Polymerization step-wise

The basic sol-gel reaction can be viewed as a two-step network-forming polymerization process. Initially a metal alkoxide (usually TEOS, Si(OCIl2CH )4) is hydrolyzed generating ethanol and several metal hydroxide species depending on the reaction conditions. These metal hydroxides then undergo a step-wise polycondensation forming a three-dimensional network in the process. The implication here is that the two reactions, hydrolysis and condensation, occur in succession this is not necessarily true (8.9). Depending on the type of catalyst and the experimental conditions used, these reactions typically occur simultaneously and in fact may show some reversibility. 

A wide variety of monomers, such as (3,5-dibromophenyl)boronic acid, 3,5-bis(trimethylsiloxy)benzoyl chloride, 3,5-diacetoxybenzoic acid, and 2,2-dimethylol propionic acid have been used for the synthesis of hyperbranched polymers. A selection of these polymers are described in Sect. 3. The majority of the polymers are synthesized via step-wise polymerizations where A B monomers are bulk-polymerized in the presence of a suitable catalyst, typically an acid or a transesterification reagent. To accomphsh a satisfactory conversion, the low molecular weight condensation product formed during the reaction has to be removed. This is most often achieved by a flow of argon or by reducing the pressure in the reaction flask. The resulting polymer is usually used without any purification or, in some cases, after precipitation of the dissolved reaction mixture into a non-solvent. 

The data here related on the kinetics of the propylene polymerization and of the transfer processes and the studies of the catalysts carried out with C-labelled alkylaluminums, derive from a series of researches mostly carried out some time ago, when the knowledge of the mechanism of the considered catalytic processes was still rather limited. Nevertheless, it helped remarkably to know these new processes of anionic coordinated polymerization their true catalytic nature (which regard to a-TiCU) differentiates them from the more usual polymerization processes (radicalic) which, actually, are not catalytic. They substantially contributed to demonstrate that the anionic coordinated polymerization is a step-wise addition process in which each monomeric unit inserts itself into a metal carbon bond of the catalytic complex. 

Recently, efforts have been made to produce calibration standards of higher molecular weight that are chemically similar to lignins, by step-wise syntheses (12), anion-initiated polymerization of quinonemethides (13), and preparative HPSEC of acetylated lignins (14). Knowledge of the molecular weights of these materials is either built into the method of preparation or determined by absolute methods such as sedimentation equilibrium measurements. 

That s our quick and dirty look at step-growth polymerization. The crucial feature of this type of reaction is the slow build-up of chains in a step-wise process. Now let s take a closer look at addition polymerization. 

Previously, a zigzag-type reaction mechanism was proposed for the diacetylene polymerization [55] and the step-wise [2 + 2] photopolymerization of diolefin compounds [80], which have a large stacking angle (i.e., a small tilt angle) in the columnar structure of the monomers. However, we can conclude that the polymerization of the muconates proceeds via a domino-type polymerization mechanism, irrespective of the shrinking and expanding polymerizations. 

Chain-growth polymerization involves the sequential step-wise addition of monomer to a growing chain. Usually, the monomer is unsaturated, almost always a derivative of ethene, and most commonly vinylic, that is, a monosubstituted ethane, 1 particularly where the growing chain is a free radical. For such monomers, the polymerization process is classified by the way in which polymerization is initiated and thus the nature of the propagating chain, namely anionic, cationic, or free radical polymerization by coordination catalyst is generally considered separately as the nature of the growing chain-end may be less clear and coordination may bring about a substantial level of control not possible with other methods. ... 

While main-chain type liquid crystalline polymers are made by step-wise polymerizations, side-group type polymers have been synthesized by both step and chain polymerizations of monomers with desired mesogenic side groups. In addition, mesogenic side groups can be introduced to polymers... 

Polymerization reactions may be divided into two major categories stepwise processes and chain-type processes. In the step-wise process, reactants are brought together and heated. Initially short chains are formed and only at the end of the reaction are long chains formed. Reactions generally require hours to form the polymers. It is by this process that condensation polymers are generally made. 

The deposition of Cr on two fluorinated poly(aryl ether) (rPAE) polymers has been investigated with x-ray photoelectron spectroscopy. Fluorine moieties were observed to be highly reactive towards the deposited Cr. Differences in polymeric fluorine chemistry (aliphatic vs. aromatic) did not affect the reaction pathway or the final reaction products. Interfacial deposition products form in a step-wise fashion dependent upon sietal coverage. A model ie proposed whereby the formation of reaction products is initiated by electron transfer from the metal to tha polymer followed by the formation of Cr-fluorldes and finally Cr-carbldes prior to the formation of a continuous unreacted metal overlayer. 

The chain-reaction polymerizations in Sects. 3.2.4 and 3.2.5 are methods that can be used to make narrower molar mass distributions than are possible with step reactions or chain reactions with random termination. This problem of producing pure polymers was mentioned above when discussing the step-wise reaction in Sect 3.1. 

The polymerization proceeds in a step-wise manner with the initial formation of dimers, trimers, tetramers, etc. In equimolar concentrations of the diol and diacids and in the absence of any exogenous catalysts, the polymerization is found to catalyzed by the diacid itself. Under these conditions, the rate of polymerization can be given as [3, 4] ... 

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