Controlled polymerization definition

An aqueous colloidal polymeric dispersion by definition is a two-phase system comprised of a disperse phase and a dispersion medium. The disperse phase consists of spherical polymer particles, usually with an average diameter of 200-300 nm. According to their method of preparation, aqueous colloidal polymer dispersions can be divided into two categories (true) latices and pseudolatices. True latices are prepared by controlled polymerization of emulsified monomer droplets in aqueous solutions, whereas pseudolatices are prepared starting from already polymerized macromolecules using different emulsification techniques. 

The reason for this poor definition of materials is found in the process of their formation, namely difficult to control polymerization reactions. Such reactions also occur in catalytic reactions with small organic molecules. The nature of carbon deposits therefore reflect all the complexity of the bulk carbon materials One aim of this article is to describe the structural anc chemical complexity of carbon or soot in order tc provide an understanding of the frequently observec complexity of the chemical reactivity (e.g. in reac tivation processes aiming at an oxidative removal o deposits). 

The production of polyurethane involves the controlled polymerization of an isocyanate, a long-chain-backbone polyol and a shorter-chain extender or cross-linker. The reaction rates can be controlled through the use of specific catalyst compounds, well known in the industry, to provide sufficient time to pour or otherwise transfer the mix and to cure the polymer sufficiently to allow handling of the freshly demolded part. The use of blowing agents allows the formation of a definite cellular core (thus the term microcellular elastomer ) as well as a non-porous skin, producing an integral sandwich-type cross section. 

All CRP methods rely on a dynamic equihbration between tiny amounts of propagating radicals and various types of dormant species. The essence of the process is a rapid reversible deactivation of growing radicals. Radicals always terminate and therefore CRP is never living in the pure sense of the living polymerization definition. Indeed, lUPAC recommends to avoid using term Uving for the radical polymerization and suggests to use the term controlled reversible deactivation radical polymerization . 

The above two examples of the polymerization of styrene contrast the prototypical controlled and uncontrolled polymerizations. Controlled polymerizations offer simple molar mass control, the ability to define the polymer end groups, and give polymer samples with narrow molar mass distributions. Molar mass definition in uncontrolled polymerizations is more difficult, polymer end groups are determined by inherent termination (and transfer) reactions, and the molar mass distributions are typically broader. There is much contemporary interest in developing polymerization reactions that are controlled because of the precision with which macromolecules can be designed. Many chapters are dedicated to these endeavors with controlled radical polymerization receiving the most attention recently. 

In this and many other chapters in this volume as well as in other volumes of this comprehensive, expressions living polymerization and controlled polymerization are used. lUPAC and ACS definitions are given in Section 3.02.11. The expression controlled was not used in its present sense when the first edition of Comprehensive Polymer Science was published. In the past decades, there have been a number of discussions on what kind of a process could bear the corresponding names. 

The process itself has certain features and it is up to the interested researchers how to name a certain group of processes that all behave kinetically in the same way. Finally, however, lUPAC gave the definition of controlled polymerization as given below. 

More extensive discussion, however, is given in Reference 11, which also describes major criteria for botb living and controlled polymerizations. In addition to the lUPAC definitions, the AGS definitions are quoted bdow. In prindple, there is no difference between the viewpoints of these two groups of experts. 

The potentiometry sensor (ion-selective electrode) controls application for determination of polymeric surface-active substances now gets the increasing value. Potentiometry sensor controls are actively used due to simple instmment registration, a wide range of determined concentrations, and opportunity of continuous substances contents definition. That less, the ionometry application for the cation polymeric SAS analysis in a solution is limited by complexity of polycation charge determination and ion-exchanger synthesis. 

Table 8 is a comprehensive compilation of AEmv values obtained in this research together with those available from the literature. An examination of the data led to a definition of the effect of initiating system, solvent and temperature, and to a general hypothesis on molecular weight control in isobutylene polymerization. 

Control of the particle size while retaining precise control over the release rate is enabled by compartmentalization of the sol-gel solution into droplets of definite size. This can be achieved by emulsification of the sol-gel solution by mixing it with a solution composed of a surfactant and a non-polar solvent (Figure 2.13). When an active molecule is located in the aqueous droplet of a W/O emulsion, encapsulation occurs as the silicon precursors polymerize to build an oxide cage around the active species. By changing the solvent-surfactant combination, the particle size can be varied from 10 nm to 100 pm as the size of the particles is controlled by the size of the emulsion droplet, which acts as a nano-reactor for the sol-gel reaction (Figure 2.13). 

Automated polymer-based synthesis comes into its own when a stepwise polymerization is required with precise control over the addition of particular monomers in a specific sequence. This is almost a definition of peptide synthesis, Nature attaches each amino acid to a different polymer (transfer RNA) and uses a computer program (the genetic code) to assemble the polymers in the right order so that the amino acids can be joined together while bound to another polymer (a ribosome). No protection of any functional groups is necessary in this process. 

Fundamental studies have led to a detailed insight into the mechanism of the polymerization and the control of the microstructure through the substituents on the cyclopentadienyl ligands. For a survey of these studies and the dominant effects playing a role the reader is referred to recent contributions by Ewen [38] and Erker [48]. In the present chapter we will summarize a few of the features that play a role in determining the microstructure the field is still in development and obviously the definite answer to several questions has not yet been given. 

Polymerization of vinyl ethers (VE) has been the subject of a considerable amount of theoretical studies. These monomers can be polymerized through radical initiation but the reaction is very slow and leads only to oligomers. Cationic polymerization initiated by a wide variety of Lewis acids is much more efficient and definitely preferred for homopolymer synthesis. Detailed theoretical aspects, and particularly recent developments concerning the controlled/living cationic polymerization of these monomers, have been discussed as well in previous exhaustive review [1,13,98,99] as in the present book (Chapters 4 and 5), and they will no longer be considered here. 

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