Initial placement offer

For the majority of developers of a new technology or system, and in particular those outside the mainstream of large corporations, a crucial element of the development plan becomes that of who will pay for itV With development capital in short supply and the venture capital (VC) market not necessarily willing to invest where there is a limited long-term potential for an initial placement offer (IPO) exit to the stock market and relatively low capital needs, the developer must be aware of the financial market... 

The Cossee-Arlman mechanism as originally proposed has a weakness—the back-flip is required to explain isoselective placement since the two active (coordination) sites are assumed to be enantiotopic. However, the structure of the traditional Ziegler-Natta heterogeneous initiators is not sufficently understood to either support or reject the assumption of enantiotopic sites. Further, even if the sites are enantiotopic, there is no overwhelming reason why the polymer chain is more stable at one site than the other—which is the rationale for the back-flip. The mechanism of isoselectivity with various metallocene initiators is much better understood since these are initiators whose molecular structures are well-established [Busico and Cipullo, 2001 Busico et al., 1997, 1999 Cavallo et al., 1998 Ewen, 1999 Rappe et al., 2000 Resconi et al., 2000], Considerable advancements in understanding heterogeneous Ziegler-Natta initiators occur if one assumes that the active sites in these initiators mimic those in metallocene initiators. Two types of metallocene initiators offer possible models... 

Several mechanisms were offered to explain steric control in polymerizations of polar monomers. Furukawa and co-workers based their mechanism on infrared spectroscopy data of interactions between the cations and the growing polymeric chains in polymerizations of methyl methacrylate and methacrylonitrile. They observed a correlation between the tacticities of the growing molecules and the carbonyl stretching frequencies. The higher the frequency, the higher is the amount of isotactic placement in the resultant chains. The adducts, as in the initiation reactions, are resonance hybrids of two structures, A and B ... 

Flatbed printers might offer the closest approximation of printing on paper, as the operator can smooth the cloth out initially with sometimes a gentle vacuum from beneath the bed to hold the piece in place. Patterns can be printed along the length or as a placement print in a specific spot. This system works best when sections... 

Various mechanisms were proposed to explain steric placement in cationic polymerization. Most of them pertain to vinyl ethers. There is no general agreement. Some of the suggested mechanisms are discussed in this section. Most were offered for homogeneous conditions with soluble initiators like BF3-0(C2H5)2. There are, however, also some explanations of steric control with insoluble catalysts, like A1(S04)2 H2SO4. 

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