Typical Chain Growth Polymers

Mixtures of two or more monomers can polymerize to form copolymers. Many copolymers have been developed to combine the best features of each monomer. For example, poly(vinyl chloride) (called a homopolymer because it is made from a single monomers) is brittle. By copolymerizing vinyl chloride with vinyl acetate, a copolymer is obtained that is flexible. Arrangement of the monomer units in a copolymer depends on the rates at which the monomers react with each other. Graft copolymers are formed when a monomer is initiated by free radical sites created on an already-formed polymer chain. 

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Chain-growth, or addition, polymers are made by adding one monomer unit at a time to the growing polymer chain. The reaction requires initiation to produce some sort of reactive intermediate, which may be a free radical, a cation, or an anion. The intermediate adds to the monomer, giving a new intermediate, and the process continues until the chain is terminated in some way. Polystyrene is a typical free-radical chain-growth polymer. 

Living polymerizations are limited to the realm of chain-growth polymerizations, in which a monomer is transformed to a polymer by a reactive species (an initiator, I) via a kinetic chain reaction (Scheme 15.1). An intrinsic limitation of a typical chain-growth process, such as free-radical polymerization, is the occurrence of termination reactions that lead to the formation of dead chains, chains that are incapable of further growth. 

The free-radical chain mechanism described earlier for polyethylene (Sec. 3.16) is typical of chain-growth polymers. The overall reaction is... 

At this temperature, the ethylene insertion was sufficiently slow, which allowed us to monitor the quasi-living behavior at the start of typical chain-growth polymerizations. The chain length increased linearly with time, but the increase in the amount of polymer was nonlinear. Assuming quasi-living behavior, the calculation of the number of active centers revealed that only 7% of the chromium centers were active after 10 min. This number increased to 14% after 30 min at -30 °C. 

The formation of step-growth polymers, unlike the formation of chain-growth polymers, does not occur through chain reactions. Any two monomers (or short chains) can react. The progress of a typical step-growth polymerization is shown schematically in Figure 27.2. When the reaction is 50% complete (12 bonds have formed between... 

Addition polymers, also referred to as chain growth polymers, are normally formed by a chain addition reaction process. When an adjacent monomer molecule reacts with an active site of the monomer, chain addition is said to have occurred. Here, the active site is considered the reactive end of the polymer or monomer, which participates in the polymerization process. A schematic representation of a typical chain addition mechanism is shown in Fig. 1.2. 

Typically, there are two types of building block monomers used in polymerization processes. In one case, the monomers contain carbon-carbon double bonds (C=C). When these unsaturated monomers are used for synthesizing polymers, the process is called chain growth polymerization. This name describes the way the monomers are formed into polymers—by a chain reaction, that is, one where the polymers are formed in very fast reactions to their final product. Examples of chain growth polymers typically used in coatings are acrylics and vinyls. 

Initially the polymer molecular weight distribution obeys a Poisson distribution, typical of a chain growth reaction without chain transfer. Since the reactions are reversible, at a later stage, also the equilibration between the polymers becomes important and a broad distribution of molecular weights is obtained. As can be seen from Figure 16.5 the presence of linear alkenes causes chain termination (chain transfer) and thus low molecular weights are produced if the cycloalkenes are not sufficiently pure. 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

Interfacially formed condensation polymers such as polyesters, polycarbonates, nylons, and PUs are typically formed on a microscopic level in a chain-growth manner largely because of the highly reactive nature of the reactants employed for such interfadal polycondensations. 

Chain growth continues at a rate dependent on the concentrations of monomer [M] and of active sites [MJ. Monomer exponents in the range 1.3 to 1.5 or higher had been observed (110, 123, 127) especially at low [M], but first order dependence has now been established over a broad range of [M] (21). A stationary level of [M ] is reached rapidly and is typically of the order of 10-8 molar. Chains grow rapidly by successive monomer additions until the polymer chain is terminated by transfer or by reaction with another radical. The rate constant for propagation (ft2) at 60° in DMF is 1960 m-1 Is-1 (16), which is a comparatively high value [see Table 1 and ref. (76)]. On the other hand it is only about one-tenth of that found for acrylonitrile in aqueous systems (Table 6)... 

Although the growth of polymer chains results from a free-radical reaction, the reaction rate is relatively slow, resembling that of typical step-growth polymerizations. When the number of double bonds equals the number of SH groups, the system described by Eq. (2.100) can be classified as an A2 (ene) + B4 (thiol) reaction. 

Group 4 elements (e.g., Ti, Zr) are used as typical catalyst precursors for olefin polymerization and serve as potent cationic components for polymer chain growth with the aid of aluminum (e.g., MAO) or boron co-catalysts. It would be more efficient and convenient if organoaluminum cations were used to polymerize olefins. From this viewpoint, following an earlier precedent with two-coordinate cations (Equation (98)),319,320 some three-coordinate organoaluminum cations hold promise, and their ability to promote polymerization of ethylene or terminal olefins is now... 

In step-growth polymerizations, overall costs of monomers, solvent recovery, and preparing the polymer for further processing usually dictate a preference for reactions that are slow at room temperature. (The reasons behind this generalization are summarized in Section 5.3.1.) The ratio of rates of macromolecular growth reactions in typical chain and step-growth polymerizations is often of the order of 10 . ... 

Biopolymers are naturally occurring polymers that are formed in nature during the growth cycles of all organisms they are also referred to as natural polymersJ Their synthesis generally involves enzyme-catalyzed, chain growth polymerization reactions, typically performed within cells by metabolic processes. 

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