Growth free-radical

The hydrophobe modification of acrylic acid represents an important class of hydrophobe-modified thickeners prepared by a chain-growth free-radical process. They differ slightly from other examples in that these products are generally cross-linked. 

In the chain growth free radical polymerization of a vinyl monomer (conventional polymerization), the growth reaction is the repeated reaction of a free radical with numbers of monomer molecules. According to the termination by recombination of growing chains, 2 free radicals and 1000 monomer molecules leads to a polymer with the degree of polymerization of 1000. In contrast to this situation, the growth and deposition mechanisms of plasma polymerization as well as of parylene polymerization could be represented by recombinations of 1000 free radicals (some of them are diradicals) to form the three-dimensional network deposit via 1000 kinetic... 

In both the polymerizations, free radicals are the species that are responsible for the formation of bonds in the depositing materials. The growth mechanism, however, is not by the conventional chain-growth free-radical polymerization. In a conventional free-radical chain-growth polymerization, two free radicals and 10,000 monomer molecules yield a polymer with degree of polymerization 10,000, which does not contain free radicals. In contrast to this situation, in plasma polymerization and Parylene polymerization, 10,000 species with free radical(s) recombine to yield a polymer matrix that has an equivalent degree of polymerization, and contains numbers of unreacted free radicals (dangling bonds). 

FIGURE 3.2 Schematic representation of the development of molar mass, for example, M, with monomer conversion for chain growth (free radical), chain growth (living) and step growth mechanisms for the polymer buildup reaction. 

Solomon, D.H., Rizzardo, E., and Cacioli, P. (1985) Substd. alkyl-amine cpds.— useful as controlled growth free radical polymerisation initiators for unsaturated monomers. Patent Number EP135280-A2. 

The disadvantage of the broad PSD in the effluent from a CSTR can actually prove to be an asset if one wishes to study competitive growth and transport rates among the particles. Kinetic model components for particle growth, free radical adsorption and free radical desorption must be of the proper form if one is to be able to predict the PSD of the effluent latex. Batch data will not provide an analysis of the same sensitivity. Hence, measurement of feed and effluent PSD s from a CSTR can be an effective way of measuring fundamental kinetic constants. 

Polymerization of styrene is carried out under free radical conditions often with benzoyl peroxide as the initiator Figure 1111 illustrates a step m the growth of a poly styrene chain by a mechanism analogous to that of the polymerization of ethylene (Sec tion 6 21)... 

This important equation shows that the stationary-state free-radical concentration increases with and varies directly with and inversely with. The concentration of free radicals determines the rate at which polymer forms and the eventual molecular weight of the polymer, since each radical is a growth site. We shall examine these aspects of Eq. (6.23) in the next section. We conclude this section with a numerical example which concerns the stationary-state radical concentration for a typical system. 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

There is a great deal more that could be said about emulsion polymerization or, for that matter, about free-radical polymerization in general. We shall conclude our discussion of the free-radical aspect of chain-growth polymerization at this point, however. This is not the end of chain-growth polymerization, however. There are four additional topics to be considered ... 

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

We begin our discussion of copolymers by considering the free-radical polymerization of a mixture of two monomers. Mi and M2. This is already a narrow view of the entire field of copolymers, since more than two repeat units can be present in copolymers and, in addition, mechanisms other than free-radical chain growth can be responsible for copolymer formation. The essential features of the problem are introduced by this simpler special case, so we shall restrict our attention to this system. 

These are addition polymerizations in which chain growth is propagated through an active center. The latter could be a free radical or an ion we shall see that coordinate intermediates is the more usual case. 

During the vapor deposition process, the polymer chain ends remain truly aUve, ceasing to grow only when they are so far from the growth interface that fresh monomer can no longer reach them. No specific termination chemistry is needed, although subsequent to the deposition, reaction with atmospheric oxygen, as well as other chemical conversions that alter the nature of the free-radical chain ends, is clearly supported experimentally. 

The free-radical polymerization of acrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-tail growth of the polymeric free radical by attack on the double bond of the monomer. 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

This reaction may then repeat itself many times so that several thousand monomer units are joined together in a time of the order of 1 second, leading to a long chain free radical. This is the propagation or growth stage. 

In the case of mechanism (6) there are materials available which completely prevent chain growth by reacting preferentially with free radicals formed to produce a stable product. These materials are known as inhibitors and include quinone, hydroquinone and tertiary butylcatechol. These materials are of particular value in preventing the premature polymerisation of monomer whilst in storage, or even during manufacture. 

Monomer molecules, which have a low but finite solubility in water, diffuse through the water and drift into the soap micelles and swell them. The initiator decomposes into free radicals which also find their way into the micelles and activate polymerisation of a chain within the micelle. Chain growth proceeds until a second radical enters the micelle and starts the growth of a second chain. From kinetic considerations it can be shown that two growing radicals can survive in the same micelle for a few thousandths of a second only before mutual termination occurs. The micelles then remain inactive until a third radical enters the micelle, initiating growth of another chain which continues until a fourth radical comes into the micelle. It is thus seen that statistically the micelle is active for half the time, and as a corollary, at any one time half the micelles contain growing chains. 

A mass of polymer will contain a large number of individual molecules which will vary in their molecular size. This will occur in the case, for example, of free-radically polymerised polymers because of the somewhat random occurrence of ehain termination reactions and in the case of condensation polymers because of the random nature of the chain growth. There will thus be a distribution of molecular weights the system is said to be poly disperse. 

Acrylic adhesives cure by a free radical chain growth mechanism. In contrast, epoxy and urethane adhesives cure by a step growth mechanism. This has a major impact on the cure kinetics, as well as the composition of the adhesive during cure ([9], pp. 6-9). Cyanoacrylate adhesives (such as Super Glue ) also cure by chain growth, but the mechanism is ionic with initiation by surface moisture. 

Free radical chain growth polymerization takes place through three distinct chemical steps. These are shown in Fig. 1. 

Fig. I. Mechanism for free radical chain growth polymerization.

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