Reactions in polymers

D. McGinniss, in Ultraviolet Eight Induced Reactions in Polymers (Sympos. Ser. No. 25), American Chemical Society, Washington, D.C., 1976. 

Interfacial adhesion and, thereby, compatibility can be enhanced by the selective crosslinking reaction in polymer blends. Inoue and Suzuki [26] reported the properties of blends dynamically crosslinked PP-EPDM blends. The crosslinking agent was yV,N -/w-phenylene-bismaleimide - poly(2,2,4 - trimethyl - 1,2-dihydroquino -line) system. Increase in interfacial adhesion leads to... 

Diachkovsky FS (1987) In Reactions in polymer systems, Khimiya, Leningrad, p 174... 

Hence, the phenomena of the low reaction rate in the polymer matrix cannot be explained by the limiting rate of reactant orientation (rotational diffusion) in the cage. This result becomes the impetus to formulate the conception of the rigid cage of polymer matrix [16-20]. In addition to the experiments with comparison of the rate constants in the liquid phase and polymer matrix, experiments on the kinetic study of radical reactions in polymers with different amounts of introduced plasticizer were carried out [7,9,15,21], A correlation between the rate constant of the reaction k and the frequency of rotation vOT of the nitroxyl radical (2,2,6,6-tetramethyl-4-benzoyloxypiperidine-/Y-oxyI) was found. The values of the rate constants for the reaction... 

This model explains the above-mentioned peculiarities of free radical reactions in polymers. 

MECHANISMS OF SECONDARY REACTIONS. The primary processes involved in absorption of radiation in polymers lead to the expectation of free radical and ionic mechanisms for the secondary chemical reactions. Electron spin resonance (ESR) spectroscopy has proved extremely valuable for observation of free radical reactions in polymers, where various radicals are stabilized in the solid matrix at different temperatures. 

The rates of chemical reactions increase with temperature due to the greater proportion of molecules which have energies in excess of the activation energy and this will apply to radiation-induced secondary reactions in polymers. However, solid polymers are also characterized by their glass and melting transition temperatures. Substantial changes in molecular mobility occur across these transitions and the rates of chemical reactions are frequently greatly affected. 

Pulse Radiolysis. Surprisingly few studies have been carried out using the powerful technique of pulse radiolysis to study radiation reactions in polymers although an increasing number of studies on organic systems (67) and on monomers (68,69) are appearing. 

One particular phenomenon, often shown with oxidising reactions in polymers, is an induction period for ageing, whereby relatively little change is seen during this period but then the rate of degradation increases abruptly. It is easy to see how this can happen in materials protected by antioxidants in that with time the protective additives will be consumed. 

Oxidising reactions in polymers often have an induction period for ageing (see Section 4.12.1) during which relatively little change is seen, but after which the rate of degradation increases abruptly. In materials protected by antioxidants the induction time ends when the antioxidants have been consumed and oxidation begins in earnest. 

Figure 3. Principles of photochemical modification of polymer (e.g. PTFE) by ultraviolet (UV) light in ammonia or acetylene atmosphere (A-B). Basic processes of photochemical modification of polymer by UV light (hv) in atmosphere are (a) surface reactions, (b) reactions in atmosphere and (c) reactions in polymer. [5].

Not always is the PFR a side undesired reaction in polymers. In some cases, it has been used for practical purposes like selective image development [248], Another important use of the PFR is the photostabilization of polymers [249]. Aromatic esters and polyesters are sometimes mixed with other polymers in order to protect them from light. This delays aging of the polymer because the light is... 

Reactions on synthetic polymers often mimic similar reactions involving small molecules where size is the main difference. There are some exceptions where nearby groups may hinder or assist in the reaction in which differences occur. Here, the main differences are often kinetic, though some geometrical differences are found for specific cases. Reactions in which the rate of reaction in polymers is enhanced by the presence of neighboring groups are called anchimeric assistance. 

Table 2.1. Criteria for chain and stepwise reactions in polymer synthesis...

Charged Particle and Photon-Induced Reactions in Polymers... 

RADIATION-INDUCED REACTIONS IN POLYMERS 3.1. Fundamentals of Radiation Effects on Polymers... 

Possibly the most convincing evidence for positive ion-molecule reactions in polymers is the high rate of decay of vinyl unsaturation during the radiolysis of polyethylene, as recently discussed by Dole, Fallgatter, and Katsuura (13). The ideas of these authors with respect to the carbonium ion mechanism for vinyl decay by means of a dimerization reaction were largely suggested by the mechanisms proposed by Col-linson, Dainton, and Walker (5) for vinyl decay (polymerization) in the radiolysis of n-hexa-l-decene, Reactions 3 and 4 of Table I. 

Let us consider the easiest example of oxygen diffusion and reaction in polymer film having the thickness 2/. When oxygen pressure is low (see Chapter 2), the rate of chain oxidation of polymer... 

For reactions in polymers (like proteins) whether the static dielectic constant is the appropriate parameter is also questionable. Recent data for electron transfer in dry lexan films (Dg - 2.A) suggest X - 1.0 V, while classical calculations require X < 0.3 V. s s... 

J. F. Rabek, Mechanisms of Photophysical Processes and Photochemical Reactions in Polymers, Wiley, New York, 1987. 

Heskins, M., Reid, W. J., Pinchin, D. J., Guillet, J. E., "Photodegradation of Vinyl Chloride - Vinyl Ketone Copolymer, " Chapter 19 UV-Light Induced Reactions in Polymers, S. S. Labana, Ed., No. 50, ACS Symposium Series, 1976. 

Olefin metathesis has become a very important reaction in polymer chemistry and natural product synthesis [47-49]. Garber et al. have used the physical properties of dendrimers in order to improve the separation between the dendritic metathesis catalyst and products on silica gel column chromatography [50]. The Van Koten group has reported on the synthesis of different generations of carbosilane dendrimers functionalized with ruthenium metathesis catalysts [51]. 

Rabek JF (1987) Mechanism of photochemical processes and photochemical reactions in polymers. Wiley, Chicester... 

PHOTO-INDUCED ELECTRON TRANSFER REACTIONS IN POLYMER-BOUND RUTHENIUM BIPYRIDYL COMPLEXES... 

However, these polymers do degrade when subjected to terrestrial ultraviolet radiation, and this has been attributed to the presence of small amounts of impurities which absorb light and initiate oxidative chain reactions within the polymer. In the oxidation process, compounds containing peroxy and keto groups are formed, and these absorb in the long wavelength region and accelerate the breakdown of the polymer chains. Obviously a detailed study of such reactions in polymers will be difficult because of the frequently unknown nature of the trace impurities which initiate the reactions and of the multiplicity of products formed in the photooxidation. 

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