Copolymerization oxidation reduction

The values of these ratios change appreciably by passing from the heterogeneous (suspension) to the homogeneous (DMF) system. In the case of copolymerization in suspension in the presence of the K2S208—AgN03 oxidation-reduction system at 30—40 °C, the ratios were found to be ry = 0,77 0,2 and r2 = 1,09 0,04, whereas in the case of the copolymerization in solution they are = 0,52 and r2 = 1,7. The difference in these values seems to be the result of the different solubility of the monomers in water and of the different rate of diffusion of the monomers to the surface of the precipitated copolymer20. From this it follows that 4 is the more reactive monomer in this binary system. 

In a recently published paper6, on the investigation of AN copolymerization with the quartemary salt of l,2-dimethyl-5-vinylpyridinium sulfate (DMVPS) in dimethyl sulfoxide (DMSO) with 2,2 -azoisobutyronitrile as initiator, and in aqueous medium in the presence of the potassium persulfate/sodium metabisulfite oxidation-reduction system at 60 °C, the authors found the reactivity of the monomers, especially that of MVPS (methylvinylpyridin sulfate) to depend significantly on the polarity of the medium. 

As it was shown in73, 74), methods that can be used to synthesize these copolymers of PAN are those of radical AN block copolymerization in the presence of an oxidation-reduction system in which the hydroxyl end groups of polyethylene oxide) (PEO)73) and polypropylene oxide) (PPO)74- oligomers serve as the reducing agents and tetravalent cerium salts as the oxidizing agents. 

Compounds 43-46 are representative examples of the polymers prepared by the oxidation-reduction copolymerization route . 

Free-radicals generated in many oxidation-reduction (or redox) reactions can be used to initiate chain poymerization. An advantage of this type of initiation is that, depending on the redox system used, radical production can occur at high rates at moderate (0-50°C) and even lower temperatures. Redox systems are generally used in polymerizations only at relatively low temperatures, a significant commercial example being the production of styrene-butadiene rubber by emulsion copolymerization of butadiene and styrene at 5-10°C ( cold recipe ). 

Ethanal is the most important of these compounds (Table 2.6). The many ways it can be produced and its high reactivity (the CHO radical has extensive chemical affinities), as well as its rapid combination with sulfur dioxide at low temperatures and its organoleptic properties, make ethanal a very important component of wine. The presence of ethanal, produced by the oxidation of ethanol, is closely linked to oxidation-reduction phenomena. It is involved in the alcoholic fermentation mechanism. Furthermore, ethanal plays a role in the color changes occurring in red wines during aging by facilitating the copolymerization of phenols (anthocyanins and catechins) (Section 6.3.10). 

Copolymerizations can proceed in an extraordinary multiplicity of ways. For example, two different monomers may dimerize to a zwitterion or charge transfer complex before the actual polymerization step. Conventional transition states are crossed during the propagation step in the majority of cases but oxidation-reduction processes may also occur. In certain circumstances, the joint polymerization of two different monomers does not lead to copolymers at all, but to polymer mixtures sometimes at all yields and sometimes only when the more reactive monomer is completely, or nearly completely, consumed. 

In the case of copolymerization of vinylphosphonic acid monoethyl ester (92) with cyclic phosphonites (86), the alternating copolymer (93) having two kinds of phosphorus atoms in the main chain was formed. During the copolymerization, monomer 92 was reduced involving a hydrogen-transfer process and monomer 86 was oxidized the oxidation state of the phosphorus atom of monomer 86 changed, therefore, from trivalent to pentavalent ( oxidation-reduction copolymerization ). ... 

Modification of lignocellulosic materials surface by copolymerization with vinyl monomers has been reported. The polymerization reaction is initiated at the surface of the fibers by incorporation of peroxides or oxidation-reduction agents, or by treatment with gamma radiation or cold plasma [49]. These reactions form free radicals on the fibers, which initiates the free chain reaction with the vinyl monomers. Different types of properties can be conferred to the fibers using different vinyl monomers, such as increased hydrophilicity with poly(vinyl alcohol), increased hydrophobicity with polystyrene or polyvinylacetate, increased reactivity with polyvinylamine etc. 

According to Eq. (25), a cyclic phosphite monomer (MN) 38 is oxidized to a phosphate unit yielding copolymer 40 whereas the a-keto acid monomer (ME) 39 is reduced to the corresponding a-hydroxy acid ester. Thus, the term redox copolymerization has been proposed to designate this type of copolymerization in which one monomer is reduced and the other monomer oxidized. The redox copolymerization clearly differs from the so-called redox polymerization in classical polymer chemistry where the redox reaction between the two catalyst components (oxidant and reductant) is responsible for the production of free radicals. 

It is important to pay attention to the potential role of peroxides created on the surface of plasma-treated, including plasma polymer-coated, TPOs in the formation of durable bonds between the substrate and primer. It has been known for decades that the peroxides formed on the irradiated polymers (by y-ray. X-ray, electron beams, etc.) can be utilized in graft copolymerization of various monomers. This method is known as the peroxide method of radiation copolymerization [27]. The trunk polymer is first irradiated by ionizing radiation in a vacuum or in an inert gas environment. The irradiated polymer is exposed to air or oxygen to convert free radicals to peroxides. Thus created peroxides-containing polymers were used as the initiator of the free radical polymerization of the second monomer. The polymer peroxides are decomposed by heat or by the use of reduction/oxidation accelerator, i.e., peroxides are converted to free radicals. 

The presence of a butyl substituent is necessary for the polymer to be soluble. The molecular weight (M ) of 35 (R = n-Bu) was determined to be 40,000 by gel permeation chromatography. The [3]ferrocenophanes 34 (R = H and n-Bu) can be copolymerized to give soluble approximately 1 1 copolymers with 25,000. Remarkably, the polymers 35 can be reversibly reductively cleaved with Li[BEtiH) (to give 36) and then regenerated on oxidation with ->. 

In contrast to the catalysts containing Ti that produce polymers with a broad MW distribution of 5 to 30, the compounds containing V produce polyethylene with a narrow MW distribution of 2-4. The V systems are suited to polymerization of higher a-olefins and for copolymerization. Therefore, these systems are used technically to make a rubber (EPDM type) by copolymerization of C2H4, propylene, and as diene ethylidene norbor-nene (ENB). These catalysts are initially very active because of the presence of V(III), which seems to be the active oxidation form. However, some of them lose activity by reduction after a short polymerization time. They can be reactivated by weak oxidizing agents (activator) like chlorobenzene. 

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