Kinetic and ESR studies on the radical polymerization of α‐n‐(α′‐methylbenzyl) β‐ethyl itaconamates derived from racemic and (s)‐α‐methylbenzylamines

The polymerization of α‐N‐(α′‐methylbenzyl) β‐ethyl itaconamate derived from racemic α‐methylbenzylamine (RS‐MBEI) by initiation with dimethyl 2,2′‐azobisisobutyrate (MAIB) was studied in methanol kinetically and with ESR spectroscopy. The overall activation energy of polymerization was calculated to be 47 kJ/mol, a very low value. The polymerization rate (R_p ) at 60 °C was expressed by R_p = k[MAIB]^0.5±0.05[RS‐MBEI]^2.9±0.1. The rate constants of propagation (k_p ) and termination (k_t ) were determined by ESR. k_p was very low, ranging from 0.3 to 0.8 L/mol s, and increased with the monomer concentration, whereas k_t (4–17 × l0⁴ L/mol s) decreased with the monomer concentration. Such behaviors of k_p and k_t were responsible for the high dependence of R_p on the monomer concentration. R_p depended considerably on the solvent used. S‐MBEI, derived from (S)‐α‐methylbenzylamine, showed somewhat lower homopolymerizability than RS‐MBEI. The k_p value of RS‐MBEI at 60 °C in benzene was 1.5 times that of S‐MBEI. This was explicable in terms of the different molecular associations of RS‐MBEI and S‐MBEI, as analyzed by ¹H NMR. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4137–4146, 2000     

High-energy radiation forming chain scission and branching in polypropylene

The degradation of high molecular weight isotactic polypropylene (iPP) subjected to gamma rays irradiation up to 100 kGy in inert atmosphere was analyzed. The investigation relied upon complex viscosity, elastic modulus, gel fraction, morphology of the insoluble fraction and deconvoluted molecular weight distribution (MWD) curves. At low irradiation doses, already at 5 kGy, the MWD curve is strongly shifted to the low molecular weight side showing chain scission, which is confirmed using the calculated chain scission distribution function (CSDF). At high dose levels, the appearance of a shoulder in the high molecular weight side of the MWD curve indicates the formation of chain branching. The presence of a considerable insoluble fraction at these high dose levels indicates also the formation of cross-linking, which has different morphology then the insoluble fraction present in the original iPP. The rheological results show changes in the molecular structure of irradiated samples in agreement with the gel content data. The chromatographic and rheological data has shown that gamma irradiation of iPP produces chain scission, branching and cross-linking.     

Initiation Mechanism of Alternating Copolymerization of Styrene with Some Electron-Accepting Monomers in the Presence of Zinc Chloride by Means of Spin Trapping Technique

Abstract

The initiation mechanism of spontaneous alternating copolymerizations of styrene (St) and some electron-accepting monomers such as methyl methacrylate (MMA), methyl acrylate (MA), methacrylonitrile (MAN), and acrylonitrile (AN) in the presence of ZnCl₂ was studied by the spin trapping technique, in which 2-methyl-2-nitrosopropane (BNO) was used as a spin trapping reagent. When this technique was applied to the alternating copolymerization systems of St-MMA-ZnCl₂, St-MA-ZnCl₂, and St-MAN-ZnCl₂, the 2-phenylvinyl radical (·CH[dbnd]CH[sbnd]C₆H₅) was trapped as nitroxide. The structure of this nitroxide, which showed a large coupling constant (19～20 G) by β-hydrogen, was confirmed by comparison with the result of authentic experiment Accordingly it was concluded that this nitroxide was formed through proton migration from the St cation radical to the acceptor monomer anion radical in the charge- or electron-transfer complex, followed by reaction with BNO.

In the St-AN-ZnCl₂ system, however, a nitroxide derived from a cyclic radical was observed together with the nitroxide from 2-phenylvinyl radical. This cyclic radical seemed to be produced via the Diels-Alder adduct between St and AN.     

