The Start Step

The actual polymerization-starting species reacts with the monomer in the start step. Two cases can be distinguished transfer of two electrons with the formation of a bond between the polymerization starter and the monomer molecule, and transfer of one electron without formation of a bond. 

In the two-electron mechanism, the starting species is always joined to the monomer unit. This is not a transfer reaction in terms of macromolec-ular terminology, where the concept of transfer is reserved for a reaction where an active species (cation, anion, radical) reacts with another molecule with the formation of an active species from this molecule (see below). Two-electron mechanisms can be subdivided into three subgroups  

An anion adds on to a monomer molecule and thereby forms a monomer anion by this electrophilic reaction. An example is the styrene polymerization started by potassium amyl  

A cation adds on to a monomer molecule and forms a monomer cation by this nucleophilic reaction. An example of this is the isobutene polymerization started by boron trifluoride/water, HPF3OH], where H is the actual initiator  

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The starting step of the present work is a specific analysis of the solution of the Schrodinger equation for atoms (section 1). The successive steps for the application of this analysis to molecules are presented in the section 2 (description of the optimised orbitals near of the nuclei), 3 (description of the orbitals outside the molecule), and 4 (numerical test in the case of H ). The study of other molecules will be presented elsewhere. 

The oxidative polymerization of 5,6-dihydroxyindole (1) and related tyrosine-derived metabolites is a central, most elusive process in the biosynthesis of eumelanins, which are the characteristic pigments responsible for the dark color of human skin, hair, and eyes. Despite the intense experimental research for more than a century,36 the eumelanin structure remains uncharacterized because of the lack of defined physicochemical properties and the low solubility, which often prevents successful investigations by modem spectroscopic techniques. The starting step of the oxidative process is a one-electron oxidation of 5,6-dihydroxyindole generating the semiquinone 1-SQ (Scheme 2.7). 

Another cationic-cationic domino reaction which follows the way of biosynthesis is used by Koert et al for the synthesis of the whole left part of the polyether etheromydn.1141 The starting step is an add catalyzed cyclization of a diepoxide to give a bistetrahydrofuran with creation of three new oxa-cycles in over 30 % yidd. 

Among the well known polymers, the ozonolysis of PVC was subject to a great number of studies, especially by A. Michel et al. [113-115]. These authors showed that the sensitivity of this polymer depends closely at first on the temperature, and also on the structure of the starting polymer, and thus, on its way of synthesis. So, the presence of double bonds on the polymer backbone constitutes a weak point which will constitute the starting step of the degradation leading to a very quick decrease in molecular weights. 

For the synthesis of an antibacterial clerodane 262, ozonolysis of a substituted optically pure (-)-2-decalone 261 with (57 ,97 ,1070-configuration was the starting step (Scheme 80) <1995J(P1)757>. 

In this respect results for ethanol are more reliable. According to Iwasita and Pastor, the starting steps of ethanol adsorption are as follows ... 

Bromine-lithium interconversion reactions of 3,4-dibromoselenophene were the starting steps for... 

Termination and transfer reactions are absent in living copolymerizations. All active species, once formed, remain active over the whole of the polymerization time. If, in addition, the starting step is very fast compared to the propagation step, then all initiator molecules are immediately converted into active species. Thus, in such a case, the sum of the active species concentrations is constant with time and equal to the original initiator concentration ... 

Another complication occurs for greatly differing polarities of monomers and macroions. For example, macroion Mf only adds on monomer B slowly, but macroion M f does not add on monomer A. Thus, all incorporated monomer A must have reacted in the start step with initiator I and in the immediately following propagation steps. Thus, the increase in macroion concentration for living polymerizations is given by... 

According to Section 18.1.2.2, the starting step of an anionic polymerization can occur via addition of an anion onto a monomer, formation of a zwitterion, or electron transfer to a monomer. Whether a specific initiator will start polymerization in a given monomer depends primarily on the basicity of the monomer and initiator. Monomers with strong accepting groups need only weak bases as initiators. In olefin derivatives CH2=CHR, therefore, the tendency toward anionic polymerization falls in the sequence of the substituents R ... 

In many cases, the polarities of the monomers or the macroions are very different for example, in the anionic copolymerization of styrene (1) and methyl methacrylate (2). A styryl anion can add methyl methacrylate therefore /ci2 7 0 but styrene will not add on to the methyl methacrylate anion (/c2i = 0). All the styrene incorporated into the polymer must therefore come from the starting step and the propagation step immediately following it. Thus, for the unipolymerization. 

The simplest chain structure -f-CH2—S- occurs through the polymerization of thioformaldehyde, CH2S, or its cyclic trimer (trithiane). Aliphatic polysulfides with two or more carbon atoms per monomeric unit are available through the polymerization of cyclic sulfides. The start step with organolithium compounds deviates from that of the epoxides in that initially the sulfur atom is attacked and subsequently the carbon atom is attacked. For example, the starting step with propylene sulfide and ethyl lithium initially gives propylene and lithium ethane thiolate ... 

The development of such a craze-like deformation band is shown in Fig. 6.3 in SEM in a sequence from an in situ deformation test of HDPE filled with 28 wt.% of about 1 pm AI2O3 particles [9]. The starting step is phase separation and void formation at the larger filler particles and agglomerates (a). Thin matrix strands between closely connected voids are plastically stretched and transformed into long fibrils (b). With increasing strain, craze-like deformation bands appear in the sample (c). Under an optimum balance of size and volume content of particles, as well as of matrix ductility, a remarkable Increase In toughness of such composites can be realized [9,10]. 

It is also the starting step for the cold process for the manufacmrer of sulphuric acid, liquid sulphur trioxide, oleum under high pressure conversion with zero emission of sulphur dioxide. 

Before you can solve any stoichiometry problem, you must have the reaction equation and the conversion factors between moles and quantities of Given and Wanted substances. For convenience, we will use the expression starting steps to describe these items. Thus, complete the starting steps means to write and balance the equation, if it is not given, and determine the conversion factors. Molar mass is the conversion factor in this section others will appear later. 

After you complete the starting steps, the solution of a stoichiometry problem usually falls into a three-step mass-to-mass path. The mass-to-mass path is... 

First, we complete the starting steps for all stoichiometry problems. We write the reaction equation and calculate the molar mass or other conversion relationships between moles and the measured quantities of the substances in the problem. In this example the equation is given. We must calculate the molar masses of the given and wanted substances. They are C2Hg 2(12.01 g/mol) + 6(1.008 g/mol) = 30.07 g/mol O2. 2(16.00 g/mol) = 32.00 g/mol. The Plan for the problem is... 

Complete the starting steps. Be sure to read the problem carefully. 

Now we are ready to use the mass-to-mass path to find the amount of MgO that is required to produce 984 kg Mg(OH)2. The starting steps require the molar masses of Mg(OH)2 and MgO as conversion factors. Once these are calculated, the Plan and calculation follow ... 

The starting steps will help you Plan your strategy for solving this problem. Our Plan will be for a single dimensional-analysis setup from the given quantity to the wanted quantity. 

This new model is indeed a very old one (Hogan, 1956) [5] and includes a simple formulation of the starting step of the polymerization cycle yielding an active species with Cr(lV),... 

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