Cyclizations and Intramolecular Rearrangements

Cyclization reactions of natural rubber and other polymers from conjugated dienes have been known for a long time. The reactions occur in the presence of Lewis and strong protonic acids. They result in loss of elastomeric properties and some unsaturation. Carbon cations form in the intermediate step and subsequent formation of polycyclic structures [108, 109]:

In a similar manner, polymers with pendant unsaturation undergo cyclization reactions in benzene in the presence of BF3 or POCl3, yielding ladder structures. The exact nature of the initiation process is not clear. Water may be needed for the initiation step [110, 111]:

The reactions result in formations of six-membered monocyclic and fused polycyclic units. These reactions of carbon cations should also lead to molecular rearrangements, like 1,2 shifts of protons, resulting in formations of five-membered rings and spiro cyclopentane repeat units [111]. Cyclization reactions of polyisoprene can be catalyzed by TiCl4 [112] and by sulfuric acid [113–115, 117]. The products appear similar in the infra-red spectra [110] with only a few minor differences. Also, there are only a small number of fused rings in the product. Polyacrylonitrile converts to a red solid when heated above 200C[116]. Only a small amount of volatile material is given off. Further heating of the red residue to about 350C or higher converts it to a black brittle material. This black material has a ladder structure:

Heating of polyacrylonitrile in the presence of oxygen yields some quinone-type structures:

Further heating of the polymer, at veryhigh temperatures, in excess of 2,000C, results information of graphite-like structures [116]. All, or almost all nitrogen is lost, probably as HCN and N2. Migration of double bonds is a well-established phenomenon in polymers from conjugated dienes. It occurs, for instance, during vulcanization of rubber (discussed in a later section). It also occurs upon simply heating some polymers [117–119]. Thus, the double bonds shift in natural rubber when it is heated to temperatures of 150C or above [120]:

Hydro chlorination of rubber in solution causes a different kind of double bond shirt [121]:

In polymers with pendant double bonds, facile cycloadditions occur upon heating [123–125]:

Many other polymeric structures can rearrange under proper conditions. For instance, poly(4,40 diphenyl propane is phthalate) rearranges upon irradiation with UV light to a structure containing o-hydroxy benzophenones [126]:

The mechanism is believed to be that of a photo-induced Fries rearrangement. It may be similar to one catalyzed by Lewis acids. Another example is a spontaneous rearrangement in the solid state of poly(a-phenylethylisonitrile) [128]:

Based on studies of molecular models, it was concluded [128] that the substituents force progress sive, one-handed twisting of the helix shape of the molecules. This occurs to such an extent that the vicinal imino double bonds are out of conjunction with each other. As a result , each benzylic hydrogen atom becomes localized over the electrons of the imino groups. It proceeds in a constant screw direction around the axis of the helix. Such steric confinement causes tautomeric rearrangements, as shown above, which results in some relaxation of the compression. An additional driving force is a gain in conjugation between the imino double bonds and the aromatic rings of each substituent. Intramolecular rearrangements of polymers with ketone groups were subjects of several studies. The Schmidt and Beckmann rearrangements were carried out on copolymers of ethylene and carbon monoxide [129]:

The starting materials were low and medium molecular weight copolymers. Infra-red spectra of the products from the Schmidt reaction show a high degree of conversion [129]. The yields of the oximes and subsequent Beckmann rearrangements are also high. A Schmidt reaction on poly (p-acetyl styrene) yields a product containing acetyl amino styrene groups in high yields and the products are surprisingly pure [130]. The solvent is acetic acid and the converted material precipitates out as a sticky mass:

Another rearrangement reaction is isomerization of unsaturated polyesters upon heating. Thus, for instance, maleate polyesters rearrange to the fumarate analogs at elevated temperatures [131]:

It was reported that polymers with cis-stilbene moieties in the backbone can undergo photocylization that is almost quantitative [473]:

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قسم الشؤون الدينية يطلق الدورة التطويرية والفقهية الخامسة لملاكات العتبة العباسية المقدسة

قسم الشؤون الفكرية ينظّم ختمة قرآنية رمضانية لطلبة العلوم الدينية الأفارقة في النجف

شعبة الخطابة تقيم مجلسا للوعظ والإرشاد في الصحن الشريف تزامناً مع حلول شهر رمضان المبارك

قسم التطوير يختتم دورة إعداد المدربين لملاكات مديرية تربية كربلاء

المزيد

الآخبار الصحية

مفاجأة.. تحسين القدرة على التحمل لا يعتمد العضلات فقط

المرتفعات والأكسجين.. دراسة "مبشرة" لمرضى السكري

علماء يبتكرون "آلية" تتوقع موعد ظهور أعراض الزهايمر

ماذا يحدث لجسمك عند التوقف عن تناول الخضراوات والفواكه؟

المزيد

الآخبار التكنلوجية

رصد "القرش النائم" في أحد أقسى الأماكن على وجه الأرض

أسماك أعماق البحر تكشف نظاما بصريا يتجاوز "المألوف"

زوكربيرغ أمام القضاء في قضية إدمان المراهقين

"علي بابا" تكشف عن نموذج جديد للذكاء الاصطناعي

المزيد

مواضيع ذات صلة

Chloromethylation Reactions

Reactions of Polystyrene

Substitution Reactions of Halogen-Bearing Polymers

Substitution Reactions of Saturated Polymeric Hydrocarbons

Isomerization Reactions

Epoxidation Reactions

Diels–Alder Condensations

1,3-Dipolar Additions

Electrophilic Additions of Aldehydes

Addition of Carbenes

Hydrogenation

Halogenation