Carbon-13 NMR microstructural characterization of dichlorocarbene adducts of polybutadiene

¹³C-NMR has been used to examine a number of dichlorocarbene adducts of cis- and trans-polybutadiene prepared in a two-phase system. Dichlorocarbene was generated by reaction of aqueous or solid NaOH or KOH with CHCl₃ in the presence of a phase transfer catalyst. Monomer compositions, comonomer sequence lengths, and stereochemical information were obtained for the resulting polymers. The polymers examined here were stereochemically pure and were treated as simple copolymers. Samples prepared using aqueous NaOH can be described as essentially random copolymers over the entire range of monomer composition. Samples prepared using solid alkali-metal hydroxides contain a higher fraction of blocked units than a polymer of comparable composition prepared using aqueous NaOH. This blockiness can coincide with the presence of two glass transition temperatures and a two-phase morphology as seen by transmission electron microscopy. Fractionation of a substantially blocked sample yielded a chlorine-poor fraction which was a random copolymer and a chlorine-rich fraction which was more blocked than the original unfractionated material.     

1,4 polymerization of isoprene by titanate catalyst systems

Titanates are versatile in the 1,4 polymerization of isoprene. The (R′O)₄Ti/RAlCl₂ catalyst gives either cis- or trans-1,4-polyisoprene, depending on the nature of both the titanate and the solvent. Primary titanates give cis-1,4-polyisoprene in both aliphatic and aromatic solvents. Secondary titanates give cis-polyisoprene in aliphatic solvents, and trans-1,4-polyisoprene in aromatic solvents. Tertiary titanates give trans-polyisoprene in both aliphatic and aromatic solvents. A mechanism is postulated which takes into consideration the role of the solvent. ESR studies of the various titanate–RAlCl₂ catalysts were made; the paramagnetic structures are related to polymerization mechanisms.     

First measurement of the double spin asymmetry in (-->)e(-->)p-->e(prime)pi(+)n in the resonance region

The double spin asymmetry in the (-->)e(-->)p --> e(prime)pi(+)n reaction has been measured for the first time in the resonance region for four-momentum transfer Q2 = 0.35-1.5 GeV(2). Data were taken at Jefferson Lab with the CLAS detector using a 2.6 GeV polarized electron beam incident on a polarized solid NH3 target. Comparison with predictions of phenomenological models shows strong sensitivity to resonance contributions. Helicity-1/2 transitions are found to be dominant in the second and third resonance regions. The measured asymmetry is consistent with a faster rise with Q(2) of the helicity asymmetry A1 for the F(15)(1680) resonance than expected from the analysis of the unpolarized data.     

Q2 Dependence of quadrupole strength in the gamma*p --> Delta(+)(1232) --> p pi(0) transition

Models of baryon structure predict a small quadrupole deformation of the nucleon due to residual tensor forces between quarks or distortions from the pion cloud. Sensitivity to quark versus pion degrees of freedom occurs through the Q2 dependence of the magnetic (M1+), electric (E1+), and scalar (S1+) multipoles in the gamma*p-->Delta(+)-->p pi(0) transition. We report new experimental values for the ratios E(1+)/M(1+) and S(1+)/M(1+) over the range Q2 = 0.4-1.8 GeV2, extracted from precision p(e,e(')p)pi(0) data using a truncated multipole expansion. Results are best described by recent unitary models in which the pion cloud plays a dominant role.     

Proton source size measurements in the eA-->e'ppX reaction

Two-proton correlations at small relative momentum q were studied in the eA(3He,4He,C,Fe)-->e(')ppX reaction at E(0)=4.46 GeV using the CLAS detector at Jefferson Lab. The enhancement of the correlation function at small q was found to be in accordance with theoretical expectations. Sizes of the emission region were extracted, and proved to be dependent on A and on the proton momentum. The size of the two-proton emission region for He was measured in eA reactions for the first time.