Polymers, radical derivatives

Ti represents the polarity of the polymer radical derived from monomer 1, and M2 represent polarity of monomer 2... 

This is the simplest of the models where violation of the Flory principle is permitted. The assumption behind this model stipulates that the reactivity of a polymer radical is predetermined by the type of bothjts ultimate and penultimate units [23]. Here, the pairs of terminal units MaM act, along with monomers M, as kinetically independent elements, so that there are m3 constants of the rate of elementary reactions of chain propagation ka ]r The stochastic process of conventional movement along macromolecules formed at fixed x will be Markovian, provided that monomeric units are differentiated by the type of preceding unit. In this case the number of transient states Sa of the extended Markov chain is m2 in accordance with the number of pairs of monomeric units. No special problems presents writing down the elements of the matrix of the transitions Q of such a chain [ 1,10,34,39] and deriving by means of the mathematical apparatus of the Markov chains the expressions for the instantaneous statistical characteristics of copolymers. By way of illustration this matrix will be presented for the case of binary copolymerization ... 

Although it has been suggested that crosslinking in the presence of MAH involves coupling of appended MAH radicals with other appended MAH radicals or with polymer radicals (7) the former is improbable due to the tendency for disproportionation rather than coupling between radicals derived from strong electron acceptor monomers such as MAH. 

There have also been several papers [61-63] on the importance of carefully establishing the reaction mechanism when attempting the copolymerization of olefins with polar monomers since many transition metal complexes can spawn active free radical species, especially in the presence of traces of moisture. The minimum controls that need to be carried out are to run the copolymerization in the presence of various radical traps (but this is not always sufficient) to attempt to exclude free radical pathways, and secondly to apply solvent extraction techniques to the polymer formed to determine if it is truly a copolymer or a blend of different polymers and copolymers. Indeed, even in the Drent paper [48], buried in the supplementary material, is described how the true transition metal-catalyzed random copolymer had to be freed of acrylate homopolymer (free radical-derived) by solvent extraction prior to analysis. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

Numerous reports published in recent years have focused on carbon-centered radicals derived from compounds with selected substitution patterns such as alkanes [40,43,47], halogenated alkanes [43,48,49,51-57], alkenes [19], benzene derivatives [43,47], ethers [51,58], aldehydes [48], amines [10,59], amino acids [23,60-67] etc. Particularly significant advances have been made in the theoretical treatment of radicals occurring in polymer chemistry and biological chemistry. The stabilization of radicals in all of these compounds is due to the interaction of the molecular orbital carrying the unpaired electron with energetically and spatially adjacent molecular orbitals, and four typical scenarios appear to cover all known cases [20]. 

D. Axelson The low density (branched) polyethylenes were free radical initiated the linear polymers were derived from commercial sources and purified and characterized as described (Macromolecules 10, 550(1977)). 

Acrylonitrile. - Used widely in the polymer industry, acrylonitrile is considered to be a possible human carcinogen. DNA damage is believed to involve metabolism to an epoxide, followed by adduct formation with DNA. There is also evidence for radical-mediated damage to DNA. For example, Ohnishi et al. have trapped a nitrogen-centred radical derived from acrylonitrile, the formation of which was associated damage to DNA.22... 

Rabek and Ranby (22) have shown that a free radical induced degradation of polystyrene occurs in the presence of oxygen, that leads to rapid chain scission. Benzoyl radicals derived from Type 1 initiators abstract hydrogen atoms from the polymer and thereby start a chain degradation process. Berner, Kirchmayr and Rist (6) and others have shown that when initiator I is irradiated in solution, the benzoyl radical abstracts a hydrogen from the surrounding solvent to form benzalkdehyde and a free radical. 

The radiotracer method for estimating efficiency of initiation was applied by Bevington and Eaves (32) to polymerization in benzene and in carbon tetrachloride. Whereas they had calculated that about 47% of the radicals from AIBN initiate polymer chains in DMF solvent, efficiency in benzene was about 50% and in carbon tetrachloride about 30%. This low efficiency in carbon tetrachloride is attributed to attack of radicals from AIBN on the carbon tetrachloride solvent, especially at high concentrations of solvent. Chains initiated by secondary radicals derived in this way from the solvent would not be detected by the tracer method. 

In accordance with the Smith-Ewart theory, the nucleation of particles takes place solely in the monomer-swollen micelles which are transformed into polymer particles [16]. This mechanism is applicable for hydrophobic (macro)mon-omers (see Scheme 2). The initiation of emulsion polymerization is a two-step process. It starts in water with the primary free radicals derived from the water-soluble initiator. The second step occurs in the monomer (macromonomer)-swollen micelles by entered oligomeric radicals. 

