Cyanoisopropyl Radical Research Articles

Solvent and external pressure effects on the ratio of the cyanoisopropyl radical recombination and disproportionation rates

A GC-MS analysis of the azobisisobutyronitrile thermal decomposition products of in solutions at 80°C showed that the ratio of recombination and disproportionation rates of the cyanoisopropyl radical does not depend on the medium viscosity, but increases when the internal pressure of the solvent increases according to the log(k dispr/k rec) = −1.25 + 0.096 P int 0.5 law. This means that the activation volume corresponding to recombination is larger than that corresponding to disproportionation. It follows from the relationship log(k dispr/k rec) = (ΔV rec ≠ − Δv dispr ≠ )ΔP/RT that, for the decomposition of the substrate in benzene under a pressure of 0.5–4.0 kbar, the difference between the activation volumes is ΔV rec ≠ − ΔV dispr ≠ = 8 cm3/mol.

Thiocarbonylthio end group removal from RAFT‐synthesized polymers by a radical‐induced process

AbstractThe removal of thiocarbonylthio end groups by radical‐addition‐fragmentation‐ coupling from polymers synthesized by RAFT polymerization has been studied. We found that a method, which involves heating the polymer with a large excess (20 molar equivalents) of azobis(isobutyronitrile) (AIBN), while successful with methacrylic polymers, is less effective with styrenic or acrylic polymers and provides only partial end group removal. This is attributed to the propagating radicals generated from the latter polymers being poor radical leaving groups relative to the cyanoisopropyl radical. Similar use of lauroyl peroxide (LPO) completely removes the thiocarbonylthio groups from styrenic or acrylic polymers but, even with LPO in large excess, produces a polymer with a bimodal molecular weight distribution. The formation of a peak of double molecular weight is indicative of the occurrence of self‐termination and ineffective radical trapping. We now report that by use of a combination of LPO (2 molar equivalents) and AIBN (20 molar equivalents) we are able to completely remove thiocarbonylthio end groups of styrenic or acrylic polymers and minimize the occurrence of self termination. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6704–6714, 2009

Formation and Interconversion of Organo-Cobalt Complexes in Reactions of Cobalt(II) Porphyrins with Cyanoalkyl Radicals and Vinyl Olefins

Observation of the formation and interconversion of organo-cobalt complexes ((TMP)Co-R) is used to reveal mechanistic features in the living radical polymerization (LRP) of methyl acrylate (MA) mediated by cobalt porphyrins. Both dissociative and associative exchange of radicals in solution with organo-cobalt complexes contribute to controlling the radical polymerization. The sequence of organo-cobalt species formed during the induction period for the (TMP)Co-R mediated LRP of MA indicates that homolytic dissociation is a prominent pathway for the interconversion of organo-cobalt complexes which contrasts with the corresponding vinyl acetate (VAc) system where associative radical exchange totally dominates these processes. The dissociation equilibrium constant (K(d(333 K))) for organo-cobalt complexes formed in methyl acrylate polymerization ((TMP)Co-CH(CO(2)CH(3))CH(2)P) was estimated as 1.15 x 10(-10) from analysis of the polymerization kinetics and (1)H NMR. The ratio of the rate constants (333 K) for the cyanoisopropyl radical (*C(CH(3))(2)CN) adding with monomer (k(1)) to the process of transferring a hydrogen atom to (TMP)Co(II)* (k(2)) was evaluated for the methyl acrylate system as 2 x 10(-3) which is larger than that for vinyl acetate LRP (9 x 10(-5)). Kinetic analysis places the rate constant for associative radical interchange (333 K) at approximately 7 x 10(5) M(-1) s(-1). The larger radical stabilization energy and lower energy of the singly occupied molecular orbital (SOMO) for methyl acrylate based radicals (*CH(CO(2)CH(3))CH(2)P) compared to vinyl acetate contribute to the observed prominence of organo-cobalt homolytic dissociation and much smaller chain transfer which result in substantially better control for living radical polymerization of methyl acrylate than that observed for vinyl acetate.

Hydrogen‐Atom Transfer in Reactions of Organic Radicals with [CoII(por)]. (por=Porphyrinato) and in Subsequent Addition of [Co(H)(por)] to Olefins

