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Abstract It is generally assumed that hexavalent chromium in wood treated with chromated copper arsenate (CCA) preservative is substantially reduced to trivalent chromium during fixation and that the potential to [...]

P.A. COOPER [*] ABSTRACT Evaluation of residual preservative levels in poles removed from service provides useful information on in-service leaching behavior, re-use options, and the need for in-service remedial treatment. [...]

P.A. COOPER [*] ABSTRACT It is known that organic acids are effective at extracting chromated copper arsenate (CCA) components from wood, so the effect of natural humic acids on leaching [...]

A variety of monocyclopentadienyl and mono(pentamethylcyclopentadienyl) complexes of titanium, zirconium, and hafnium are assessed for abilities to initiate polymerization of ethylene, styrene, and, in part, α-methylstyrene. In general, little or no activity was found for either neutral species of the types CpMMe3 and CpMMe2OR or for cationic 12- and 14-electron species of the types [CpMR2L]⁺ and [CpMR2L2]⁺, respectively (Cp = η⁵-cyclopentadienyl; R = alkyl; L = amine, phosphine ligands). In contrast, much better olefin polymerization initiators result from abstraction of a methyl carbanion from Cp*MMe3 (Cp* = η⁵-pentamethylcyclopentadienyl) by B(C6F5)3, a reaction which gives cationic, 10-electron species of the type “[Cp*MMe2][BMe(C6F5)3]”. Of these, the complex [Cp*TiMe2][BMe(C6F5)3] (A) is an excellent initiator or initiator precursor for the polymerization of ethylene and styrene, resulting in high yields respectively of high molecular weight polyethylene and atactic (a-PS) and/or syndiotactic polystyrene (s-PS), depending on conditions; the tacticity of purified s-PS, as judged by ¹³C{¹H} NMR spectroscopy, approaches 100%. While the polymerization of ethylene probably involves a classical Ziegler−Natta process, polymerization of styrene to s-PS and a-PS apparently involves respectively a Ziegler−Natta process and carbocationic initiation. High yields of essentially syndiotactic poly(α-methylstyrene) are obtained by utilizing the same initiator system, also apparently via a carbocationic process.

The compounds Cp*TiMe2C6F5, Cp*TiMe2OC6F5, and Cp*TiMe2Cl (Cp* = η⁵-C5Me5) react with the borane B(C6F5)3 to form the thermally unstable, chiral complexes Cp*TiMe(C6F5)(μ-Me)B(C6F5)3, Cp*TiMe(OC6F5)(μ-Me)B(C6F5)3, and Cp*TiMeCl(μ-Me)B(C6F5)3, respectively, which are similar to the known Cp*TiMe2(μ-Me)B(C6F5)3. All four μ-Me compounds behave as sources of the highly electrophilic species [Cp*TiMeE]⁺ (E = Me, Cl, C6F5, OC6F5) when treated with the borane, the last three being chiral, and all four systems exhibit catalytic activities for the polymerization of ethylene to high-molecular-weight polyethylene. Despite the chirality at titanium of three of the compounds, polymerization of propylene by all of them results in the formation of atactic, elastomeric polypropylene. NMR analyses of the propylene polymers formed show that initiation involves 1,2-insertion into a Ti−Me bond, and while propagation involves primarily head-to-tail 1,2-insertions, an unusually high (by metallocene standards) proportion of the insertions also involves 2,1-misinsertions but essentially no 1,3-enchainment. The major olefinic end groups are vinylidene (CH2&dbd;CMe−), resulting from β-hydrogen transfer following a 1,2-insertion, and vinyl (CH2&dbd;CH−), resulting from β-hydrogen transfer from the methyl group following a 2,1-insertion or, more likely, β-methyl transfer following a 1,2-insertion. Small amounts of internal olefins are also formed via β-hydrogen transfer following a 2,1-insertion. An EPR study of the Cp*TiMe3/B(C6F5)3 system in toluene indicates that <0.01% of the titanium is occasionally present during polymerization as a complex of titanium(III), suggesting that a contribution to catalysis by titanium(III) species is unlikely.

Treatment of the neutral trimethyl compounds Cp*MMe3 (M = Ti, Zr, Hf) with the highly electrophilic borane B(C6F5)3 in methylene chloride in the presence of various arenes results in methyl carbanion abstraction and coordination of the arene to form complexes of the type [Cp*MMe2(η⁶-arene)][MeB(C6F5)3] (M = Ti, Zr, Hf; arene = benzene, toluene, m- and p-xylene, anisole, styrene, mesitylene). Indications of relative metal−arene affinities have been gleaned from a variety of low-temperature ¹H NMR experiments, and it seems that both steric and electronic factors play significant roles in determining stabilities. The crystal and molecular structures of [Cp*HfMe2(η⁶-toluene)][MeB(C6F5)3] have been determined; as anticipated, there are no significant contacts between anion and cation, and the latter assumes a bent-metallocene type of structure.

The present work focused on the buckling of composite plates subjected to biaxial compression using classical laminated plate theory (CLPT). The two composite plates are bonded by an elastic medium. Classical laminated plate theory (CLPT) is utilized for deriving the governing equations. Difference between 4-ply symmetric lamination scheme with different material properties and fiber orientation angle (0deg and 30deg) is shown. The study show that the mechanical properties of the composite materials depending on the fiber orientation angle and lamina positions.