Mixture, monomers

In the end the conclusion can be made, that reaction cessation in low-temperature polycondensation process is limited by pnrely physical factor, namely, by macromolecular coil density reaching of reactive medium density (mixture monomer — polymer solution) [121], that is possible for fractal objects only. Such conclusion follows from the simple analysis, adduced below. As it is known [122], the fractal object density pfr can be calculated according to the Eq. (73) ... 

In the feed stage, the remainder of the monomer and initiator are added to the reaction mixture. Monomer diffuses from droplets through the aqueous phase to the growing polymer particles and propagates the polymerization. Radicals can also enter the particles, causing termination or re-initiation of polymerization. As the particles become larger, more surfactant from the aqueous phase is absorbed onto the surface to stabilize the dispersion. 

M. average molecular weight of the reaction mixture (monomer and polymer)... 

Structure and Nomenclature The principal method of polymerizing monomers by the chain kinetic scheme involves the opening of double bonds to form a linear molecule. In a reacting mixture, monomer, fully reacted polymer, and only a small amount of rapidly reacting species are present. Once the polymer terminates, it is dead and cannot react further by the synthesis scheme outlined previously. 

Figure 3.4 shows a reactor used for the plasma graft-polymerization. The reactor diameter is 30 mm. and the length is 240 mm. A square polypropylene substrate membrane (Celanese. Celgard 2400.200 x 2000 X and 2500,400 x 4000 X, 6 X 6 cm) is placed in the reactor and is subjected to plasma treatment for 30 s in the presence of residual air under the pressure of 30 Pa. A low-temperature plasma is excited by a 13.56-MHz power source at less than 20 W to prevent the polypropylene substrate membrane from heat damage. Then an aqueous solution of methacrylic acid-2-hydroxyethyl and acrylic acid mixture (monomer concentration 5 wt%) is transferred from the monomer solution reservoir to the reactor. The reactor is immersed into a temperature-controlled bath where polymerization is carried out for 2 h at 50 to 70 C [32]. 

For the simple case of only two monomers, the problem of calculating F, as a function of degree of conversion and initial monomer composition is analogous to the situation in differential distillation of a binary mixture. Monomer is converted to polymer irreversibly just as volatile liquids are distilled out from a pot irreversibly. A material balance gives the Rayleigh equation [51] (also called the Skeist equation) ... 

Povodyrev et aJ [30] have applied crossover theory to the Flory equation ( section A2.5.4.1) for polymer solutions for various values of N, the number of monomer units in the polymer chain, obtaining the coexistence curve and values of the coefficient p jj-from the slope of that curve. Figure A2.5.27 shows their comparison between classical and crossover values of p j-j for A = 1, which is of course just the simple mixture. As seen in this figure, the crossover to classical behaviour is not complete until far below the critical temperature. 

Surfactants provide temporary emulsion droplet stabilization of monomer droplets in tire two-phase reaction mixture obtained in emulsion polymerization. A cartoon of tliis process is given in figure C2.3.11. There we see tliat a reservoir of polymerizable monomer exists in a relatively large droplet (of tire order of tire size of tire wavelengtli of light or larger) kinetically stabilized by surfactant. 

A polymer is a macromolecule that is constructed by chemically linking together a sequent of molecular fragments. In simple synthetic polymers such as polyethylene or polystyrer all of the molecular fragments comprise the same basic unit (or monomer). Other poly me contain mixtures of monomers. Proteins, for example, are polypeptide chains in which eac unit is one of the twenty amino acids. Cross-linking between different chains gives rise to j-further variations in the constitution and structure of a polymer. All of these features me affect the overall properties of the molecule, sometimes in a dramatic way. Moreover, or... 

Polymerization takes place, in the following manner in the presence of suitable peroxide catalyst these compounds polymerize with themselves (homopolymerizatiOn) in aqueous emulsion. When the reaction is complete, the emulsified polymer may be used directly or the emulsion coagulated to yield the solid polymer (312). A typical polymerization mixture is total monomer (2-vinylthiazole), 100 sodium stearate, 5 potassium persulfate, 0.3 laurylmercaptan, 0.4 to 0.7 and water, 200 parts. 

The elastomer produced in greatest amount is styrene-butadiene rubber (SBR) Annually just under 10 lb of SBR IS produced in the United States and al most all of it IS used in automobile tires As its name suggests SBR is prepared from styrene and 1 3 buta diene It is an example of a copolymer a polymer as sembled from two or more different monomers Free radical polymerization of a mixture of styrene and 1 3 butadiene gives SBR... 

Copolymerization. Copolymerization occurs when a mixture of two or more monomer types polymerizes so that each kind of monomer enters the polymer chain. The fundamental structure resulting from copolymerization depends on the nature of the monomers and the relative rates of monomer reactions with the growing polymer chain. A tendency toward alternation of monomer units is common. 

At any p, very small and very large values of n contribute a lower weight fraction to the mixture than do intermediate values of n. This arises because of the product nN in Eq. (5.29) is large for monomers, in which case n... 

We define the problem by assuming the polymerization involves AA and BB monomers and that the B groups are present in excess. We define and to be the numbers of A and B functional groups, respectively. The number of either of these quantities in the initial reaction mixture is indicated by a superscript 0 the numbers at various stages of reaction have no superscript. The stoichiometric imbalance is defined by the ratio r, where... 

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

What we seek next is a quantitative relationship between the extent of the polymerization reaction, the composition of the monomer mixture, and the point of gelation. We shall base our discussion on the system described by reaction (5.U) other cases are derived by similar methods. To further specify the system we assume that A groups limit the reaction and that B groups are present in excess. Two parameters are necessary to characterize the reaction mixture ... 

For a fixed extent of reaction, the presence of multifunctional monomers in an equimolar mixture of reactive groups increases the degree of polymerization. Conversely, for the same mixture a lesser extent of reaction is needed to reach a specified with multifunctional reactants than without them. Remember that this entire approach is developed for the case of stoichiometric balance. If the numbers of functional groups are unequal, this effect works in opposition to the multifunctional groups. 

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