Substitution Reactions of Halogen-Bearing Polymers

Procedures for commercial chlorinations of poly (vinyl chloride) vary. Low temperature chlorinations are done on aqueous dispersions of the polymers that are reacted with chlorine gas in the presence of swelling agents, like chloroform. These are light catalyzed reactions, usually carried out at about 50C. They result in substitutions of methylene hydrogens [158, 159].

Some breakdown of the polymers accompanies the reactions in suspension [154]. Irregularity in the structures and the release of HCl significantly contribute to this. It appears that the degradations are initiated by HCl that is released as a result of the chlorination. In the process, double bonds form. Immediately upon their formation, they become saturated with chlorines. A study was conducted on solution chlorination of poly (vinyl chloride) in the presence of free radicals [155] generated from azobisisobutyronitrile. The reaction appears to proceed in two stages. The first one takes place until 60% of all the CH₂ groups have reacted. After that, in the second stage the original–CHCl groups are attacked. Some unreacted–CH₂– groups, however, remain [155].

Photo chlorination can achieve 90% conversion [156, 157]. Typical commercially chlorinated poly (vinyl chloride), however, has a chlorine content of 66–67%. The material contains methylene groups and is in effect a copolymer of vinyl chloride and 1,2-dichloroethylene. Chlorination raises the softening temperature of poly(vinyl chloride). The products exhibit poorer heat stability than the parent material and are higher in melt viscosity. High-temperature chlorinations in chlorinated solvents at 100C result in extensive substitutions of the methylenic hydrogens as well. The reactions, however, are accompanied by extensive chain scissions. The products are soluble in solvents like acetone and methylene chloride and have low softening points, low impact strength, and poor color stability. Chlorination of poly(vinylidene chloride) can be carried out with sulfuryl chloride using azobisiso butyronitrile [155]. The reaction appears to proceed in three steps:

1. Formation of SO₂Cl• free radicals by abstraction.

 2. Formation of free radicals on the polymeric backbones:

3. Transfer reactions of the active sites:

Chlorination of poly(vinyl fluoride) yields a product with 40–50% chlorine content [158]. Fluorination of poly(vinylidene fluoride) was reported [160]. When mixtures of fluorine and nitrogen gases are used, the reactions are limited by the amount of diffusion of fluorine into the polymer network. X-ray photoelectron spectroscopy shows presence of–CF2–,–CHF–, and–CH₂ groups in the product [161]. Many attempts at other modifications of poly (vinyl chloride) were reported in the literature. Often the reactions are based on expectations that the polymers will react like typical alkyl halides. Unfortunately, in place of nucleophilic substitutions, the polymers often undergo rapid and sequential eliminations of HCl along the chains. Nevertheless, many substitution reactions are still possible and can be successfully carried out. One example is a replacement of 43% of the chlorine atoms with azide groups [162]:

The reaction can be carried out at 60C for 10 h and the polymer does not cross-link. It proceeds faster in DMF than in other solvents. Once substituted, the polymer becomes photosensitive and cross-links when irradiated with ultraviolet light. The presence of azide groups allows further modifications. One such modification is a reaction with an isocyanate [162]:

By a similar reaction, phosphinimine groups can be formed on the polymer backbone [162]:

Some other modifications of the azide group containing polymers are [163, 164]:

Note: The illustration of the reactions with LiAlH4, as shown above, implies that all chloride atoms remain intact on the polymer backbones. It appears likely, however, that some of them might get removed and double bonds might form instead in the backbone.

Poly (vinyl chloride) reacts with various thiols in ethylene diamine to produce monosulfide derivatives [165]:

When sulfur is added to the reaction mixture, disulfides form instead [165]:

The thiolate ions can serve as strong nucleophiles and also as weak bases [166]. When poly (vinyl chloride) is suspended in water in the presence of swelling agents and phase transfer catalysts, it reacts with mercaptans, like 3-[N-(2-pyridyl) carbamoyl]-propylthiol:

Acetoxylation of poly(vinyl chloride) can be carried out under homogeneous conditions [167]. Crown ethers, like 18-crown-6, solubilize potassium acetate in mixtures of benzene, tetrahydrofuran, and methyl alcohol to generate unsolvated, strongly nucleophilic “naked” acetate anions. These react readily with the polymer under mild conditions [167]. Substitutions of the chlorine atoms on the polymeric backbones by anionic species take place by SN2 mechanism. The reactions can also proceed by SN1 mechanism. That, however, requires formations of cationic centers on the backbones in the rate-determining step and substitutions are in competition with elimination reactions. It is conceivable that anionic species may (depending upon basicity) also facilitate elimination reactions without undergoing substitutions [167]. Reactions of poly (vinyl chloride) with aromatic amines, amino alcohols, or aliphatic amines in DMFsolution result in both substitutions and in eliminations [168]. Reactions with aniline yield the following structure [168]:

Carbanionic reaction sequences can be used to introduce various photosensitive groups [169]. The process consists of creating carbanionic centers on the backbone and then reacting them with various halogenated derivatives:

A redox cyclopentadienyl iron moiety can also be introduced into the poly (vinyl chloride) backbone by a similar technique [170]. Many other attempts were reported at replacing the halogens of poly (vinyl chloride), poly (vinyl bromide), and poly (vinyl iodide) with an alkali metal or with a hydrogen. For instance, in an effort to form poly (vinyl lithium), the polymers were reacted with organolithium compounds and with metallic lithium. The reactions with alkyllithium, however, resulted in substitutions by the alkyl groups, similarly to the reactions shown previously [171]:

Reactions with metallic lithium lead to formations of polyenes [171]. On the other hand, when poly(vinyl chloride) is reacted with metal hydrides, like lithium aluminum hydride in a mixture of tetrahydrofuran and decalin at 100C, macroalkanes form [172]:

Replacement of the chlorine with N,N-dialkyl dithiocarbamate was reported to occur at 50–60C in DMF solvent [173]:

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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

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الآخبار التكنلوجية

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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 Saturated Polymeric Hydrocarbons

Cyclizations and Intramolecular Rearrangements

Isomerization Reactions

Epoxidation Reactions

Diels–Alder Condensations

1,3-Dipolar Additions

Electrophilic Additions of Aldehydes

Addition of Carbenes

Hydrogenation

Halogenation