Free radical copolymerization monomer parameters

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. 

The Q-e scheme is an attempt to express free radical copolymerization data on a quantitative basis by separating reactivity ratio data for monomer pairs into parameters characteristic of each monomer. Under this scheme, radical-monomer reaction rate constant k, is written as ... 

To determine the Q and e values of a monomer, the Q and e values of another monomer must be known. Styrene has been chosen as reference monomer for free radical copolymerizations, since it can be copolymerized with many other different monomers. Its values have been arbitrarily been given as 0 = 1 and e = —0.8. The Q and e values determined in this way are empirical values that often reproduce experimentally observed behavior quite satisfactorily. Large deviations are occasionally observed, however (see Table 22-4), especially for e values determined via the exponents, and so correspond to variations in r values that differ by large amounts. The Q-e scheme allows the copolymerization parameters of unknown monomer pairs to be estimated, and so allows their copolymerization capacity to be assessed. For this, the following guidelines apply (1) monomers with very different Q values cannot... 

It appears to hold quite generally that the polarity of monomer or macroions is more important than their resonance stabilizations. The reverse is true for free radical copolymerizations. Since cations and anions exhibit opposed polarities (electronegativities), an r > re in cationic copolymerizations lead to an ta re in anionic copolymerizations, and vice versa (Table 22-15). In most ionic copolymerization cases, one copolymerization parameter is always greater than unity and the other is less than unity (Tables 22-15 through 22-17). Thus, ionic copolymerizations cannot be carried out azeotropically. The product mostly has a value of about unity for the ionic copolymerization of two resonance-stabilized monomers or non-resonance-stabilized monomers that is, more or less ideal nonazeotropic copolymerizations occur. On the other hand, ionic polymerization of a resonance-stabilized monomer with a non-resonance-stabilized monomer often yields rA B values that are much greater than unity. In such cases, an accentuated tendency toward block polymerization is expected and observed. 

Anionic copolymerization of macromonomers with low molecular weight monomers could offer an interesting alternative to access graft copolymers of well-controlled structural parameters and composition. Provided that no deactivation takes place, quantitative copolymerization yields are to be expected. Compared to free-radical copolymerization, the rate constants are higher (due to the higher selectivity of carbanions over radicals). As a result, it should be easier to determine the influence of the parameters quoted on the macromonomer reactivity. Another problem is that it is not always easy to purify the macromonomers, since distillation is not possible. 

The composition of copolymers obtained in a free radical polymerization can be predicted based on several kinetic parameters of copolymerization reaction. For a copolymer starting with two species of monomer P and P , it can be assumed that the rate of addition of the monomer to a growing free radical depends only on the nature of the end group. If the chain is indicated by X, this is equivalent with the assumption that the radical Ri will act equivalently with X-R, and the radical R will act equivalently with X-Ri . The following reactions will take place in the system ... 

T t has previously been shown that the rate of the free-radical-initiated copolymerization of maleic anhydride and vinyl monomers is faster in poor solvents than in good solvents, and that the copolymerization rates in these poor solvents decrease as the difference in the solubility parameter values between that of the copolymer and the solvent increases (6, 10). Other investigators have suggested that this free-radical initiation involves hydrogen abstraction, and that the free-radical precursor increases the yield of the alternating copolymer regardless of the monomer charge (3). 

Graft Copolymers. In graft copolymerization, a preformed polymer with residual double bonds or active hydrogens is either dispersed or dissolved in the monomer in the absence or presence of a solvent. On this backbone, the monomer is grafted in free-radical reaction. Impact polystyrene is made commercially in three steps first, solid polybutadiene rubber is cut and dispersed as small particles in styrene monomer. Secondly, bulk prepolymerization and thirdly, completion of the polymerization in either bulk or aqueous suspension is made. During the prepolymerization step, styrene starts to polymerize by itself forming droplets of polystyrene with phase separation. When equal phase volumes are attained, phase inversion occurs. The droplets of polystyrene become the continuous phase in which the rubber particles are dispersed. R. L. Kruse has determined the solubility parameter for the phase equilibrium. 

Jaisinghani and Ray (40) also predicted the existence of three steady states for the free-radical polymerization of methyl methacrylate under autothermal operation. As their analysis could only locate unstable limit cycles, they concluded that stable oscillations for this system were unlikely. However, they speculated that other monomer-initiator combinations could exhibit more interesting dynamic phenomena. Since at that time there had been no evidence of experimental work for this class of problems, their theoretical analysis provided the foundation for future experimental work aimed at validating the predicted phenomena. Later studies include the investigations of Balaraman et al. (43) for the continuous bulk copolymerization of styrene and acrylonitrile, and Kuchanov et al. (44) who demonstrated the existence of sustained oscillations for bulk copolymerization under non-isothermal conditions. Hamer, Akramov and Ray (45) were first to predict stable limit cycles for non-isothermal solution homopolymerization and copolymerization in a CSTR. Parameter space plots and dynamic simulations were presented for methyl methacrylate and vinyl acetate homopolymerization, as well as for their copolymerization. The copolymerization system exhibited a new bifurcation diagram observed for the first time where three Hopf bifurcations were located, leading to stable and unstable periodic branches over a small parameter range. Schmidt, Clinch and Ray (46) provided the first experimental evidence of multiple steady states for non-isothermal solution polymerization. Their... 

