in the reaction mixture. The propagation constants were found to decrease in the order k12>k11>k21>k22. Block copolymers with an alternating block and a poly (methyl methacrylate) block are formed from a monomer feed having MMA in excess [398]. A variation on the group transfer polymerization of n-butyl acrylate was reported [399]. Here, 1-methoxy-1(trimethylsiloxy)-2-methyl-1-propene was used as an initiator and HgI2 as a catalyst. This differs from the nucleophilic-catalyzed group transfer polymerizations described above.

The half-lives were reported to be in the range of minutes to hours. Induction periods were observed. Formation of trimethylsilyl iodide was proposed to be the cause of an induction period [399]. The polymerization follows first-order kinetics with respect to the concentrations of the initiator and the catalyst. With respect to concentration of the monomer the reaction is described as being first-order internally. It follows, however, an external reaction order of 1.52 due to the higher polarity of the reaction medium at higher monomer concentrations. The authors tentatively propose that the active species are formed from initiator, catalyst and trimethylsilyl iodide [399]:

Part of the proposed mechanism is that TMSI forms from MTS and iodine. It is assumed, therefore, that TMSI forms a complex with HgI₂, which in turn activates MTS to form an active center capable of initiating the polymerization [399]:

Because group transfer polymerization is another form of a living polymerization, attempts have been made to write kinetic equations to include all forms of such polymerizations in one unified scheme. Livinenko and Muller [400] carried out a general kinetic analysis and compared molecular weight distributions for various mechanisms of activity exchange in living polymerizations. They concluded that molecular weight distributions in many living, e.g. anionic, group transfer, cationic, and radical polymerizations strongly depend on the dynamics of various equilibria between chain ends of different reactivity [400]. They also concluded that a very important special case is the equilibria between active and dormant centers [400]. Mechanisms that include uni- and bimolecular isomerizations (or activations/deactivations), aggregation, direct bimolecular activity exchange, and for both fast and slow monomer addition can be unified. The averages of the molecular weight distributions and polydispersity indexes, Pw/Pn were derived by them and the dependencies of these averages on three universal parameters were analyzed: (1) on the reactivity ratio of the two species, λ =kp/kp, (2) on the fraction of the more active species, α = P*/I₀, which is determined by the initial concentrations of reagents, and (3) on a generalized exchange rate parameter, b, which quantifies the rate of exchange relative to that of propagation. The dependence of b on the initial concentrations of reagents is defined by the mechanism of exchange and can be used as a mechanistic criterion to distinguish between various possible mechanisms. Litvinenko and Miller concluded that for the cases where b > 1, the polydispersity indexes decrease with monomer conversions. This is a common observation in many living polymerizations where, 10 < b < 100. At full conversions, a simple relation, Pw/Pn≈1+o/B, is valid, where Y depends on a and l. For the common cases where one type of species are dormant this was simplified further to Pw/Pn≈1 + 1/B. Generally, the molecular weight distributions are narrower if the monomer is added slowly [400].

Scholten et al. [401] reported that N-heterocyclic carbenes are potent catalysts for group transfer polymerization of acrylic esters. The reaction is illustrated as follows:

0.1–0.5 moles of catalyst are used in tetrahydrofuran. The reaction is quenched with methyl alcohol. The product had a narrow molecular weight distribution.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تطلق فعاليات يوم الخدمة تزامنًا مع ذكرى ولادة أبي الفضل العباس (عليه السلام)

العتبة العباسية المقدسة تشارك بأكثر من 730 إصداراً في معرض كربلاء الدولي للكتاب

العتبة العباسية المقدسة تشارك في فعّاليات مهرجان ربيع الشهادة الثقافي الدولي الثامن عشر

قسم الشؤون الدينية يواصل إقامة دوراته الفقهية لتأهيل ملاكات مجموعة مشاتل الكفيل

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

Copolymerizations by Ionic Mechanism

Polymerization of Ketones and Isocyanates

Polymerizations of Di Aldehydes

Polymerization of Unsaturated Aldehydes

Anionic Polymerization of Aldehydes

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