Novel multigraft copolymers of poly(methyl methacrylate-graft-polystyrene) (PMMA-g-PS) in which the number of graft PS side chains was varied were prepared by a subsequent two-step living radical copolymerization approach. A polymerizable 4-vinylbezenyl 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) monomer (STEMPO), which functioned as both a monomer and a radical trapper, was placed in a low-temperature atom transfer radical polymerization (60°C) process of methyl methacrylate with ethyl 2-bromopronionate (EPNBr) as an initiator to gain ethyl pronionate-capped prepolymers with TEMPO moieties, PMMA-STEMPOs. The number of TEMPO moieties grafted on the PMMA backbone could be designed by varying STEMPO/EPNBr, for example, the ratios of 1/2, 2/3, or 3/4 gained one, two, or three graft TEMPO moieties, respectively. The resulting prepolymers either as a macromolecular initiator or a trapper copolymerized with styrene in the control of stable free-radical polymerization at an elevated temperature (120°C), producing the corresponding multigraft copolymers, PMMA-g-PSs. The nitroxyl-functionalized PMMA prepolymers produced a relatively high initiation efficiency (Ͼ0.8) as a result of the stereohindrance and slow diffusion of TEMPO moieties connected on the long PMMA backbone. The polymerization kinetics in two processes showed a living radical polymerization characteristic. The molecular structures of these prepolymers and graft copolymers were well characterized by combining Fourier transform infrared spectroscopy, gel permeation chromatography, chemical element analysis, and 1 H NMR.

There is much current interest in the nonlinear optical pmperties of quantum confined semiconductor nanocryslals, particularly for their potential use in nonlinear photonic devices. Recently we have succe~sfully synthesizcd an optically homogenous PbS-polymer nanocomposite by in-situ polymerization of bicantinuous micmcmulsions [I]. The PbS nanocryslalr with an average size of 5 nm were formed in aqueous domains randomly embedded in the methyl methacrylate (MMA)iacronitrile (AN)/m-mcthoxy poly(efhylene oxid.& ~mdecyl-u-melhacrylaa (CI-PEO-R-MA-40) copolymer nctwark. Here we present an experimental rNdy of third+der o p t i d nanlincarity in the PbS-polymer nanucomposite, using the Z-scan technique with linear polarized 120-fs laser pulses at 780-m wavelength.Thc mechanisms responsible for the observed optical nodinrzty are also discussed.In OUI Z-swn expenmenl. the laser b m was provided by B Ker-lens mode-locked Ti:sapphire loser operating 81 76 MHz repetition mls, with a nearly Gaussian TEMm mode spatial profile. The beam waist wo at the focal point was 19 pm. n~e thickness ofthe sample was 0.1 mm. The measured open-aperture Z scans exhibit that the Z-scan trace at the irradiance up to 0.9 GWiem' is nearly flat, indicating that no nonlinca absorption occurred in the PbS-polymer nanocompoaite at 780-nm wvvclenglh The closed-apemrc 7 scans indicate B negative sign of thc nmlincar rcfraclivc index ni for the sample By fining the 2-scan theory to the measured closed-apenure Z-scan data, n2 value for the nanwumposite is extracted to be -6.8 x 10." cm'iW, which is about 3 orders of magnitude higher than those of commercially available bulk semiconductors, such m CdS and ZnS, measured with fmioseurrud pulses ut 780-nm wavelength 121. We also urnducted Z xans on the MMA/AN/CI-PEO-R-MA-40 copolymer reference without PbS nanocryslalr, and no optical nonlineadties are measurable. This indicates that PbS nannocrystals give rise to the observed refractive nonlinearity in the PbS-polymer nanocomposite. Since the l i n w absorption rpeclrum of the sample shows that no obvious absorption occurs at 780-m wavelength. thc observed optical nonlinearity in o w Z-scan measurement is non-resonant. The obtained large p m l y refractive nonlinearity of the nanocomposite may mainly he amihuted to the Optical Stark cffmt and contribution of the surface-trapped states of the PbS ~110crystaIs. The expximenlal results suggest that PbS-polymer nanocompositr with large refcactivr nodinearity and negligible absorptive nonlinearity is potential materials far applications iii optical switching devices. References: [I] R. We used new P*O~Na*O-ZnO-AIF,-Ca*O, glass system for the Carmvtiun of PbSe and PbS Semiconductor Quantum Do6 (SQD). Thermal lreatmemt of initially colorle.~~ glasses at 390440T led to their coloring to brown and black depending OD the annealing. X-ray d i h t o m e t r y ofthe annealed glass samples pmved existenco of PbSe, PbS SQD with NaCl cubic Ermchlre in the glasses Transmission electron microscopy of the samples demonslral...