Polymerization chain/addition

Low-density polyethylene (LDPE) is produced under high pressure in the presence of a free radical initiator. As with many free radical chain addition polymerizations, the polymer is highly branched. It has a lower crystallinity compared to HDPE due to its lower capability of packing. 

Major fiber-making polymers are those of polyesters, polyamides (nylons), polyacrylics, and polyolefins. Polyesters and polyamides are produced by step polymerization reactions, while polyacrylics and polyolefins are synthesized by chain-addition polymerization. 

Thermal Rimaway in Chain-Addition Polymerizations and Copolymerizations... 

The objectives of this presentation are to discuss the general behavior of non isothermal chain-addition polymerizations and copolymerizations and to propose dimensionless criteria for estimating non isothermal reactor performance, in particular thermal runaway and instability, and its effect upon polymer properties. Most of the results presented are based upon work (i"8), both theoretical and experimental, conducted in the author s laboratories at Stevens Institute of Technology. Analytical methods include a Semenov-type theoretical approach (1,2,9) as well as computer simulations similar to those used by Barkelew LS) ... 

Frequently function R can be written as a single term having the simple form of equation 1. For Instance, with the aid of the long chain approximation (LCA) and the quasi-steady state approximation ((JSSA), the rate of monomer conversion, I.e., the rate of polymerization, for many chain-addition polymerizations can be written as... 

During the development of these criteria the Semenov analysis was extended to systems with heat-exchanger reservoir temperatures different from feed temperatures (Tr < Tq) and with delayed runaway (larger value of e), which resulted In significant concentration drift prior to runaway. Since values of e for chain-addition polymerizations are not nearly as small as those for the gaseous explosions Investigated by Semenov, R-A Is not as sensitive nor Is It as early In terms of extent of reaction. 

Peaking and Non-isothermal Polymerizations. Biesenberger a (3) have studied the theory of "thermal ignition" applied to chain addition polymerization and worked out computational and experimental cases for batch styrene polymerization with various catalysts. They define thermal ignition as the condition where the reaction temperature increases rapidly with time and the rate of increase in temperature also increases with time (concave upward curve). Their theory, computations, and experiments were for well stirred batch reactors with constant heat transfer coefficients. Their work is of interest for understanding the boundaries of stability for abnormal situations like catalyst mischarge or control malfunctions. In practice, however, the criterion for stability in low conversion... 

Polymerizations are classified as either step (condensation) or chain (addition) polymerizations. The two differ in the time-scale of various reaction events, specifically in the length of time required for the growth of large-sized molecules. The synthesis of polysulfides (Eq. 1) and polyurethanes (Eq. 2) are... 

For the synthesis of block copolymers chain addition polymerization (ionic or radical) as well as condensation polymerization and stepwise addition polymerization can be used. 

A very important characteristic of polymerization reactors is their thermal stability as discussed by Sebastian (6). Chain addition polymerizations are thermally simple reactions, in that the polymerization exotherm is attributable almost in its entirety to the chain propagation step. For chain addition polymerization reactors the rate of reaction r is proportional to the product of the square root of initiator concentration, cu and to monomer concentration, cm... 

Chain addition polymerizations have a typical value of e 4 x 10-2, and for such batch reactions re increase 106 times from 0 = 10 1 to 0 = 10, with the plateau region at 0 values that are 1010 times higher. It is for this reason that chain addition polymerization reactions, although experimentally studied, as with methyl methacrylate (41,42), are rarely carried out in reactive polymer processing equipment. 

The ratio of the adiabatic heat generation characteristic time to that of heat removal is now a = tAD/(tR + tn) and related to the Nusselt number. For the chain addition polymerization this relation is shown graphically in Fig. 11.7. 

Hiese, M. Martin, G. Gotro, J. Gelation in thermosets formed by chain addition polymerization. Polym. Eng. Sci. 1990, 30 (2), 83. 

Photocondensation Polymerization.—This term is used in the sense that the polymerization requires the absorption of a photon for every propagation step, as distinct from the photoinitiation of the free-radical chain addition polymerization discussed above. The photodimerization reaction (14) of maleimides in... 

With this backgrotmd, let us explain the mechanism of chain (addition) polymerization. If we write the electronic stmcture of the main group, vinyl ... 

Three broad types of polymerization processes are generally recognized chain-addition polymerization, step-growth polymerization, and coordination polymerization. 

Chain-Addition Polymerization. The characteristic pattern of chain-addition polymerization is the one-by-one attachment of individual monomer units to the growing polymer chain. Addition of each successive monomer unit regenerates a... 

The most important chain-addition polymerizations are processes in which the reacting end of the polymer is a free radical. This type of polymerization is commonly called radical-chain polymerization. The basic mechanistic concepts are those introduced in Part A, Chapter 12, where free-radical chemistry was considered. 

Many commercially important polymers are copolymers resulting from copolymerization, which is the polymerization of mixtures of two (or more) monomers. Copolymerization will be discussed somewhat later, after the other important mechanisms for chain-addition polymerization have been considered. 

Polyethylene (low density) CH2=CH2 radical-chain addition polymerization... 

Poly(methyl methacrylate) Polyacrylonitrile Poly(vinyl acetate) CO2CH3 / CH2=C CHa CH2=CHCN 0 CH2=CHOCCHa radical-chain addition polymerization radical-chain addition polymerization in aqueous suspension radical-chain addition polymerization... 

The chain addition polymerizations require monomers with double bonds. They require free radical or ionic initiators to open the double bond and form the polymerization path in the manufacture of polymers such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride which together constitute the majority of polymers, about 70 % of all polymers produced. A wide range of copolymers or terpolymers are produced by chain addition polymerization of two or three different monomers with double bonds. 

Even though not as extensive as chain addition polymerization, polymerization by step growth mechanisms in supercritical fluid media is also gaining attention. 

This contribution discusses the molecular characteristics of a series of original nonlinear and yet well-defined architectures based on PS, PB and PEO that were obtained by ROM polymerization or copolymerization of the corresponding macromonomers. Unlike other "living" chain addition polymerization, ROMP is robust and versatile and could successfully be applied to shape very common polymers into particular forms. 

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