Reactions of Polymers Polymer Modification

Until comparatively recent times polymers have been utilized almost exclusively as materials so that greater interest has been focused on their physico-mechanical properties than on their chemical properties. The exceptions to this rather sweeping generalization are cases where a certain polymer could be prepared only by a chemical transformation of a precursor or where desired physical properties in a polymeric material could be developed only by alterations to the molecular architecture. For example, poly(vinyl alcohol) has been prepared on a commercial scale for many years by hydrolysis (or alcoholysis) of poly(vinyl acetate), whilst most applications of natural and many synthetic rubbers are feasible only after some crosslinking or curing reaction - processes which are still the subject of chemical investigation. 

The state of the subject up to the early 1960 s is covered by the volume edited by Fettes. Several collections of articles on reactions of polymers have been published since then, as well as specialized reviews on particular topics including syntheses on solid supports, reactions on unsaturated polymers, and synthetic biologically active polymers.  

The foregoing remarks indicate the diversity of subjects which may be covered by a review on polymer reactions. Because of space limitations, however, several topics within this range, such as photochemistry, crosslinking reactions and syntheses of block copolymers, have been omitted. The formation of graft copolymers, where grafting is preceded by chemical modification of the back bone, is included but not radiation-induced or other direct grafting reactions. Some of these topics are covered elsewhere in this volume. 

An obvious application of chemical transformation processes is the preparation of polymers which cannot be synthesized by direct polymerization. Polyepi-iodohydrin (PEIH) provides a good example from the recent literature. Attempts at direct preparation were unsuccessful but partial success was achieved by nucleophilic displacement of chlorine from polyepichlorohydrin (PECH) in a 

HCl is greatest with the hard bases (fi.g. benzyloxide) and least with soft bases e,g. 4-nitrobenzoate). This method offers the possibility of synthesizing a wide range of novel polymers with a variety of functional groups attached to the backbone.  

---

The possibility of carrying out not only fast polymerisation processes in turbulent flows, but also polymer-analogous reactions of polymer modification [18,20-22] was exemplified by butyl rubber (BR) chlorination in solution with molecular chlorine. 

The Chemical analysis of polymers is very similar to the Chemical analysis of low Molecular weight organic compounds, if we make suitable modification to ensure solubility or the availability of sites for reaction (e.g., insoluble specimens should be ground to expose a large surface area). The general methods used for functional group and elemental analysis are applicable. Chemical reactions of polymers also provide means of Chemical analysis, as also their reactions of degradation. 

It was previously mentioned was that a large number of minor copolymers of PET have been developed over the past 50 years, with the intent of modifying textile fiber properties and processability [2, 3], Of broader interest is that some of these textile modifications, such as PET copolymers with metal salts of 5-sulfoisophthalic acid (SIPA), have their own rich chemistries when the extent of polymer modification is increased beyond textile levels. An example of such a modification is that changing the counterions associated with SIPA can significantly effect the kinetics of polyester transesterification reactions (the... 

There are several reasons for the appeal of polymer modification immobilization is technically easier than working with monolayers the films are generally more stable and because of the multiple layers redox sites, the electrochemical responses are larger. Questions remain, however, as to how the electrochemical reaction of multimolecular layers of electroactive sites in a polymer matrix occur, e.g., mass transport and electron transfer processes by which the multilayers exchange electrons with the electrode and with reactive molecules in the contacting solution [9]. 

Another way to achieve desirable polymer properties is the modification of preformed polymers. This modification may take place on the reactive sites of the polymer chain through alkylation, hydrolysis, sulfonation, esterification, and other various reactions of polymers. Examples of natural polymers and their modifications are cellulose and its derivatives, chitin and chitosan, and polysaccharides. These are still to this day very important polymers for pharmaceutical applications. 

In H2 plasma s reaction 3 occurs upon exposure to the atmosphere. Clearly, more work needs to be done before the surface chemistry of polymer modification can be more clearly understood. Static SIMS can play an important role in such studies. 

A large amount of theoretical work has been done on the problem of treating the kinetics of polymer modification reactions (9,10). In addition to the kinetics, the distributions of monomer units among the products of such reactions and the compositional heterogeneity of the products are of interest. The problem of treating irreversible polymer reactions is essentially solved,... 

Attenuated total reflectance infrared spectroscopy was employed to determine the possible chemical modification of the PVC specimen exposed to t-butyl alcohol and methyl t-butyl ether. Infrared spectroscopy has been used to study solvent absorption (17), oxidation (18) and other degradation reactions of polymers (19). In the studies of the hostile effects of methyl t-butyl ether and acetone, the solvent was concentrated and examined using conventional infrared techniques. 

Reactions of Polymer-Reactive Antioxidants. Three different modifications of this approach have been reported. 

Supercritical modification of polymers was studied by several scientists to improve or change the properties of polymers. Polymers can either be chemically or physically modified. Examples of chemical modifications are the functionalization of polymers (grafting) or a chemical reaction of the functional groups of polymers to obtain new materials [38, 39]. Examples of physical modifications are the preparation of polymer blends, impregnation of polymers with additives [46], or foaming of polymers [59-61]. Another studied topic of polymer modification and impregnation is the supercritical dyeing of polymer fibers [40, 41). 

The kinetic descriptions in this chapter are developed for unimolecular scission of an initiator to yield two radicals (Scheme 3.1), the most common means of generating radicals in industrial systems. Thermal initiation of monomers is an additional mechanism capable of forming primary radicals at higher temperatures, as discussed for styrene in Section 3.2.1.3. Photoinitiators that produce radicals by ultraviolet irradiation are often used to initiate crosslinking and curing reactions these polymer modification techniques are not discussed in this chapter. 

The microstructures of copolymers prepared by chemical modification are obviously amenable to study using exactly the same techniques that are applied to copolymers prepared by more normal routes. Here, part of the value of such studies is that they can impart information concerning the mechanism of the chemical modification reactions. Some examples of polymer modification are discussed in this section. 

Another important area of polymer modification with subcritical and supercritical water is the hydrolysis of polycondensation polymers such as polyethylene terephthalate (PET), polyurethanes, and nylons for conversion to their monomers [ 37]. Specifically, in supercritical water, 91 % monomer recovery (terephthalic acid) is achieved at 400 °C and 400 bar in less than 15min reaction times [38]. Studies of these reactions using a hydrothermal diamond anvil cell to follow the phase changes during the reaction of PET... 

Another example of polymer modification reaction is the hydrolysis of cellulose in subcritical and supercritical water [40]. Cellulose is shown to hydrolyze rapidly (<1 s) in supercritical water in the absence of any catalysts to glucose, fructose, and oligomers (cellobiose, cellotriose, etc) with a hydrolysis product yield of about 75 % at 400 °C and... 

Besides polymerization reactions and modification reactions of polymers, other types of reactions can also be performed in extruders (41,42) ... 

---