Free radical copolymerization polymer composition

In the model under consideration all macromolecules of fixed length Z have the same composition X(Z). However, owing to the substantial polydispersity of the products of free-radical copolymerization for length (even for polymer specimens obtained under low conversions), their composition distribution ... 

When studying the free-radical copolymerization of methacrylic and acrylic acids with vinyl monomers, it was established that the addition of catalytic amounts of SnCl and (C6Hs)3SnH has a marked effect on the copolymer composition. It was found that complexes are formed by charge transfer between unsaturated acids and the above tin compounds. It has been suggested that the change in polymer composition is caused by the interaction of the tin compounds with a transition complex resulting in a decrease of the resonance stabilization of the latter 94,). 

As is well known from free radical copolymerization theory, the composition of the copolymers will depend only on the propagation reaction. The relative ability of monomer to add to a growing chain is influenced by the nature of the last chain unit and by the relative concentration. Generally, chain transfer to monomer by polymer radicals will occur to an appreciable extent, and the final product will be made up of homopolymers, multisegment block copolymers, and branched and grafted structures. In the presence of two or more monomers,... 

Today, the majority of all polymeric materials is produced using the free-radical polymerization technique [11-17]. Unfortunately, however, in conventional free-radical copolymerization, control of the incorporation of monomer species into a copolymer chain is practically impossible. Furthermore, in this process, the propagating macroradicals usually attach monomeric units in a random way, governed by the relative reactivities of polymerizing comonomers. This lack of control confines the versatility of the free-radical process, because the microscopic polymer properties, such as chemical composition distribution and tacticity are key parameters that determine the macroscopic behavior of the resultant product. 

Figure 10.7. Instantaneous copolymer composition in free-radical copolymerization of styrene and 2-vinyl thiophene mole fraction of styrene in polymer as function of initial mole fraction and fractional conversion of styrene calculated with reactivity ratios pa = 0.35 and pb = 3.10 (from Mayo and Walling [127]).

Knot et al. (51) converted soybean oil to several monomers for use in structural applications. They prepared rigid thermosetting resins by using free radical copolymerization of maleates with styrene. The maleates are obtained by glycerol trans-esterification of the soybean oil, followed by esterification with maleric anhydride. They also synthesized several TAG-based polymers and composites and compared their properties. It was found that the moduli and glass transition temperature (Tg) of the polymers varied and depended on the particular monomer and the resin composition. They proposed that the transition from glassy to rubbery behavior was extremely broad for these polymers as a result of the TAG molecules acting both as cross-linkers as well as plasticizers in the system. 

The reactivity ratios for the free-radical copolymerization of styrene (rj = 0.4) and acrylonitrile (r2 = 0.04) result in uneven incorporation of each monomer into the copolymer as seen in Figure 3. Thus, most SAN and ABS polymers are made at the crossover point (A in Figure 3) to avoid composition drift. 

A question that continually arises when the topic of stable free radical copolymerization is discussed is what is the composition and microstructure of the copolymers Scheme 1 shows the four possible propagation reactions for a stable free radical copolymerization based on the terminal model. It is expected that if in the uncapped form, the nitroxide leaves the vicinity of the propagating chain end the copolymer microstructure should not be affected by the presence of nitroxide. Unsuccessful attempts by Sogah and Puts to influence the microstructure of polymers prepared by the SFRP process using chiral nitroxides suggest that the nitroxide does leave the vicinity of the propagating chain end (3). This is in agreement with Fukuda s results, which show that the microstructure of styrene-acrylonitrile (SAN) copolymers... 

If the nitroxide does leave the vicinity of the propagating chain end then the reactivity ratios for the radicals should also be the same as in conventional radical polymerizations. However, if the capping and uncapping rates for the two monomers are different this would lead to different concentrations of the two types of propagating chain ends relative to what would be present in a conventional radical polymerization. To address this issue, stable free radical copolymerizations of styrene-isoprene and styrene-acrylonitrile were studied in detail to compare the low conversion copolymer compositions to those prepared by conventional radical polymerization. The microstructure of the polymers was also examined. 

Polymers with pendant cyclic carbonate functionality were synthesized via the free radical copolymerization of vinyl ethylene carbonate (4-ethenyl-l,3-dioxolane-2-one, VEC) with other imsaturated monomers. Both solution and emulsion free radical processes were used. In solution copolymerizations, it was found that VEC copolymerizes completely with vinyl ester monomers over a wide compositional range. Conversions of monomer to polymer are quantitative with complete incorporation of VEC into the copolymers. Cyclic carbonate functional latex polymers were prepared by the emulsion copolymerization of VEC with vinyl acetate and butyl acrylate. VEC incorporation was quantitative and did not affect the stability of the latex. When copolymerized with acrylic monomers, however, VEC is not completely incorporated into the copolymer. Sufficient levels can be incorporated to provide adequate cyclic carbonate functionality for subsequent reaction and crosslinking. The unincorporated VEC can be removed using a thin film evaporator. The Tg of VEC copolymers can be modeled over the compositional range studied using either linear or Fox models with extrapolated values of the Tg of VEC homopolymer. 

FRP leads to the formation of statistical copolymers, where the arrangement of monomers within the chains is dictated purely by kinetic factors. The most common treatment of free-radical copolymerization kinetics assumes that radical reactivity depends only on the identity of the terminal unit on the growing chain. The assumption provides a good representation of polymer composition and sequence distribution, but not necessarily polymerization rate, as discussed later. This terminal model is widely used to model free-radical copolymerization according to the set of mechanisms in Scheme 3.11. 

Copolymerization by RAFT Unlike in a free radical copolymerization, during the RAFT process, aU the chains grow continuously throughout the reaction, allowing the composition drift to be captured within the chain structure and leading to polymer chains with similar composition. 

FIGURE 13.4 Using data from Figure 13.3, the fractional conversion of Am and VB are shown. A two-phase reaction occurs in which copolymer of Am and VB is produced up until 5500 s, after which homopolymers of Am form. Hence, a blend of copolyelectrolyte and Am homopolymer results. Also shown are the cumulative M and M. according to Equation 11.7. The mass of the polymer chains jumps up once the final Am homopolymerization period begins. Reprinted (adapted) with permission from Alb AM, Paril A, Catalgil-Giz H, Giz A, Reed WF. Evolution of composition, molar mass, and conductivity during the free radical copolymerization of polyelectrolytes. J Phys Chem B 2007 111 8560-8566. 2007 American Chemical Society. 

For the styrene-vinyl acetate-MA polymerization very significant deviations from the classical concept of free-radical copolymerization were observed.Due to almost identical rates of reaction of MA with styrene and vinyl acetate, there is strict alternation of the monomers along the polymer chain and the copolymer composition regularly corresponds to a 1 1 1 ratio. 

Considerable recent effort has been directed towards the conversion of vegetable oils into solid polymeric materials. These vegetable oil-based polymers generally possess viable mechanical properties and thus show promise as structural materials in a variety of applications. For example. Wool and coworkers have prepared rigid thermosets and composites via free-radical copolymerization of soybean oil monoglyceride maleates and styrene (10-12). The new maleate monomers are obtained by glycerol transesterification of soybean oil, followed by esterification with maleic anhydride (10). It has been... 

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