Anionic Polymerization of Aldehydes

Many patents describe polymerizations of anhydrous formaldehyde by anionic mechanism. The initiators included amines, phosphines, and metal alcoholates. Kern pictured initiations of formaldehyde polymerizations by tertiary amines as direct addition reactions [332, 334]:

R3N+CH2O → R3NR — CH2 — OR

Earlier, however, Machacek suggested [335-337] that the initiations take place with the help of protonic impurities:

Much of the evidence presented since favors the Machacek mechanism of initiation [335]. By contrast, tertiary phosphines apparently do initiate such polymerizations by a zwitter ion mechanism [338]. This may, perhaps, be due to higher nucleophilicity and lower basicity than that of the tertiary amines. Phosphorus incorporates into the polymer [338] in the process.

The propagation reactions in tertiary amine initiated polymerizations can be pictured as follows [339]:

The terminations, probably, result from chain transferring [241]:

The newly formed active species can initiate new polymerizations. Metal alkyls and metal alcoholates are very effective anionic initiators for higher aldehydes. Aldol condensation can occur, however, in the presence of strong bases. Thus, while some such initiators yield high molecular weight polymers from formaldehyde, they only yield low molecular weight polymers from higher aldehydes. Initiations by metal alkyls result from additions to the carbonyl group:

Where, Me represents a metal group. The propagation reaction is a series of successive nucleo- philic additions [340], with the alkoxide ion as the propagating species:

There are indications that many aldehyde polymerizations result in formations of "living" polymers similarly to anionic polymerizations of vinyl compounds. Termination can occur through hydride transfer via a form of a crossed Cannizzaro reaction:

The alkyl substituent has a tendency to destabilize the propagating anion by increasing the charge density on the oxygen:

As a result, weak bases, like amines, fail to initiate polymerizations of higher aldehydes. A stereospecific anionic polymerization of acetaldehyde was originally reported in 1960 [343, 344]. Two alkali metal compounds [341] and an organozinc [342] one were used as the initiators. Trialkylaluminum and triarylaluminum in heptane also yield crystalline, isotactic polymers from acetaldehyde, heptaldehyde, and propionaldehyde at -80°C [343]. Aluminum oxide, activated by diethylzinc, yields stereoblock crystalline polymers from various aldehydes [342, 344]. Lithium alkoxide formed polyacetaldehyde is insoluble in common solvents. It melts at 165°C [341].

The mechanism of stereoregulation is still debated. Some concepts are presented in the rest of this section. Natta [343] believed that polymerizations initiated by organoaluminum compounds proceed by coordinated anionic mechanism. The aluminum atoms were seen as forming complexes with the oxygen atoms on the penultimate units:

The activated complexes, which form, have steric configurations that allow minimum amounts of non- bonded interactions. Other mechanisms were proposed since. For instance, Furukawa et al. [344] concluded that metal alkyl compounds must become metal alkoxides through reactions with the aldehydes:

His polymerization mechanism, therefore, is based on known reactions of metal alkoxides with carbonyl compounds. Side reactions, like a Meerwein-Ponndorf or a Tischenko, should be expected and were included into the reaction scheme:

Each monomer addition to the growing chain requires a transfer of the alkoxide anion to the carbonyl group. This results in a formation of a new alkoxide anion. (A hydride transfer from the alkoxide group to the carbon atom of the aldehyde can take place by the Meerwein-Ponndorf reduction.) Chain growth takes place by repetition of the coordination of the aldehyde, and subsequent transfer of the alkoxide anion. In the Vogl and Bryant [345] mechanism, four oxygens are coordinated to the metal atom. These oxygens are from the penultimate and ultimate units of the growing chains and from two monomers:

A simultaneous coordination of the two aldehydes prior to addition may explain the observed sequence of isotactic dyads. Yashida and Tani derived their mechanism from investigations of isotactic aldehyde polymerizations by [R₂AlOCR0NC₆H₅] catalysts [346–348]. The bulkiness of the substituent group is considered to be the most important factor in steric control. The catalyst enhances the degree of stereo regulation by controlling the mode of approach through coordination. Also, the Lewis acidity of the catalyst must be confined to a narrow range for each particular aldehyde to yield isotactic polymers [347, 348]. If the acidity is too strong, an amorphous polymer forms. If it is too weak, no polymerization takes place [348]. In addition, special techniques, like high pressures, for instance, can result in formations of isotactic butyraldehyde and heptaldehyde [349].

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

Group Transfer Polymerization

Copolymerizations by Ionic Mechanism

Polymerization of Ketones and Isocyanates

Polymerizations of Di Aldehydes

Polymerization of Unsaturated 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