Hydrogen‐bond‐assisted stereocontrol in the radical polymerization of N‐isopropylacrylamide with primary alkyl phosphate: The effect of the chain length of the straight ester group

The radical polymerizations of N‐isopropylacrylamide (NIPAAm) were carried out in toluene at low temperatures in the presence of phosphoric acid esters such as trimethyl phosphate, triethyl phosphate (TEP), tri‐n‐propyl phosphate, and tri‐n‐butyl phosphate (TBP). Syndiotactically rich poly(NIPAAm)s were obtained from ?60 to 0 °C, and TEP provided the highest syndiotacticity (racemo dyad = 65%) at ?40 °C. On the other hand, lowering the temperature reversed the stereoselectivity of the propagation reaction so that isotactically rich poly(NIPAAm)s were obtained at ?80 °C. In particular, TBP exhibited the most isotactic specificity (meso dyad = 57%). Job's plots for NIPAAm–TBP mixtures revealed that NIPAAm and TBP formed a 1:1 complex at 0 °C and a predominantly 1:2 complex at ?80 °C through a hydrogen‐bonding interaction. Therefore, the stereospecificity of NIPAAm polymerization should depend on the stoichiometry of the hydrogen‐bond‐assisted complex. Thus, the mechanism for this polymerization system was discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 50–62, 2005     

Quantum state of muon and proton in crystalline silicon

Quantum state and dynamics of muon and proton in crystalline silicon have been studied by solving one‐particle Schrödinger equations for the impurities. The ground state wavefunctions and energies are determined as a function of local distortion of the host Si lattice, where the Si–H interaction we used is that parameterized by Ramírez and Herrero following the results of earlier pseudopotential‐density‐functional calculation. It is shown that quantum zero‐point motion of muon induces an effective potential barrier between the tetrahedral‐symmetry (T) site and the bond‐center (BC) site, which ensures bistability of muonium states observed in experiments. It is also shown that, if we fully consider the quantum effect with the present model potential, the BC site becomes less stable than the T site on the contrary to the experiments and former theoretical calculations.     

Isotope effects in the diffusion of hydrogen

Isotope effects in diffusion of hydrogen atoms are investigated theoretically. It is shown that isotope effect is reduced by a nonadiabatic effect of the heat bath so that the classical‐quantum crossover temperature and quantum tunneling rate lose their mass dependence. On the other hand, isotope effect is reversed in classical hopping rate under strong spatial confinement at the barrier top. These results indicate that isotope effects can be the means of observing influences of many degrees of freedom characterizing environment in diffusion process. This revised version was published online in August 2006 with corrections to the Cover Date.     

Regioselective cycloaddition reaction of alkene molecules with the asymmetric dimer on Si(100)c(4 x 2)

We investigated the adsorption states of 2-methylpropene and propene on Si(100)c(4 x 2) using low-temperature scanning tunneling microscopy. We have found that regioselective cycloaddition reactions (di-sigma bond formation) occur between the asymmetric alkene molecules and the asymmetric dimers on Si(100)c(4 x 2). First-principles calculations have elucidated that the regioselectivity is closely related to the structures of precursor species and these precursor species have carbocation-like features. Thus, we conclude that Markovnikov's rule is applicable for the cycloaddition of asymmetric alkene with the asymmetric dimer on Si(100)c(4 x 2).     

Long-lived polymer radicals. III. Synthesis of block copolymer by the reaction of living poly(N-methylacrylamide) radical with some vinyl monomers

N-Methylacrylamide (NMAAm) was polymerized quantitatively by using di-tert-butyl peroxide as photosensitizer to be, for the most part, incorporated in living poly(NMAAm) radical. The living polymer radical reacted effectively with acrylate monomers to yield block copolymer. Longer alkyl chain of the acrylate monomer caused a decrease in the conversion of the second monomer. Methacrylate monomers, such as methyl methacrylate and cyclohexyl methacrylate, showed relatively low reactivities in comparison with acrylates. Styrene exhibited a much lower conversion. The resulting block copolymers showed different thermochromic behaviors in methyl benzoate from that of poly(NMAAm). This is explained on the basis of the difference between refractive indexes of the block copolymers and poly(NMAAm).     

IR study on the intensity of ν1 of carbon disulfide

The IR intensity of ν₁ of carbon disulfide is reported in various states. The intensity increases with a decrease in temperature. Less than 10% of the observed intensity can be explained in terms of an electric field applied from an adjacent molecule. On the other hand, nearly half of the intensity is explained in terms of a vibrational coupling of ν₁ with ν₃ or ν₂, which is estimated from dimer models, on the assumption that the observed IR and Raman frequencies can be attributed to dimers.