Grollmann U, Schnabel W (1980) On the kinetics of polymer degradation in solution, 9. Pulse radiolysis of polyethylene oxide). Makromol Chem 181 1215-1226 Hamer DH (1986) Metallothionein. In Richardson CC, Boyer PD, Dawid IB, Meister A (eds) Annual review of biochemistry. Annual Reviews, Palo Alto, pp 913-951 Held KD, Harrop HA, Michael BD (1985) Pulse radiolysis studies of the interactions of the sulfhydryl compound dithiothreitol and sugars. Radiat Res 103 171-185 Hilborn JW, PincockJA (1991) Rates of decarboxylation of acyloxy radicals formed in the photocleavage of substituted 1-naphthylmethyl alkanoates. J Am Chem Soc 113 2683-2686 Hiller K-O, Asmus K-D (1983) Formation and reduction reactions of a-amino radicals derived from methionine and its derivatives in aqueous solutions. J Phys Chem 87 3682-3688 Hiller K-O, Masloch B, Gobi M, Asmus K-D (1981) Mechanism of the OH radical induced oxidation of methionine in aqueous solution. J Am Chem Soc 103 2734-2743 Hoffman MZ, Hayon E (1972) One-electron reduction of the disulfide linkage in aqueous solution. Formation, protonation and decay kinetics of the RSSR radical. J Am Chem Soc 94 7950-7957... 

When the polymer is charged, the repulsive forces of the charges prevent an approach of the radicals, and the lifetime of the radicals increases dramatically. In the case of poly(acrylic acid), for example, the decay of the poly(acrylic) acid radicals is fast and follows the same kinetics as any radical derived from neutral polymers as long as the polymer is fully protonated (low pH) (Ulanski et al. 1996c). With increasing pH and concomitant dissociation of the polymer, however, the polymer assumes a rod-like shape, its segments become less flexible, and repulsive forces increasingly prevent their approach. Some radicals survive even for hours under such conditions. 

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

The presence of free radicals deriving from carbon black could also complicate the interpretation of NMR data in the case of filled rubbers, because radicals may cause a substantial decrease in T2. Two types of radicals have been detected in carbon-black-filled rubbers localised spins attributable to the carbon black and mobile spins deriving from rubbery chains [86]. Mobile spins are formed because of the mechanical breakdown of polymer chains when a rubber is mixed with carbon black. The concentration of mobile spins increases linearly with carbon black loading [79, 87]. 

Voluminous literature data exists on the thermal degradation of polymers. It was, e.g., established that characteristic parameters of the polymer degradation are determined by the way of preparation of a particular sample. It has been postulated that the less stable samples contain weak links in the macromolecule. However, the nature of these links is not well defined it is known that the weakest links in polymer chains are peroxidic bonds. In hydrocarbon polymer stored in air, a certain concentration of peroxides will always be present as well as of free radicals derived from the decomposition of peroxides. 

In order to get further insight into the reaction mechanism for the degradation of PMMA, we have studied the nature and behavior of radical entities in irradiated PMMA by using the ESR and ESE techniques complementarily [37]. Two PMMA samples, a commerical PMMA and an initiator-free PMMA prepared by the radiation-polymerization of bulk monomer, were used, but no difference was found in the results. Residual monomer was carefully removed from the PMMA samples, because the monomer molecule readily modifies the radicals derived from the polymer. The samples were irradiated in vaccum. Figure 9 demonstrates the dose-yield curve we obtained by irradiating PMMA in vacuum at 273 K. The G value for the radical formation is determined to be 3.0 from the slope of the linear portion below 12 kGy. 

The simplest explanation for the formation of high molecular weight polymer through oxidative coupling of aryloxy radicals involves the successive addition of monomer units to the radicals derived from polymer phenols (Reaction 4). 

So far, detailed information on the photochemistry of these combinations is not available. It can be expected that similar reaction pathways hold as given in Scheme 3. In some cases, end-group analyses of polymers prepared clearly indicate that initiation of polymerization takes place by radicals derived from both partners. The radical mechanism is also dedded through the inhibitory effect of air or benzoquinone. For some combinations, the kinetic non-ideality with respect to the low initiator exponent O < 0.5) can be interpreted on the basis of significant initiator dependent termination, particularly by degradative initiator transfer mechanism. 

All these spectra were acquired at elevated temperatures ( 100°C), that is, where the observation of fast motion spectra is expected. In Fig. 14.4A, the TREPR spectrum of the main-chain polymer radical from photolysis of /-PMMA is repeated from the bottom left side of Fig. 14.2, as it is the starting point for comparisons of spectral features such as hnewidths and hyperfine coupling constants. The nomenclature used throughout this section is derived using the notations indicated in Scheme 14.1 and Chart 14.1. For example, a main-chain radical from PMMA will be denoted la, whereas the oxo-acyl radical from PFOMA will be designated as radical 6b, and so on. For all radicals simulated, the parameters used are listed in Table 14.1. 

Dependence of rate coefficients on polymer chain length. The rate equations in Example 10.3 were derived with the assumption that the rate coefficients do not depend on the degree of polymerization of the polymer radicals and remain constant as more polymer molecules are formed. There are two major exceptions ... 

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