The concept of copolymerizing with a functional comonomer that is soluble in the continuous phase can virtually be extended to any vinyl functional monomer, provided that under such conditions the copolymerization parameters will allow a copolymerization to occur. The functionalities available using hydrophobic monomers with functional monomers in direct miniemulsions are summarized in Table 15.1. Latexes with a double functionality were prepared via a free-radical polymerization of divinylbenzene in miniemulsion [49, 50] after polymerization, the remaining vinyl bond might be reacted with a thiol-functionalized PEG via the thiol-ene chemistry [49]. 

Figure 22-1. Change in mole fraction of acrylonitrile (AN), butyl acrylate (BA), and butadiene (BU) in monomer mixture (atm ") and in copolymer (x ) as a function of yield for the free radical terpolymerization of an [AN]o [BA]o [BU]o - 0.5 0.25 0.25 mixture at 60 C. The copolymerization parameters are rANBU = 0.1, 2- buba 9.9, tbuan 3.5,...

The monomer pairs in free radical polymerizations can be arranged in a series according to the products of the copolymerization parameters (Table 22-3). On the left-hand side in this series are monomers with electron-donating groups, such as butadiene, styrene, or vinyl acetate and on the right are monomers with electron-attracting substituents, such as maleic anhydride, acrylonitrile, vinylidene chloride, etc. The product r rg decreases from one to zero in the vertical series, whereas in the horizontal series it increases from low values (left) to values up to unity (right). 

Table 22 5. The Product of the Binary Copolymerization Parameters for the Free Radical Terpolymerization of the Conjugated Monomers Methyl Acrylate, Methyl Methacrylate, Acrylonitrile, and Styrene, as well as for the Nonconjugated Monomers Vinyl Acetate, Vinyl Chloride, and Vinylidene Chloride...

Most copolymerizations in the presence of a free radical initiator obey the simple copolymerization equation. Equation (22-22). Consequently, the copolymerization parameters calculated from this equation can be interpreted directly as the ratios of two rate constants. Since they are relative reactivities, they must be influenced by the polarity, the resonance stabilization, and the steric effects of the monomers. In these cases, resonance stabilization effects are generally stronger than polarity influences, and these, in turn are greater than effects due to steric hindrance. 

The influence of polarity is especially noticeable when both monomers produce resonance-stabilized polymer free radicals. The resonance-stabilized styrene with the electrodonating phenyl group always has a copolymerization parameter of less than unity when copolymerized with resonance-stabilized comonomers with electron-attracting groups (i.e., acrylic compounds) (see Table 22-10). The unlike monomer is then preferentially added on, which is easily understandable for the copolymerization of two monomers with opposed polarities. The relationships are also similar in the copolymerization... 

If, however, one polymer free radical is resonance stabilized and the other is not, the resonance-stabilized monomer is preferentially added on to the resonance stabilized free radical, since, a new resonance species is formed. That is why styrene has a copolymerization parameter much greater than unity and vinyl esters have copolymerization parameters of much less than unity when these two monomers are copolymerized together. 

According to the Q e scheme, copolymerization parameters are influenced by the polarity of the monomers or their polymer free radicals. Assuming the partial charges are localized, the polarity term e e (and, correspondingly, the other three polarity terms) can be expressed by the charges z of the free radical and Zg of the monomer, the distance L between these charges in the transition state complex, the relative permittivity c the Boltzmann constant k, and the absolute temperature T ... 

Copolymerization. Acrylic and methacrylic acids readily copolymerize free radically with many vinyl monomers. This versatility results from a combination of their highly reactive double bonds and their miscibility with a wide variety of water- and solvent-soluble monomers. Reactivity ratios derived from copolymerizations with many monomers are tabulated in many books on polymerization, for example in Wiley s Polymer Handbook (14) (see also Wiley s Database of Polymer Properties). Q and e values are parameters that may be established for a monomer based on a large number of reactivity ratios with other monomers. These parameters are associated with interactions between the monomer and the growing chain via resonance (Q) and polar effects (e). 

The reactivity of vinylojy and allylo q monomers derived from polyols and simple carbohydrates was quantitatively evaluated in free-radical UV-initiated copolymerization with acceptor monomers such as DEF or DEM. Kinetic data confirmed the lower reactivity of allylethers compared with vinylo>qr analologs as described in literature. Interestingly, allyl ribosides exhibited the highest reactivity in the allylojgr series combined with the highest final conversion levels. A correlation between the donor monomer structure described by Hansen parameter 8h, corresponding to H-bonding interactions, and the initial polymerization rate has been established. 

The severity of the chemical heterogeneity strongly depends on the copolymerization parameters. In free-radical polymerization there is just one pair of parameters, which may depend somewhat on temperature, for one pair of monomers whereas in ionic polymerization these parameters for every pair of monomers strongly depend on the counter ion and solvent polarity (see Table 7.6). 

The aim of this review is to summarize and discuss the kinetic data of the emulsion polymerization and copolymeiization of vinyl chloride. The current understanding of the kinetics of free-radical polymerization of conventional monomer is briefly described and kinetic data of radical polymerization and copolymerization of vinyl chloride in the presence of hydrophobic and hydrophilic additives are summarized. Efiects of the initiator type and concentration, the reaction conditions and the type of diluent are evaluated. Variation of kinetic and molecular weight parameters in the heterogeneous polymerizations with emulsifier type and concentration are discussed. 

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