The mechanisms for hydrogen-atom transfer from the cyanoisopropyl radical (*)C(CH(3))(2)CN to [Co(II)(por)](*) (yielding [Co(III)(H)(por)] and CH(2)=C(CH(3))(CN); por = porphyrinato) and the insertion of vinyl acetate (CH(2)=CHOAc) into the Co-H bond of [Co(H)(por)] (giving [Co(III){CH(OAc)CH(3)}(por)]) were investigated by DFT calculations. The results are compared with experimental data. These reactions are relevant to catalytic chain transfer (CCT) in radical polymerization of olefins mediated by [Co(II)(por)](*), the formation and homolysis of organo-cobalt complexes that mediate living radical polymerization of vinyl acetate, and cobalt-mediated hydrogenation of olefins. Hydrogen transfer from (*)C(CH(3))(2)CN to [Co(II)(por)](*) proceeds via a single transition state that has structural features resembling the products [Co(H)(por)] and CH(2)=C(CH(3))CN. The separated radicals approach to form a close-contact radical pair and then pass through the transition state for hydrogen-atom transfer to form [Co(III)(H)(por)] and CH(2)=C(CH(3))CN. This process provides a very low overall barrier for the hydrogen-atom transfer reaction (DeltaG(double dagger) = +3.8 kcal mol(-1)). The reverse reaction corresponding to the addition of [Co(H)(por)] to CH(2)=C(CH(3))CN has a low barrier (DeltaG(double dagger) = +8.9 kcal mol(-1)) as well. Insertion of vinyl acetate into the Co-H bond of [Co(III)(H)(por)] also proceeds over a low barrier (DeltaG(double dagger) = +11.4 kcal mol(-1)) hydrogen-transfer step from [Co(III)(H)(por)] to a carbon atom of the alkene to produce a close-contact radical pair. Dissociation of the radical pair, reorientation, and radical-radical coupling to form an organo-cobalt complex are the culminating steps in the net insertion of an olefin into the Co-H bond. The computed energies obtained for the hydrogen-atom transfer reactions from (*)C(CH(3))(2)CN to [Co(II)(por)](*) and from [Co(H)(por)] to olefins, as well as the organo-cobalt bond homolysis energies correspond well with the experimental observations. The mechanism of alkene insertion into the Co-H bond of [Co(III)(H)(por)] is of general interest, because the species does not contain any cis-vacant sites to the hydride and the usual migratory insertion pathway is not available. The low barrier predicted here for the multistep insertion process suggests that (depending on the bond strengths) even for systems that do have a cis-vacant site, the radical-type insertion might compete with classical migratory insertion.

Exchange of Organic Radicals with Organo-Cobalt Complexes Formed in the Living Radical Polymerization of Vinyl Acetate

Exchange of organic radicals between solution and organo-cobalt complexes is experimentally observed and the reaction pathway is probed through DFT calculations. Cyanoisopropyl radicals from AIBN (2,2'-azobisisobutyronitrile) enter solutions of cobalt(II) tetramesityl porphyrin ((TMP)Co(II)*, 1) and vinyl acetate (VAc) in benzene and react to produce transient hydride (TMP)Co-H and radicals (*CH(OAc)CH2C(CH3)2CN (R1*)) that proceed on to form organo-cobalt complexes (TMP)Co-CH(OAc)CH3 (4, Co-R2) and (TMP)Co-CH(OAc)CH2C(CH3)2CN (3, Co-R1), respectively. Rate constants for cyanoisopropyl radical addition with vinyl acetate and hydrogen atom transfer to (TMP)Co(II)* are reported through kinetic studies for the formation and transformation of organo-cobalt species in this system. Rate constants for near-degenerate exchanges of radicals in solution with organo-cobalt complexes are deduced from (1)H NMR studies and kinetic modeling. DFT computations revealed formation of an unsymmetrical adduct of (TMP)Co-CH(OAc)CH3 (4) with *CH(OAc)CH3 (R2*) and support an associative pathway for radical interchange through a three-centered three-electron transition state [R...Co...R]. Associative radical interchange of the latent radical groups in organo-cobalt porphyrin complexes with freely diffusing radicals in solution that is observed in this system provides a pathway for mediation of living radical polymerization of vinyl acetate.

Correlation of binder mechanical properties with microstructure for poly(butadiene–acrylonitrile–methacrylic acid) terpolymer: 13C‐NMR and pyrolysis‐GC studies

Abstract13C‐NMR spectroscopy (22.5 MHz) was used to characterize poly(butadiene (B)–acrylonitrile (A)–methacrylic acid (M)) liquid terpolymers prepared by emulsion polymerization. The sequence distribution of the monomers in the terpolymers was described in terms of triads BBB, ABA, ABB, BBA, MBB, and AMB and was found to vary with the mode of addition of methacrylic acid monomer. The NMR data was found to be in good agreement with a mechanism of polymerization in which methacrylic acid is preferentially involved in initiation reactions by a thiyl radical arising from the reaction of the chain modifier, 1‐dodecanethiol, and cyanoisopropyl radical generated from AIBN initiator. Binders obtained by curing the liquid terpolymers with an epoxy resin showed widely varying mechanical properties with tensile strength varying from 4.3 to 0.6 kg/cm2 and elongation at break from 130 to 450%. It was found that tensile strength increases and elongation decreases with the number of acrylonitrile units between the methacrylic acid crosslinks. Good correlation was obtained between triad population ratio [ABB + BBA] [BBvB]/[ABA][MBB] and the mechanical properties of the binders. Pyrolysis‐GC data at 550 and 600°C confirmed the results obtained from 13C‐NMR, and the mole ratio of butadiene to arylonitrile in the pyrolyzates showed correlation with the properties of binders.

