Chain growth process

The interest in hyperbranched polymers arises from the fact that they combine some features of dendrimers, for example, an increasing number of end groups and a compact structure in solution, with the ease of preparation of hn-ear polymers by means of a one-pot reaction. However, the polydispersities are usually high and their structures are less regular than those of dendrimers. Another important advantage is the extension of the concept of hyperbranched polymers towards vinyl monomers and chain growth processes, which opens unexpected possibilities. 

A chain growth process is suggested as the polymerization mechanism on the basis of studies of polymer molecular weight vs. extent of reaction. 

The reaction of AgOTf with a flexible diimine ligand in 1 2 and 1 1 ratio leads to the mono- or dinuclear complex, respectively. They further react with AgOTf to give coordination polymers. These complexes are models for the chain-growth process required in the formation of coordination polymers (see Scheme 2).642,643 Similar complexes are obtained with the 1,4-di-t-butyldia-zabut- 1,3-diene derivative.644... 

Why are the kinetics of chain growth polymerization more difficult to study than those of step growth polymerization What simplification do we use to treat the kinetics of the chain growth process How does this simplification reduce the complexity of the problem and what are the limitations of this method ... 

The surface carbide is hydrogenated in subsequent steps, which leads to an appreciable coverage of the surface by CHx(ads) intermediates. Chain growth processes are assumed to occur by radical pathways such as Eqs. V and VI. 

In the present work the unassisted CO dissociation version of the carbide theory has been adopted to describe CO conversion, while the alkyl mechanism has been adopted for describing the chain growth process. Accordingly, as proposed in our previous work,10 the FT reaction pattern has been detailed as follows ... 

N. O. Elbashir and C. B. Roberts, Enhanced Incorporation of a-Olefins in the Fischer-Tropsch synthesis Chain-Growth Process over an Alumina-Supported Cobalt Catalyst in Near-Critical and Supercritical Hexane Media, Ind. Eng. Chem. Res., 2005, 44, 505-521. 

Nearly all synthetic polymers are synthesized by the polymerization or copolymerization of different "monomers." The chain growth process may involve the addition chain reactions of unsaturated small molecules, condensation reactions, or ringopening chain-coupling processes. In conventional polymer chemistry, the synthesis of a new polymer requires the use of a new monomer. This approach is often unsatisfactory for Inorganic systems, where relatively few monomers or cyclic oligomers can be Induced to polymerize, at least under conditions that have been studied to date. The main exception to this rule is the condensation-type growth that occurs with inorganic dl-hydroxy acids. 

The early stages in the chain growth process have been mimicked by Braunstein with the use of a Pd-Me complex supported by an acetamido-derived P-O ligand. Four catalytic intermediates were intercepted by the sequential addition of CO-ethene-CO-ethene, and the occurrence of reversible and irreversible steps was established (Scheme 7.9). Unlike diphosphine ligands [10], the insertion of ethene into the y-chelate acyl complex was found to be a facile process occurring without the intervention of CO [25]. 

The migratory insertion reactions of Pd(acyl) (alkyl acrylate) and Pd(acyl) (vinyl acetate) have been investigated by several authors and valuable mechanistic information on this important elementary step of the chain growth process has been obtained by NMR spectroscopy [li, jj. Of particular relevance are recent NMR studies by Braunstein dealing with the stereochemistry of insertion of methyl acrylate into Pd-acyl bonds derived from the reaction of CO into Pd-Me or... 

For the evaluation of the SEC measurements, a second plot can be created where the obtained values o M (left coordinate, should increase linearly with conversion) and the polydispersity indices (PDI, right coordinate, should be lower than 1.3) are plotted vs. the conversion.This plot again can be used to discuss the degree of control, and the time period where control was achieved during the chain growth process.This time is in general approx. 2 h. 

The general outline of steps leading to the primary oxygenated products presented above for cobalt catalysts (a chain growth process which proceeds through aldehyde intermediates) may also apply to the rhodium system. Certainly, the same array of products is observed in both systems, although secondary reactions are evidently less predominant in most of the rhodium... 

A mechanism possibly involving intermolecular hydride transfer in this promoted ruthenium system is thus very different from the reaction pathways presented for the cobalt and unpromoted ruthenium catalysts, where the evidence supports an intramolecular hydrogen atom transfer in the formyl-producing step. Nevertheless, reactions following this step could be similar in all of these systems, since the observed products are essentially the same. Thus, a chain growth process through aldehyde intermediates, as outlined earlier, may apply to this ruthenium system also. 

There are, however, synthetic constraints on the use of anhydro sugars as a source of polysaccharides, because chain-growth processes generally lead to the formation of homopolymers only, or, if two or more monomers are used, to the formation of random copolymers. In these processes, the polymer composition and mer sequence-... 

Rare-earth oxides (La2C>3, Dy203) have been shown to be active in isosynthesis.283 284 It was also observed that zirconia may catalyze the highly selective formation of isobutylene under appropriate conditions.285 Two chain growth processes, CO insertion into aldehydic Zr-C bonds and condensation between methoxide and... 

At the Mellon Institute he applied l4C tracers to examine the behavior of intermediates in Fischer-Tropsch synthesis over iron catalysts. By adding small amounts of radioactively labeled compounds to the CO/H2 synthesis gas mixtures, he was able to prove that some of these compounds (e.g., small alcohols) are involved in the initiation step of the chain growth process that leads to larger hydrocarbon products. It was during this era that his associates first placed a catalytic reactor into the carrier gas stream of a gas chromatograph and developed the microcatalytic pulse reactor, which is now a standard piece of equipment for mechanistic studies with labeled molecules. While at Mellon Institute Emmett began editing his comprehensive set of seven volumes called Catalysis, which he continued at Hopkins. 

It must be pointed out that the only fundamental difference between the synthesis of block copolymers and that of graft copolymers consists in the selective localization of the reactive groups along the chains in the first case, these groups must be located at the ends of the chains, while for grafting they will be present as side groups. The further chain growth process is however essentially identical for both cases. 

Cyclic siloxanes can undergo a ring-opening polymerization that is a chain-growth process. Free radicals are not useful as initiator species, because of the nature of the siloxane bond, but anionic and cationic initiators are very effective. The reaction is illustrated using the most common cyclic oligomers, the trimer hexamethylcyclotrisiloxane... 

The epoxy or oxirane group is characterized by its reactivity toward both nucleophilic and electrophilic species and it is thus receptive to a wide range of reagents. Epoxy monomers polymerize through step-growth and chain-growth processes. The ionic polymerization initiated by both Lewis bases or acids will be discussed later (Sec. 2.3.4). The case of polyaddition polymerizations is mainly represented by epoxy-amine reactions. 

Manufacture of Higher Straight-Chain Alcohols by the Ethylene Chain Growth Process... 

A polymer that results from the addition reactions of alkenes, dienes, or other compounds with double and triple bonds. Most addition polymers form by a chain-growth process, (p. 1223)... 

Through readsorption, the olefin can either be hydrogenated on the catalyst surface or be reinserted into the chain growth process. Diffusion... 

Branching can resnlt from the chain-growth process or from branching gronps in the monomers. For example, ethylene can be polymerized by a radical process to a highly branched low-density polyethylene (LDPE) or copolymerized with small amounts of a-alkenes like 1-hexene or 1-octene nsing a metal-mediated catalyst the resnlt is a linear polyethylene punctuated by short-chain branches and known as linear-low-density polyethylene (LLDPE). 

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