To obviate the destructive interaction of highly reactive metallocene catalysts with classical silica-based supports while retaining the advantage of supported catalysts, a noninteracting polystyrene support was developed. Supported catalysts for the polymerization of alpha-olefins are prepared by treating lightly cross-linked, chloromethylated polystyrene beads consecutively with a secondary amine, an ammonium salt of a weakly coordinating anion, and a neutral dialkylmetallocene. Catalytic sites are distributed homogeneously throughout the support particle, and the polymerization occurs within the bead, in contrast to traditional surface-supported metallocene catalysts. The copolymerization of ethylene and 1-hexene at 40 degreesC affords discrete spherical polyolefin beads with a size (0.3 to 1.4 millimeters) that varies according to the polymerization time.

The self-assembling adsorption/quaternization of a (dialky1amino)stilbazole nonlinear optical chromophore precursor onto benzyl chlorideprefunctionalized surfaces is studied in situ by polarized second harmonic generation techniques. The measurements reveal that the self-assembly kinetics are approximately biexponential ( I x 2 -(1 --a l k z t ) ) with a rapid initial phase, followed by a slower second phase. The in situ measurements also indicate that the average tilt angle of the chromophore dipole moment direction with respect to the surface normal increases significantly ( -20" -42O) in the initial phases of chromophore adsorption and then remains nearly constant. These results suggest substantial chromophor-hromophore electrostatic and steric repulsion as monolayer growth proceeds. The maximum chromophore coverage observed corresponds to -49 A2/ molecule and x(2) = 3 X lO-' esu at X = 1064 nm.

The progress and extent of nucleophilic substitution and ion exchange reactions of self-assembled chromophoric monolayers are studied by X-ray photoelectron (XPS) and second harmonic generation (SHG) spectroscopy. Self-assembled monolayers prepared from 2-[4-(chloromethyl)phenyl]ethyl trichlorosilane (1) on glass substrates are susceptible to nucleophilic substitution of ∼90% of the surface-confined benzylic chloride functionalities with the “hypernucleophile” 4-(dimethylamino)pyridine; however, only ∼60% of the densely packed benzyl chloride groups undergo reaction with the high-β chromophore precursor 4‘-[4-[N,N-bis(3-hydroxypropyl)amino]styryl]pyridine (2a). Quaternization of a benzylic monolayer with this molecule yields a monolayer having a bulk second-order NLO response (χ(2)) of 3 × 10-7 esu at λ0 = 1064 nm, corresponding to a near-maximum chromophore coverage of ∼2 × 1014 molecules/cm2. The kinetics of this substitution reaction and associated structural modifications are studied in real time by in situ polarized SHG techniques, which reveal non-Langmuirian kinetics and a rapidly increasing chromophore tilt angle with increasing coverage. The quaternization kinetics can be fit to a phenomenological biexponential rate equation with k‘1 ≈ 2 × 10-2 L mol-1 s-1 and k‘2 ≈ 2 × 10-3 L mol-1 s-1 and to a coverage-dependent activation energy model (E A = E 0 + E bθ), yielding a perturbative energy E b of 6−8 kJ mol-1. Both models are compatible with increasing repulsive interactions between chromophores at high coverages. The charge-compensating chloride counterions within monolayers having dense chromophore packing can be ion exchanged with iodide, up to a maximum of ∼40% of available chloride ions. The introduction of larger anions (sulfanilate, ethyl orange, eosin B) is observed in less densely packed films; however, the ion exchange process is completely inhibited in monolayers capped with a siloxane overlayer. In all cases, exchange of the chloride leads to significant increases in the second-harmonic generation efficiency, up to 45% on exchange with eosin B. In the case of iodide and sulfanilate substitution for chloride, the increase in the second-order response upon ion exchange is attributable to the incoming anion assuming a position within the monolayer microstructure different from that of the displaced anion.

The communication describes an approach to the synthesis of thin-film second-order nonlinear optical (NLO) materials in which self-assembled chromophore-containing multilayer structures are built up on clean glass or quartz surfaces in the repeating reaction sequence p-ClsSUCH^CelLCHsCl (coupling layer), 4-[lV,N-bis(3-hydroxypropyl)amino]phenylethynyl-4'-pyridine (chromophore layer), CI3SÍOSÍCI2OSÍCI3 (capping layer). The multilayer structures have been characterized by optical and X-ray photoelectron spectroscopy, advancing contact angle measurements, and polarized second harmonic generation (SHG) properties. It is found that the initially formed coupling-chromophore monolayer exhibits a rather weak SHG response which increases substantially upon capping with the Si reagent. The proposed structural reorganization of the chromophore dipoles (j8mz direction) to an average orientation more coincident with the surface normal is supported by optical spectroscopy. The square root of the SH intensity scales linearly with the number of chromophore layers up to nine layers. For a single coupling-chromophore layer, ® = 6 X 10~8 esu at hu = 1.17 eV.Challenges in the construction of efficient moleculebased thin film second-order nonlinear optical (NLO) materials1 include maximizing the number density of constituent high-d chromophores as well as achieving and preserving maximum microstructural acentricity. We recently reported an approach to such materials2•3 which employs established silicon self-assembly technology4 as well as a bifunctional precursor which is transformed into(1) (a) Materials for Nonlinear Optics: Chemical Perspectives; M arder,