The Comprehensive Closed-Loop Model

The model is closed-loop as it accounts for all the recoverable products formed from all the starting materials (i.e., from the initiators HX or RX, the coinitiator MtXn, and the monomer M). 

The model is based on the proposition that all carbocationic polymerizations of olefins, alkyl vinyl ethers, etc., involve a spectrum of species with differing ionicitieS connected by equilibria formally expressed by the Winstein ionicity spectrum 1] the Winstein spectrum starts with a covalent species and progresses through increasingly polarized and ionized species, to fully ionized solvated ( free ) ion pairs  

For the sake of simplicity the large number of possible entities in the Winstein spectrum can be reduced to three representative species [1, 24]  

First we will consider the events indicated on the right side of the scheme, the part which concerns polymerizations induced by ubiquitous protic impurities 

The last entity in the first line on the right shows the species H8 — 6  

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Experiments have been carried out to substantiate the comprehensive closed-loop model described in Sect. 2. The experiments included the determination of kc and kp under various conditions including a study of the effect of temperature on these parameters, an examination of the effect of experimental conditions on M and Mw/M together with an analysis of the GPC traces obtained. The raw data are shown in Tables 2-8 in the Appendix. 

Regarding the comprehensive closed-loop model, a new methodology was developed to simulate kinetic responses of real-life carbocationic polymerizations to various events or combinations of events. Basic scenarios were set up to analyze numerically the above events and to demonstrate their effects on N and Mn vs Wp, and — ln(l — C) and C vs time diagnostic plots, respectively. 

A closed-loop comprehensive model uniting impurity-induced and purposely-added initiator-induced isobutylene (IB) and styrene (St) polymerizations was developed. Both impurity-induced and purposely-induced olefin polymerizations can be both conventional or living, and the reaction conditions will determine whether the prevailing mechanism will be conventional or living. The model was used to elucidate the detailed mechanism of olefin polymerizations and to provide guidance toward preparative advances. The heart of the model is the Winstein tonicity spectrum which in its simplest form consists of three fundamental entities connected by two equilibria a) a dormant species (in fact the initiator) which can be either a protic impurity or a purposefully added... 

Under the leadership of Kathy Boardman, the Core writing program conducted a comprehensive portfolio assessment of the English 102 course in 2000, with a follow-up study in 2001 that focused on a few features that the initial study had revealed as possible areas for improvement in the curriculum. This was well in advance of our accrediting body s new interest in assessment, and Kathy s closing of the loop by changing instructor preparation and inservice training provided an impressive model of effective assessment practice. 

Building a complete inventory of plant proteins and metabolites remains a major challenge. It is now obvious that even in a model plant such as Arabidopsis this inventory is very far from comprehensive, and that only the most abundant/extractable constituents have been described. Due to an additional level of complexity, the complete description of the main plant polymers will take several decades. The last step for closing the loop will be to better understand how these small metabolites impact gene and protein expression/activity, and plant homeostasis. 

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