chain termination

Five reaction schemes are suggested for the initiated nonbranched-chain addition of free radicals to the multiple bonds of the unsaturated compounds. The proposed schemes include the reaction competing with chain propagation reactions through a reactive free radical. The chain evolution stage in these schemes involves three or four types of free radicals. One of them is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. Based on the suggested schemes, nine rate equations (containing one to three parameters to be determined directly) are deduced using quasi-steady-state treatment. These equations provide good fits for the nonmonotonic (peaking) dependences of the formation rates of the molecular products (1:1 adducts) on the concentration of the unsaturated component in binary systems consisting of a saturated component (hydrocarbon, alcohol, etc.) and an unsaturated component (alkene, allyl alcohol, formaldehyde, or dioxygen). The unsat...

Five reaction schemes are suggested for the initiated nonbranched-chain addition of free radicals to the multiple bonds of the unsaturated compounds. The proposed schemes include the reaction competing with chain propagation reactions through a reactive free radical. The chain evolution stage in these schemes involves three or four types of free radicals. One of them is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. Based on the suggested schemes, nine rate equations (containing one to three parameters to be determined directly) are deduced using quasi-steady-state treatment. These equations provide good fits for the nonmonotonic (peaking) dependences of the formation rates of the molecular products (1:1 adducts) on the concentration of the unsaturated component in binary systems consisting of a saturated component (hydrocarbon, alcohol, etc.) and an unsaturated component (olefin, allyl alcohol, formaldehyde, or dioxygen). The unsaturated compound in these systems is both a reactant and an autoinhibitor generating low-reactive free radicals. A similar kinetic description is applicable to the nonbranched-chain process of the free-radical hydrogen oxidation, in which the oxygen with the increase of its concentration begins to act as an oxidation autoingibitor (or an antioxidant). The energetics of the key radical-molecule reactions is considered.

Five reaction schemes are suggested for the initiated nonbranched-chain addition of free radicals to the multiple bonds of the unsaturated compounds. The proposed schemes include reactions competing with chain propagation reactions through a reactive free radical. The chain evolution stage in these schemes involves three or four types of free radicals. One of them is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. Based on the suggested schemes, nine rate equations (containing one to three parameters to be determined directly) are deduced using quasi-steady-state treatment. These equations provide good fits for the nonmonotonic (peaking) dependences of the formation rates of the molecular products (1:1 adducts) on the concentration of the unsaturated component in binary systems consisting of a saturated component (hydrocarbon, alcohol, etc.) and an unsaturated component (olefin, allyl alcohol, formaldehyde, or dioxygen). The unsaturated compound in these systems is both a reactant and an autoinhibitor generating low-reactive free radicals. A similar kinetic description is applicable to the nonbranched-chain process of the free-radical hydrogen oxidation, in which the oxygen with the increase of its concentration begins to act as an oxidation autoingibitor (or an antioxidant). The energetics of the key radical-molecule reactions is considered.  C НОН, etc.), is included for the first time in the chain propagation stage. This reaction yields a low-reactive radical (such as СН2=С(СН3)  C Н2 or Н  C =О) and thus leads to chain termination because this radical does not continue the chain and thereby inhibits the chain process . We will consider kinetic variants for the case of comparable component concentrations with an excess of the saturated component and the case of an overwhelming excess of the saturated component over the unsaturated component .

Yields in methyl methacrylate (MMA) polymerization as a function of triethylaluminum (TEA) concentration have been determined a t a constantbenzoquinone (BQ) concentration. The polymerization is negligible a t [TEA]/[BQ] concentration.ratios smaller than 1, but reaches a maximum yield and then decreases for larger TEA concentrations. Molecular weight measurements show a similar trend, although the maximum is shifted with respect to that corresponding to polymer yield. The existence of two reaction mechanisms is shown by rate measurements a t constant initial concentrations. One is a fast reaction, over in a few minutes, which accounts for most of the yield after 30 min reaction time. The other is a slower photoinitiated reaction of the products of the fast reaction. The mechanism proposed is based on a chain-transfer reaction between the inhibitor radical, formed by the addition of the polymer radical (M-) to BQ and TEA, giving a reactive ethyl radical: Me + BQ + MBQ. MBQ-+ MTEA-MBQAl(C2H5)2 + CiH; + MMA The competition between a pair of reactions of TEA and BQ, i.e., a free-radical reaction corresponding to the conjugate addition and a molecular reaction leading to the BQ reduction product, is held responsible for the observed yield maximum, since the molecular reaction prevails a t larger TEA concentrations. The observed drop in the molecular weight is the result of a chain-transfer reaction on TEA, i.e., the substitution of an ethyl group by the growing polymer radical.

