Reactivity and Chemical Modification of Polymers

this chapter will be devoted to the presentation of typical reactions that are specific to linear polymers and to the modifications undergone by polymers molecular stmcture under the effect of physical or chemical attacks. Because the reaction mechanisms are generally identical (or close) to those described for simple molecules, they will not be elaborated in detail in this chapter, except for a few cases. 

Organic and Physical Chemistry of Polymers, by Yves Gnanou and Michel Fontanille Copyright 2008 John Wiley Sons, Inc. 

Because each reaction is a particular case, it is difficult to establish general rules. Certain reactions on polymers can be strongly accelerated by the macromolecular nature of either the reagent or the substrate. Such behavior is generally accounted for by the presence of neighboring groups, or the influence of the chain stereochemistry, or the effect of electrostatic attractions or repulsions, and so on but in most cases the apparent reactivity of polymers is definitely lower than that of simple model molecules, due to a reduced accessibility to the reaction sites. 

The viscosity of polymers is very high in the condensed state, and only small molecules can freely move and react in the reaction medium. The variety of reagents (or substrates) that can be used under these conditions is thus limited, and most of the reactions involving polymers require the presence of a solvent. The lack of solubility of some of the polymers restricts the possibilities of modifying them. 

The reactions carried out on polymers enter into two main categories  

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REACTIVITY AND CHEMICAL MODIFICATION OF POLYMERS is also responsible for the degradation through cycloaddition ... 

Stmctural and chemical modification of urethane containing polymer matri-ces with macrocycles - calixarenes having reactive hydrazide groups have been carried out and stmcture, physico chemical and sensor properties of polyure-thanesemicarbazides (PUS) synthesised have been studied. The polymers obtained (on the base of polypropylene glycol MM 1000 and polysiloxane diol MM 860, hexamethylene diisocyanate and calixarene dihydrazide) are identified by IR-spectroscopy, size exclusion chromatography (SEC), DSC, WAXS and SAXS methods. 

Reactive extrusion is the chemical modification of polymer while it is being transported in an extruder. In this work, polypropylene is intentionally degraded by the addition of a free radical initiator (a peroxide) during extrusion. The product has improved flow properties because of the removal of the high molecular weight tail and the narrowing of the molecular weight distribution. 

The Flory principle is one of two assumptions underlying an ideal kinetic model of any process of the synthesis or chemical modification of polymers. The second assumption is associated with ignoring any reactions between reactive centers belonging to one and the same molecule. Clearly, in the absence of such intramolecular reactions, molecular graphs of all the components of a reaction system will contain no cycles. The last affirmation concerns sol molecules only. As for the gel the cyclization reaction between reactive centers of a polymer network is quite admissible in the framework of an ideal model. 

Chemical modification of polymers (J.) still remains a field of continuously increasing importance in macromolecular chemistry. In spite of its high diversification, it may be divided into 2 distinct but complementary main research lines a) the fundamental study of the chemical reactivity of macromolecular chains b) the synthesis of new homopolymers and copolymers, and the functionalization of linear or crosslinked polymers. Some of these facets have been reviewed in the last years (2-6), and the purpose of this presentation is to illustrate a number of characteristic topics both from fundamental and applied points of view, through some literature data and through our own studies on nucleophilic substitution of polymethylmethacrylate (PMMA). 

In general, reactive extrusion is the term applied to chemical modification of polymers in the presence of an initiator such as peroxides. During chemical modification, there can be other undesired side reactions taking place, for example, cross-linking, degradation into low MW fractions and so on. Good process control is an essential requisite for attaining a balance of desired properties. 

The net result of the presence of ions, firee-radicals, and ultraviolet radiation is a gas plasma environment that has an extremely high chemical reactivity. The proper plasma can convert any organic material to volatile products. This is the basis of the use of plasma for cleaning or stripping of organic coatings and the modification of polymer surfaces. Plasma can also be used to etch some metals. 

The interactions and adhesion involved is basically a surface phenomena, and the nature and characteristics of interfaces and interphases are of prime importance. They can be promoted by use of proper compatibilizing agents (5) or directly, by modifying surface properties (6). Permanent chemical modification of polymer surfaces is a rather difficult task, and for polyolefins, which are poorly reactive this difficulty is even greater. On the other hand, if activated, polyolefins can react with oxygen of the atmosphere easily which is a chain reaction with... 

Physicochemical properties of polymers, their processing conditions, and feasible applications for creating composites of different purposes are in many respects determined by the nature of functional groups contained in polymer macromolecules. The chemical modification of polymers via reactive groups of macromolecules makes it possible to obtain new polymeric products with a wide variety of properties and, thus, to widen the application areas of the modified pol5mers. 

Although most of the work on the use of phase transfer catalysis for the chemical modification of polymers was carried out in the last five years, the first report of the application of this technique to the modification of reactive polymers originated ten years ago from the laboratory of Okawara, one of the great innovators of polymer chemistry, and was concerned with the chemical modification of poly(vinyl chloride) (Ref. 10, 11). This was followed by simultaneous brief reports by Roovers (Ref. 12) and by Roeske et al. (Ref. 13) describing the use of 18-crown-6 in the reaction of carboxylates with poly(chloromethyl styrene). Our own first report (Ref.14) in this field described the use of phase transfer catalysis to control site-site interactions in the... 

Various strategies for the syntheses of either aliphatic or aromatic functional fluorinated monomers have been proposed in the hterature. Because of their costs, they have been involved in copolymerization with fluoroalkenes, and although a lack of basic research is noted (e.g., no assessment of the reactivity ratios), many apphed investigations have been developed. In fact, most companies producing fluorinated monomers and derivatives have solved the challenge to prepare fluorocopolymers bearing sulfonic acid side groups. Nevertheless, quite a few studies concern phosphonic acid function. Compared with direct copolymerization, the alternative to prepare fluorofunctional copolymers by chemical modification of polymers is often employed. 

Miscenko, N., Groeninckx, G., Reynaers, H., Koch, M., Pracella, M. 4th ATPS Conference on Chemical Modification of Polymers and Reactive Blending, Gargnano, Italy, 1996, Abs. p. 46. 

This chapter includes the following sections main chemical modifications of polymers reactivity and mechanisms kinetics techniques use in the chemical modification of polymers and main applications of the chemical modification of polymers. 

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