Polymerization, free-radical addition step-growth

The inherent driving force of the partition of reactants between phases has a strong impact on both the kinetics and the product properties, especially if the polymerization mechanism has strict stoichiometric requirements. This is the main reason why heterophase polymerizations via step-growth mechanisms frequently face serious problems. Similar issues may be valid for some radical polymerization techniques where active reactants (e.g., control agents) must partition equally between aU particles. The most common type of polymerization mechanism applied in the production of emulsion polymers is free-radical addition, and this will form the focus of the present chapter. 

Find the chemical structures of sebacic acid and propylene glycol (use your textbook, a chemistry handbook, or the Internet). Will these react by free radical (addition) polymerization or condensation (step growth) polymerization Write the order of the atoms along the repeat unit on the polymer backbone (do not include pendant groups or groups foimd only at the end of the polymer chain). 

Note that the conversion X used here is a very different quantity than the p used in Chapter 8 for step-growth polymers. Here, X is the fraction of monomer molecules that have reacted to form polymer. The quantity p represents the fraction of functional groups that have reacted (whether on a monomer or part of a growing polymer chain). In step-growth polymerization, each monomer molecule has several functional groups that react independently, so p is not the fraction of reacted monomer. (Compare to free-radical addition, where both sides of the double bond are reacted in sequence.)... 

As in step-growth polymerization, a distribution of chain lengths is always obtained in a free-radical addition polymerization because of the inherently random nature of the termination reaction with regard to chain length. Expressions for the number-average... 

It is instructive to compare the results of Example 9.10 with Equation 8.6 for step-growth polymerization. There, x increases monotonically with conversion ip), and high conversions are necessary for high chain lengths. This is not true for free-radical addition, where chains formed early in the reaction have fully developed chain lengths. 

From a practical standpoint, most copolymers are made by free-radical addition polymerization, although step-growth polymerization can also be used. For the sake of simplicity in our discussion, we will focus only on the free-radical meehanism A quantitative treatment of random copolymerization is based on the assumption that the reactivity of a growing chain depends only on its active terminal unit. Therefore, when two monomers, Mj and M2, are copolymerized, there are four possible propagation reactions ... 

The photoinduced addition of a thiol (RSH) to an olefinic double bond has been used to produce polymer networks by taking multifunctional mono-mers. " The thiol-ene polymerization proceeds by a step growth addition mechanism that is propagated by a free radical, chain transfer reaction involving the thiyl radical (RS ). The initial thiyl radicals can be readily generated by UV irradiation of a thiol in the presence of a radical-type photoinitiator. The overall reaction process can be schematically represented as follows ... 

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]). 

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

In polyolefins, the chain is propagated by an intermediate free-radical species or by an alkyl species adsorbed onto a solid. Both the free radical and the alkyl have the possibility of termination, and this creates the possibility of growth mistakes by chain transfer and chain-termination steps that create dead polymer before all reactants are consumed. The presence of termination steps produces a broader molecular-weight distribution than does ideal addition polymerization. 

In general, there are two distinctively different classes of polymerization (a) addition or chain growth polymerization and (b) condensation or step growth polymerization. In the former, the polymers are synthesized by the addition of one unsaturated unit to another, resulting in the loss of multiple bonds. Some examples of addition polymers are (a) poly(ethylene), (b) poly(vinyl chloride), (c) poly(methyl methacrylate), and (d) poly(butadiene). The polymerization is initiated by a free radical, which is generated from one of several easily decomposed compounds. Examples of free radical initiators include (a) benzoyl peroxide, (b) di-tert-butyl peroxide, and (c) azobiisobutyronitrile. 

Chain growth differs from step growth in that it involves initiation and usually also termination reactions in addition to actual growth. This makes its kinetic behavior similar to that of chain reactions (see Chapter 9). However, the chain carriers in chain-growth polymerization need not be free radicals, as they are in ordinary chain reactions. Instead, they could be anions, cations, or metal-complex adducts. While the general structure of kinetics is similar in all types of chain-growth polymerizations, the details differ depending on the nature of the chain carriers. 

The way in which a plasma polymer is formed has been explained by the rapid step growth polymerization mechanism, which is depicted in Figure 5.3. The essential elementary reactions are stepwise recombination of reactive species (free radicals) and stepwise addition of or intrusion via hydrogen abstraction by impinging free radicals. It is important to recognize that these elementary reactions are essentially oligomerization reactions, which do not form polymers by themselves on each cycle. In order to form a polymeric deposition, a certain number of steps (cycle) must be repeated in gas phase and more importantly at the surface. The number of steps is collectively termed the kinetic pathlength. 

Polymer formation reqnires that many monomers mnst be attached to a growing polymer molecnle. This requires that highly reactive functional groups must be available at each growth step. This is achieved by two main mechanisms. Addition polymerization requires monomers to join the polymer without net loss of atoms. This usually involves free radical reaction of molecules that have C=C double bonds, and proceeds throngh three steps initiation, propagation, and termination. 

Bulk polymerizations, such as addition (free-radical- and ionic-based) and step-growth types. Grafting polymerization by small molecules. Interchain copolymer formation, based on chain cleavage, graft copolymerization, and end-group block copolymerization. 

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