Yields in methyl methacrylate (MMA) polymerization as a function of triethylaluminum (TEA) concentration have been determined a t a constantbenzoquinone (BQ) concentration. The polymerization is negligible a t [TEA]/[BQ] concentration.ratios smaller than 1, but reaches a maximum yield and then decreases for larger TEA concentrations. Molecular weight measurements show a similar trend, although the maximum is shifted with respect to that corresponding to polymer yield. The existence of two reaction mechanisms is shown by rate measurements a t constant initial concentrations. One is a fast reaction, over in a few minutes, which accounts for most of the yield after 30 min reaction time. The other is a slower photoinitiated reaction of the products of the fast reaction. The mechanism proposed is based on a chain-transfer reaction between the inhibitor radical, formed by the addition of the polymer radical (M-) to BQ and TEA, giving a reactive ethyl radical: Me + BQ + MBQ. MBQ-+ MTEA-MBQAl(C2H5)2 + CiH; + MMA The competition between a pair of reactions of TEA and BQ, i.e., a free-radical reaction corresponding to the conjugate addition and a molecular reaction leading to the BQ reduction product, is held responsible for the observed yield maximum, since the molecular reaction prevails a t larger TEA concentrations. The observed drop in the molecular weight is the result of a chain-transfer reaction on TEA, i.e., the substitution of an ethyl group by the growing polymer radical.

The polymerization of MMA mediated by symmetric trithiocarbonate as a reversible additionfragmentation chain transfer agent is studied. It is shown that the process proceeds according to the two stage pseudoliving radical mechanism. The polymeric reversible addition fragmentation chain transfer agent is more efficient than its low molecular mass analog. The use of the polymeric reversible addition fragmenta tion chain transfer agent makes it possible to synthesize narrowly dispersed homopolymers of MMA and related copolymers with a controllable molecular mass. Both chain transfer agents have practically no effect on the initial rate of copolymerization but allow weakening or even suppression of the gel effect at high con versions.

The posteffects in the polymerization of methyl methacrylate at low monomer conversions are studied. The experimental two-step method consists of a short initial photoinduced polymerization period and subsequent register of the monomer concentration evolution in the dark. After switching off the light, the radical concentration drops sharply, but a nonnegligible concentration of persistent radicals continues the polymerization reaction for long periods of time, yielding ultrahigh molecular weight materials (over 10 000 monomer units). By a fitting procedure applied to the conversion-time curves obtained during the dark period, average values of the termination rate coefficients, 〈k t 〉, scarcely exceeding 10 000 L mol-1 s-1 , and radical concentrations at the beginning of the postpolymerization about 10-8 mol L-1 are determined. The reliability of these fitting results is checked by the design of reactions with similar polymerization rate but under steady-state conditions. The molecular weight of the polymers synthesized during the dark period is controlled by the transfer reactions to the monomer and to the initiator. Both transfer reaction constants are estimated by using a method based on the analysis of the instantaneous chain length distribution. The extremely low 〈k t 〉 values obtained by the analysis suggest that the power-law exponents which describe the chain-length dependency of k t should be higher than the unity for long radicals even in dilute solution (i.e., at low monomer conversions).

The posteffects in the polymerization of methyl methacrylate at low monomer conversions are studied. The experimental two-step method consists of a short initial photoinduced polymerization period and subsequent register of the monomer concentration evolution in the dark. After switching off the light, the radical concentration drops sharply, but a nonnegligible concentration of persistent radicals continues the polymerization reaction for long periods of time, yielding ultrahigh molecular weight materials (over 10 000 monomer units). By a fitting procedure applied to the conversion-time curves obtained during the dark period, average values of the termination rate coefficients, 〈k t 〉, scarcely exceeding 10 000 L mol-1 s-1 , and radical concentrations at the beginning of the postpolymerization about 10-8 mol L-1 are determined. The reliability of these fitting results is checked by the design of reactions with similar polymerization rate but under steady-state conditions. The molecular weight of the polymers synthesized during the dark period is controlled by the transfer reactions to the monomer and to the initiator. Both transfer reaction constants are estimated by using a method based on the analysis of the instantaneous chain length distribution. The extremely low 〈k t 〉 values obtained by the analysis suggest that the power-law exponents which describe the chain-length dependency of k t should be higher than the unity for long radicals even in dilute solution (i.e., at low monomer conversions).

The kinetics of the radical polymerization of methyl methacrylate initiated by an AIBN-hexaphenylethane initiating system was studied, and the molecular mass characteristics of the resulting polymers were investigated within the entire range of monomer conversions. It was found that, when hexaphenylethane was in excess with respect to AIBN, the mechanism of polymerization changed to pseudoliving, and the process was characterized by a successive increase in the molecular mass of the polymers and by suppression of the gel effect at high conversions.

The response to antiretroviral therapy in human immunodeficiency virus-infected patients is limited by the emergence of drug resistance. This resistance is a consequence of the high rate of HIV mutation, the high rate of viral replication (especially when potent multidrug therapies are not used or taken reliably), and the selective effect of these drugs, which favors the emergence of mutations that can establish clinical drug resistance. The introduction of highly active antiretroviral therapy, which typically includes at least 2 nucleoside reverse transcriptase inhibitors and a protease inhibitor or a nonnucleoside reverse transcriptase inhibitor, for most treatment-naive patients results in a reduction of viral load below the limit of detection determined by currently available HIV RNA assays. It is this marked reduction that results in durable viral suppression, usually only possible by the simultaneous use of 3 or 4 drugs. The reverse transcriptase inhibitor components of highly active antiretroviral therapy are crucial for such benefits of combination therapy. Specific amino acid changes are associated with resistance to several reverse transcriptase inhibitors, but new mutation complexes have been observed that can confer broad cross-resistance within this class. Genotypic and phenotypic resistance assays to measure drug resistance are being developed, but refinements in both methodology and our ability to interpret results of these assays are necessary before they are introduced into widespread clinical use.