Poly (vinyl alcohol) and Poly (vinyl acetal) s

Vinyl alcohol monomer does not exist because its keto tautomer is much more stable. Poly (vinyl alcohol) can be prepared from either poly (vinyl esters) or from poly (vinyl ethers). Commercially, however, it is prepared exclusively from poly (vinyl acetate). The preferred procedure is through a transesterification reaction using methyl or ethyl alcohols. Alkaline catalysts yield rapid alcoholyses. A typical reaction employs about 1% of sodium methoxide and can be carried to completion in 1 h at 60C. The product is contaminated with sodium acetate that must be removed. The reaction of transesterification can be illustrated as follows:

The branches of poly(vinyl acetate) that form during polymerization as a result of chain transfer ring to the acetate groups cleave during transesterification. As a result, poly(vinyl alcohol) is lower in molecular weight than its parent material. Poly(vinyl alcohol) is very high in head to tail structures, based on NMR data. It shows the presence of only a small amount of adjacent hydroxyl groups. The polymer prepared from amorphous poly(vinyl acetate) is crystalline, because the relatively small size of the hydroxyl groups permits the chains to line-up into crystalline domains. Synthesis of isotactic poly(vinyl alcohol) was reported from isotactic poly(vinyl ethers), like poly(benzyl vinyl ether), poly(t-butyl vinyl ether), poly (trimethylsilyl vinyl ether), and some divinyl compounds. Poly(vinyl alcohol) is water soluble. The hydroxyl groups attached to the polymer backbone, however, exert a significant effect on the solubility. When the ester groups of poly(vinyl acetate) are cleaved to a hydroxyl content of 87–89%, the polymer is soluble in cold water. Further cleavage of the ester groups results in a reduction of the solubility and the products require heating of the water to 85C to dissolve. This is due to strong hydrogen bonding that also causes unplasticized poly(vinyl alcohol) to decompose below its flow temperature. On the other hand, due to hydrogen bonding the polymer is very tough. Poly(vinyl acetals) are prepared by reacting poly(vinyl alcohol) with aldehydes. Reactions of poly (vinyl alcohol) with ketones yield ketals. These are not used commercially. Not all hydroxyl groups participate in formations of acetals and some become isolated. A typical poly(vinyl acetal) contains acetal groups, residual hydroxyl groups, and residual acetate groups from incomplete transesterification of the parent polymer. Poly(vinyl acetal)s can be formed directly from poly(vinyl acetate) and this is actually done commercially in preparations of poly(vinyl formal). A typical reaction is carried out in the presence of acetic acid, formalin, and sulfuric acid catalyst at 70C:

Poly(vinyl butyral), on the other hand, is prepared from poly(vinyl alcohol) and butyraldehyde. Sulfuric acid is used as the catalyst. Commercially only poly(vinyl formal) and poly(vinyl butyral) are utilized on a large scale in coating materials.

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خدمات صحية متكاملة قدَّمها قسم الشؤون الطبية لزائري ليلة النصف من شعبان في مجمَّع الشيخ الكليني

خدمات صحية متكاملة قدَّمها قسم الشؤون الطبية لزائري ليلة النصف من شعبان في مجمَّع الشيخ الكليني

في ذكرى ولادة الإمام المهدي (عجّل الله فرجه).. قسم الشؤون الفكرية يصدر عددًا خاصًّا من مجلة العقيدة

للاطّلاع على خدماته المقدمة للزائرين مركز السيد جعفر الحلي يستقبل المدير العام لمديرية العمليات والخدمات الطبية في وزارة الصحة

المزيد

الآخبار الصحية

دراسة تكشف "سر الوسادة".. ودورها في حماية البصر

بالأشعة تحت الحمراء.. "ثورة علمية" في علاج اللاعبين

مشروب أحمر يخفض ضغط الدم "بشكل ملحوظ"

الصحة العالمية تكشف حقيقة انتشار فيروس نيباه خارج الهند

المزيد

الآخبار التكنلوجية

دراسة تدفعك لشراء ساعة ذكية "فورا".. هذا ما تقوله عن قلبك

علماء حاصلون على "نوبل": الذكاء الاصطناعي لن يستبدل الإنسان

وانغ هو: القمة العالمية للعلماء تجسد تقدير الإمارات للعلم

رئيس رابطة كبار العلماء يعلن تأسيس مقر في الإمارات

المزيد

مواضيع ذات صلة

Kinetic Considerations

Reactions of Functional Groups

Poly (vinyl acetate)

Poly (vinylidine chloride)

Copolymers of Vinyl Chloride

Poly (vinyl chloride)

Miscellaneous Fluorine Containing Chain-Growth Polymers

Copolymers of Fluoroolefins

Poly chloro tri fluoro ethylene

Poly tetra fluoro ethylene

Polyacrylamide, Poly(acrylic acid), and Poly(methacrylic acid)

Acrylonitrile and Methacrylonitrile Polymers