Reactions polymer

Nine examples of reactions after polymerization are selected in this section. Further details of these industrially important reactions should be looked-up in the General References given in the Preface, as is also proposed for all other examples of materials mentioned in this book, and partially illustrated for the following reactions. 

Three possible cross-links Involving sulfur atoms 

The second example of a polymer reaction is the industrial cross-linking of rubber by vulcanization sketched in Fig. 3.50. The process was invented already in 1839 by C. N. Goodyear without knowledge of its chemical stracture. Natural rubber is cis-poly(l-methyl-1-butenylene) or polyisoprene with a low glass transition temperature of about 210 K. Its structure and those of other rubbers are given in Fig. 1.15. The addition of sulfur in the form of Sg rings and heating causes the vulcanization. Of the listed cross-hnks in Fig. 3.50, only the left example is an efficient network former. The sulfur introduces about 1 cross-link for each of 50 S-atoms used. Modem vulcanization involves activators and accelerators for increased efficiency. The detailed mechanism is rather complicated and not fully understood. 

Other cross-ltnkmg reactions are shown in Fig. 3.51. They can lead to products that can be first shaped by melting the flexible macromolecules (plastics) and then fixing the shape by cross-linking, approaching a rigid macromolecule as seen in 

Cross-linking of Polyethylene with Ionizing Radiation 

---

Primary aromatic amines react with aldehydes to form Schiff bases. Schiff bases formed from the reaction of lower aUphatic aldehydes, such as formaldehyde and acetaldehyde, with primary aromatic amines are often unstable and polymerize readily. Aniline reacts with formaldehyde in aqueous acid solutions to yield mixtures of a crystalline trimer of the Schiff base, methylenedianilines, and polymers. Reaction of aniline hydrochloride and formaldehyde also yields polymeric products and under certain conditions, the predominant product is 4,4 -methylenedianiline [101 -77-9] (26), an important intermediate for 4,4 -methylenebis(phenyhsocyanate) [101-68-8], or MDI (see Amines, aromatic amines, l thylenedianiline). 

V. D. McGinniss, in S. S. Labana, ed.. International Symposium of UV Eight-Induced Polymer Reactions, American Chemical Society, Washington, D.C., 1976. 

This is an exothermic reaction, and both homogeneous (radical or cationic) and heterogeneous (soHd catalyst) initiators are used. The products range in molecular weight from below 1000 to a few million (see Olefin polymers). Reaction mechanisms and reactor designs have been extensively discussed (10-12). 

In a typical process, reaction is carried out at elevated temperatures in a polar solvent. The general polymer reaction scheme is as follows ... 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

Oxidizing agents, e.g., quinones, which were shown to be able to retard oxidation [13] can function as antioxidants (via a chain breaking acceptor process, CB—A) if they can compete with oxygen for the alkyl radicals (Scheme 4). In the case of polymers, reaction 4a can... 

The use of light olefins, diolefins, and aromatic-based monomers for producing commercial polymers is dealt with in the last two chapters. Chapter 11 reviews the chemistry involved in the synthesis of polymers, their classification, and their general properties. This book does not discuss the kinetics of polymer reactions. More specialized polymer chemistry texts may be consulted for this purpose. 

Kinoshita, Imoto etal.11 14) synthesized other anionic models, 5 (APVP), CPVP, UPVP, TPVA, HPVA, THPVA, and 6 (AMPPVA), by the polymer reaction of N-eoupled(2-dihydrogenphosphate)-ethylderivatives of nucleic acid bases (or adenosine-5 -phosphate, AMP) with polyvinylaleohol. A, C, U, T, H, and TH denote adenine, cytosine, uracil, thymin, hypoxanthine, and theophylline, respectively. The authors reported the apparent hypochromities of 3 to 16% for many kinds of mixtures of the models and DNA or RNA, as compiled in Table 1. However, for the mixtures APVA + RNA, HPVA + RNA HPVA + DNA, THPVA + RNA, CPVA + DNA and CPVA + RNA, no hypochromicity was detected. 

Exothermic oxygen-polymer reactions apparently occur before ignition. 

This reverse electrochemical control of the gel composition and volume is the basis for the singular electrochemical properties and the concomitant applications of conducting polymers. Reactions and properties based on polypyrrole films can be summarized as shown in Table 5 and below ... 

During the aqueous hydrolysis of dichlorosilanes there is always a very important side reaction. It is the self-condensation of silanols which are formed initially during the hydrolysis. These reactions also give rise to the formation of cyclic siloxanes together with the linear oligomers or polymers (Reaction Scheme III). The amount of cyclic products usually depends on the hydrolysis conditions and the degree of the self-condensation attained as well as concentration considerations. 

N. Friis and A. E. Hamielec, Principles of Polymer Reactor Design, in Polymer Reaction Engineering Course Notes, McMaster University, Hamilton, Ontario, Canada, p.55. 

With such modeling efforts, coupled with some small-scale tests, we can assess the hazards of a polymer reaction by knowing certain physical, chemical and reaction kinetic parameters. 

In conclusion, we have reviewed how our kinetic model did simulate the experiments for the thermally-initiated styrene polymerization. The results of our kinetic model compared closely with some published isothermal experiments on thermally-initiated styrene and on styrene and MMA using initiators. These experiments and other modeling efforts have provided us with useful guidelines in analyzing more complex systems. With such modeling efforts, we can assess the hazards of a polymer reaction system at various tempera-atures and initiator concentrations by knowing certain physical, chemical and kinetic parameters. 

This book is an outgrowth of an earlier book, Chemical Reactor Design, John Wiley Sons, 1987. The title is different and reflects a new emphasis on optimization and particularly on scaleup, a topic rarely covered in undergraduate or graduate education but of paramount importance to many practicing engineers. The treatment of biochemical and polymer reaction engineering is also more extensive than normal. 

Burnett, G. M., Mechanism of Polymer Reactions, Interscience, New York 1954,... 

---