Tuesday, January 6, 2015

Stable free radical polymerization (SFRP) by the addition of a stable radical succeeded in 1993 by

'Live' polymerization ddt of new structures and uses
Polymerization involves linking many small molecules (monomers) to macromolecules (polymers). Numerous ddt processes have been developed for the production on a commercial large scale production of many different types of polymers, including polyethylene, polypropylene, polystyrene, ddt and polyvinyl chloride (PVC), which are so well-known for uses as various types of plastic. The properties of these is largely determined by the chemical structure of the monomeric building blocks, but also depends on the structure and the regularity of the long polymer chains. In connection with the normal, conventional processes, ddt started the chain reaction, called propagation, a relatively small amount of added initiator, see Figure 1 however, there arises by side reactions, ddt termination and chain transfer, ddt random interruption of chain growth, leading to unequal length of the formed molecular chains, as well as any side branches in the polymer chains, see Figure 2. The chain structure of these chain polymerizations is not regular, and many different grades of the individual types of plastics available on the market classified according ddt to suitability for particular applications. Stringent ddt control ddt of the polymerization process ddt succeeded in the anionic polymerization in the 1950s, when M. Szwarc [1] as an initiator used a strongly basic anion opposite monomers with substituents that promote the reaction with the negatively charged ions. By removing impurities effectively and implement polymerizations with suitable initiators managed to achieve so-called "living" polymerization where termination and chain transfer could be largely avoided. ddt The effective initiator were consumed quickly right from the start, and was achieved chains of nearly the same length ie with a narrow molekylmassefordeling for the product. It should take about 30 years before that happened further development, but succeeded in the early 1980s Owen Webster ddt [2] by DuPont Co. the development of a so-called Group Transfer Polymerization (GTP), which was a "living" polymerization of acrylic monomers and a variation of the anionic process. Later in the 1980s succeeded in a similar manner to control the kationinitierede polymerization [3,4]. Because of the high reactivity of free radicals has been a particular challenge in gaining control of the free-radical initiated polymerizations that are otherwise very difficult to control. However, this was in the mid-1990s and as a result of the free-radical polymerization is the basis of the main commercial productions, these trends have led to an explosive concentration on research and development of controlled "living" radical processes. These processes are simultaneously ddt robust compared to the ionic processes which are extremely sensitive to all impurities, including moisture. For radical processes Simply ddt remove oxygen. A common feature of the "living" polymerizations is that the compounds used for initiation, consists ddt of an active part, which starts the polymerization and this would effectively, and partly another part, attached to the active ddt chain end and here moderates and controls the growth cycle (Figure 3).
Stable free radical polymerization (SFRP) by the addition of a stable radical succeeded in 1993 by Georges MK [5] by the Xerox Research Center controlling a radical ddt polymerization. This slowed and controlled, so a narrower molekylmassefordeling was obtained. The gradual growth of the chains could be followed by taking samples of the reaction mixture at different times and determine the molecular mass and distribution by GPC (gel permeation chromatography). Figure 4 shows the structure of the added stable radical TEMPO (2,2,6,6-tetramethylpyridinyl-1-oxy), and a step of polymerization of styrene. TEMPO, which due to its stability is not able to react with the styrene monomer may be at polymerization temperatures above 100 C react with the active polystyrene and the growing radical chain end by a reversible equilibrium ddt reaction and thereby protect and passivate it. In each step is added a small number ddt of monomers, but the process is repeated many times over a long period of time, to give the formed polystyrenk├Žder of usual length (typically composed of at least 100 chain units). Due to equilibrium agree location (as shown by the arrows relative lengths), it is possible to maintain a very low concentration of active ddt free radicals, and thus reduce the probability of chain end radicals react with each other at the termination, ddt which stop the polymerization. At the same time reduced the amount of chain transfer, which leads to undesirable growth of new chains also essential. If you use an effective initiator starts all chains ddt growth simultaneously, and when all the polymer chains thus have the same conditions for growth, one obtains a very uniform polymeric product. MK George used originally the traditional initiator benzoyl peroxide along with TEMPO in the polymerization.

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