Reactions of Functional Groups

Kinetic considerations are of paramount importance in understanding the mechanism of step-growth polymerization [1]. As stated in Chap. 1, chain-growth polymerizations take place in discreet steps. Each step is a reaction between two functional groups, like, for instance, in a polyesterification reaction, it is a reaction between–COOH and–OH. The increase in molecular weight is slow. The first step is a condensation between two monomers to form a dimer:

A dimer can react next with another monomer to form a trimer:

Two dimers can combine to form a tetramer:

These step condensations continue slowly with the molecular weights of the polymers increasing with each step. In such reactions, the monomeric species disappear early from the reaction mixtures, long before any large molecular weight species develop. In most step-growth polymerizations, on a weight basis, less than 1% of monomeric species remain by the time the average chain length attains the size of ten combined monomeric units [1, 3, 4, 6]. One important characteristic of step-growth polymerizations is that any functional group on any one molecule is capable of reacting with any opposite functional group on any other molecule. Thus, for instance, if it is a reaction of polyesterification, any carboxylic acid group on any one molecule, regardless of size, can react with any hydroxy on another one. This is true of all other step-growth polymerizations. It means that the rates of step-growth polymerizations are the sums of the rates of all reactions between molecules of various sizes. A useful assumption that can be applied here is that the reactivities of both functional groups remain the same throughout the reaction, regardless of the size of the molecules to which they are attached. This allows treating step-growth polymerizations like reactions of small molecules. General observations would suggest slower reactivity of functional groups attached to large molecules. This, however, is usually due to lower diffusion rates of large molecules. The actual reactivity of the functional groups depends upon collision frequencies (number of collisions per unit of time) of the groups and not upon the rate of diffusion. Functional groups on the terminal ends of large molecules have greater mobility than the remaining portions of the molecules as a whole. In addition, the reactivity of one given functional group in a bifunctional molecule is not altered by the reaction of the other group (if there is no neighboring group effect). This implies that the reactivities of functional groups are not altered during the polymerization. The kinetics of step-growth polymerization can be derived from a polyesterification reaction that follows the same course as all acid-catalyzed esterifications [2].

1. Protonation step:

2. Reaction of the protonated carboxylic acid group with the alcohol:

The above polyesterifications, like many other reactions, are equilibrium reactions. They must be conducted in a way that allows the equilibrium to shift to the right to attain high molecular weights.

One way is by continual removal of the by-products. In such situations, the reactions take place at nonequilibrium conditions and there is no K₄.

اخر الاخبار

اخبار العتبة العباسية المقدسة

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

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

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