Hindered initiation, primary-radical termination and effects of pressure on polymerization rates

The rates of polymerization of diethyl fumarate (DEF) and trans-ethyl cinnamate (EC) have been measured at 60°C, at various concentrations of the initiators azoisobutyronitrile (AIBN) and benzoyl peroxide (BPO), and at pressures from 1 to 3000 bar. In the EC/AIBN system at 1 bar the rate is nearly independent of initiator concentration and termination occurs almost entirely by reaction between growing chains and initiator radicals. The abnormally large initial pressure-acceleration of this polymerization can be explained in terms of the appropriate rate equation. Primary-radical termination is also of significance in the system EC/BPO, but not in the polymerization of DEF. The polymerization of each monomer is faster with BPO than with AIBN. It is concluded that in the initial reaction with EC the cyanoisopropyl radical is more hindered than are phenyl or benzoyloxy radicals. In the system DEF/AIBN initiation is hindered less strongly. There is some evidence of a related steric effect in the copolymerization of these monomers with methyl methacrylate.

ELECTRON TRANSFER PROCESSES IN THE REACTION OF PHENOTHIAZINE AND 10-METHYLPHENOTHIAZINE WITH 2,2′-AZOBISISOBUTYRONITRILE

Abstract Electron transfer processes in the reaction of phenothiazine and 10-methylphenothiazine with 2,2′-azobisisobutyronitrile have been examined by UV and ESR spectroscopies. The reaction was found to proceed via one electron transfer from phenothiazine derivatives to a cyanoisopropyl radical.

13C N.M.R. spectra of pyrrolidines and piperidines. Structure of the Perhydroisoindol-5-ones and 3-Azabicyclo[3,3,1]nonan-6-imines formed by the cyanoisopropyl radical induced cyclization of N-Methyl-N,N-bis(2-alkylallyl)amines

The 13C N.M.R. chemical shifts of a series of substituted pyrrolidines and piperidines are documented and substituent parameters determined. These data have been used to assign structures to the polymers and bicyclic products formed in the cyanoisopropyl radical induced cyclizations of a series of N-methyl-N,N-bis(2-a1kylallyl)amines. The formation of perhydroisoindol-5-one and/or 3-azabi- cyclo[3,3,1]nonan-6-imine depends upon the bulk of the 2-alkyl substituent and correlates with the presence of pyrrolidine and/or piperidine units in the polymer structure.

The cyclopolymerization of N-Allyl-N-methyl(2-substituted allyl)amines. The structure of the polymers and low molecular weight products

Mass and 13C nuclear magnetic resonance spectroscopy are used to determine the structures of the poly(N-methylpyrrolidines), poly(N-methylpiperidines) and low molecular weight products obtained from N-allyl-N-methyl(2-substituted allyl)amines by cyanoisopropyl radical induced cyclizations. The structures are used to determine the preferred site of initial radical addition to the diallylamine and the subsequent direction of cyclization of the intermediate azaheptenyl radicals. Steric interactions induced by the β-substituents tend to favour attack at the unsubstituted allyl group whereas conjugated substituents favour attack at the substituted group with the consequent formation of conjugation-stabilized radicals. Kinetically controlled cyclization to the pyrrolidines occurs in all cases except that of the t-butyl derivative in which steric interactions induce cyclization to both piperidine and pyrrolidine products.

Cyanoisopropyl radical induced cyclization and cyclopolymerization of N-methyl-N-(2-alkylallyl)amines and N-methyl-NN-bis-(2-alkylallyl)amines. A 13C nuclear magnetic resonance study

Download options Please wait... Article information DOI https://doi.org/10.1039/C39750000982 Article type Paper Download Citation J. Chem. Soc., Chem. Commun., 1975, 982-983 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Cyanoisopropyl radical induced cyclization and cyclopolymerization of N-methyl-N-(2-alkylallyl)amines and N-methyl-NN-bis-(2-alkylallyl)amines. A 13C nuclear magnetic resonance study D. G. Hawthorne, S. R. Johns, D. H. Solomon and R. I. Willing, J. Chem. Soc., Chem. Commun., 1975, 982 DOI: 10.1039/C39750000982 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author David G. Hawthorne Stanley R. Johns David H. Solomon Richard I. Willing