We report the synthesis of model highly branched methacrylic copolymers by copolymerizing methyl methacrylate (MMA) with a disulfide-based dimethacrylate (DSDMA) branching comonomer via reversible addition-fragmentation chain transfer (RAFT) in toluene at 90°C using 1,1 0 -azobiscyclohexanecarbonitrile initiator and a cumyl dithiobenzoate (CDB) chain transfer agent. Selective cleavage of the disulfide bonds in the DSDMA branching comonomer using tributylphosphine leads to the formation of low polydispersity primary chains, as judged by gel permeation chromatography. The molecular weight distribution of these degraded chains is comparable to a RAFT-synthesized linear poly(methyl methacrylate) homopolymer prepared in the absence of any DSDMA brancher. This confirms that good control over the RAFT copolymerization is achieved under branching conditions and that the polydisperse highly branched chains simply comprise randomly coupled near-monodisperse primary chains, as expected. Moreover, HPLC analysis of the copolymerizing solution confirms that the consumption of DSDMA comonomer is close to that expected for a statistical copolymerization. The CDB efficiency is estimated to be 90% by GPC and 1 H NMR spectroscopy. Taking this into account and allowing for the incomplete comonomer conversions (typically 96-97%), our systematic variation of the proportion of DSDMA per primary chain indicates that this RAFT formulation conforms closely to classical Flory-Stockmayer theory. This near-ideal behavior is in marked contrast with earlier literature reports of strongly nonideal behavior, presumably because of significant levels of intramolecular cyclization. Our hypothesis is that this unwanted side reaction, which consumes the DSDMA brancher without leading to intermolecular branching, is suppressed in the present study because of the relatively high comonomer concentration (50% w/w) used in our RAFT syntheses.

The statistical copolymerization of methyl methacrylate (MMA) with varying amounts of a disulfide-based dimethacrylate (DSDMA) branching comonomer in toluene at 90°C can lead to highly branched soluble methacrylic copolymers under appropriate conditions. This model system is utilized in order to examine the following points: (i) the relative merits of using reversible addition-fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP) in such syntheses; (ii) the effect of varying the number of DSDMA units per primary chain; (iii) the effect of varying the initial monomer concentration. Kinetic studies of the linear RAFT and ATRP homopolymerizations conducted in the absence of any DSDMA confirmed their living character at 10, 30, and 50 wt % [MMA] 0 , where the former monomer concentration approximately corresponds to the critical overlap concentration, c*, for linear poly(methyl methacrylate) (PMMA) chains with a mean degree of polymerization of 50. HPLC analysis of the monovinyl and divinyl comonomers confirms that there is statistical incorporation of the DSDMA brancher into the growing poly(methyl methacrylate) chains. Cleavage of both RAFT-and ATRPsynthesized branched copolymers prepared at 50 wt % [MMA] 0 using tributylphosphine affords linear primary chains with narrow molecular weight distributions; thus these retro-syntheses demonstrate the retention of living character under branching conditions and suggest little or no chain transfer to polymer. In principle, macroscopic gelation can be avoided provided that the number of fully reacted divinyl branching comonomers per primary chain is less than unity. Taking into account the respective efficiencies of the RAFT chain transfer agent and the ATRP initiator, this hypothesis holds for both ATRP and RAFT branching copolymerizations conducted in the presence of DSDMA at 50 wt % [MMA] 0 but fails at 10 wt % [MMA] 0 . Thus, soluble branched copolymers can be prepared at 10 wt % [MMA] 0 containing up to five fully reacted DSDMA units per primary chain using RAFT chemistry and up to three fully reacted DSDMA units per primary chain with the ATRP formulation; no gelation is observed even when the overall conversion of vinyl groups exceeds 96%. These observations strongly suggest that intramolecular cyclization is prevalent at this lower monomer concentration, regardless of the precise nature of the polymerization chemistry. In contrast, intermolecular branching between primary chains is evidently favored at 50 wt % [MMA] 0 , since this concentration substantially exceeds c*. In summary, although there are no doubt some subtle differences between branched copolymers synthesized via RAFT and ATRP chemistry, physical factors are arguably much more important than the precise nature of the living radical polymerization chemistry; in particular, systematic variation of the monomer concentration clearly leads to fundamentally different behavior.

A route to the doubly unsaturated bridgehead sultam 12 has been developed. When irradiated at 350 nm, this conjugated diene is isomerized via a two-photon process to the structurally novel spiro heterocycle 17 constituted of cyclobutene, thietane dioxide, and pyrrolidine rings. A probable mechanism for the generation of 17 and select reactions thereof are reported. [reaction: see text]

An analytical friction model is presented, predicting the coefficient of friction in elastohydrodynamic (EHD) contacts. Three fully formulated SAE 75W-90 axle lubricants are examined. The effect of inlet shear heating (ISH) and starvation is accounted for in the developed friction model. The film thickness and the predicted friction are compared with experimental measurements obtained through optical interferometry and use of a mini traction machine. The results indicate the significant contribution of ISH and starvation on both the film thickness and coefficient of friction. A strong interaction between those two phenomena is also demonstrated, along with their individual and combined contribution on the EHD friction.

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