Copolymerization, reaction rate

Polymerization of modified monomers makes the polymerization itself more challenging, as polymerization parameters known for common monomers, such as copolymerization reaction rates, do not necessarily apply to pre-modified monomers. Post-polymerization functionahzation methods, however, enable the use of functionahties as the side-chain modifiers to a well-defined polymer backbone so that a variety of functional polymers can be produced through one single polymer scaffold. A major challenge of this method is that the modification step must be a clean... 

Monomer-radical reaction rates are also influenced by steric hindrance. The effect of steric hindrance in reducing monomer reactivity can be illustrated by considering the copolymerization reaction rate constants (ku) for di- and tri-substituted ethylene. Table 8.4 lists some of these values. 

Solution copolymerization of the MA-styrene pair has been studied in acetone at high pressure (1-4 kbars). At 40°C, the copolymerization reaction rate between MA and the growing polymer chain, having a MA unit preceding the active styrene moiety, increased with pressure for both bulk and solution copolymerizations condition. Thus, Enomoto and coworkers conclude the penultimate effect diminishes or disappears with pressure escalation. Composition and infrared studies show a 1 1 alternating structure for the material, over a wide range of monomer concentration of one component. 

Since 1 is a monomer with low activity, copolymers 2 obtained at any stage of the copolymerization process, irrespective of the monomer ratio in the initial mixture, always contain a smaller amount of monomeric units of 1 than that in the corresponding monomer mixture. 1 being prone to enter the chain-transfer reaction, the increase of its content in the initial monomer mixture reduces substantially the reaction rate and decreases the molecular mass of the copolymers. It was found that copolymers 2 which contain 2—8% of monomeric units of 1 and are suitable for obtaining fibres must have a molecular mass between 45 000 and 50000. Such copolymers can be obtained with a AN 1 ratio in the initial mixture between 95 5 and 85 15. Concentrated solutions of copolymers, especially those with a molecular mass smaller than the above limit, are characterized by a very low stability which is a substantial shortcoming of these copolymers. 

Reactivity ratios for the copolymerization of AN with 7 in methanol at 60 °C, proved to be equal to rx AN= 3,6 0,2 and r%n = 0 0,06, i.e., AN is a much more active component in this binary system. The low reactivity of the vinyl double bond in 7 is explained by the specific effect of the sulfonyl group on its polarity26. In addition to that, the radical formed from 7 does not seem to be stabilized by the sulfonyl group and readily takes part in the chain transfer reaction and chain termination. As a result of this, the rate of copolymerization reaction and the molecular mass of the copolymers decrease with increasing content of 7 in the initial mixture. 

The various copolymerization models that appear in the literature (terminal, penultimate, complex dissociation, complex participation, etc.) should not be considered as alternative descriptions. They are approximations made through necessity to reduce complexity. They should, at best, be considered as a subset of some overall scheme for copolymerization. Any unified theory, if such is possible, would have to take into account all of the factors mentioned above. The models used to describe copolymerization reaction mechanisms arc normally chosen to be the simplest possible model capable of explaining a given set of experimental data. They do not necessarily provide, nor are they meant to be, a complete description of the mechanism. Much of the impetus for model development and drive for understanding of the mechanism of copolymerization conies from the need to predict composition and rates. Developments in models have followed the development and application of analytical techniques that demonstrate the inadequacy of an earlier model. 

Penultimate Group Effects Copolymerization Model. This model represents an extension of the Mayo-Lewis model in which the next to last or penultimate group is assumed to affect the reaction rate. Under this assumption the eight reactions represented by the following equations are of importance ( ) ... 

Free-radical copolymerization of trimethyl- or tributylvinyltin with styrene or methyl methacrylate results in low ( 10%) yield of copolymer. Moreover, both the reaction rate and viscosity decrease considerably with higher vinyltin content in the starting mixture 49). These findings imply that organotin monomers tend to inhibit free-radical copolymerization. 

Funke et al. [34] found that on thermal curing of unsaturated polyesters (UP) and styrene the conversion of fumaric acid units decreased with an increase in temperature. A following treatment of all samples at the highest curing temperature used before, had no effect on the conversion of the fumaric acid units. By a temperature increase at an early stage of the copolymerization reaction only the reaction rate could be increased,but the final conversion was the same as that obtained after a longer time at a lower temperature. 

By using lipophilic initiators, such as 2,2 -azobis(isobutyronitrile) (AIBN), in the micro-ECP, diffusion of monomers is too slow compared with the reaction rate. Therefore, copolymerization is confined to the incoherent, lipophilic phase [112,113] and very small microgel particles with a rather uniform size result. 

Experiments were also performed with the aim of polymerizing a mixture of two monomers (69). The reaction rate and the composition of the graft copolymer conform to the rules of mechanochemical synthesis and radical copolymerization. If the two monomers have almost equal reactivities, the composition of the copolymer is approximately that of the initial monomer mixture (Fig. 20). The... 

Fig. 2. Effect of KBr on the copolymerization of equimolar mixtures of ethylene glycol carbonate with phthalic anhydrite.54) (Reproduced by courtesy of Huthig and Wepf Verlag). 1 — conductivity at 120 °C 2 — reaction rate at 200 °C 3 — reaction rate at 180 °C...

The effect of the anion on the copolymerization rate is controversial. Hilt et al.41 established for different sodium halides the following order of efficiency F < Cl < Br < J . This order was interpreted on the basis of the increase in nucleophilicity or polarizability of the anions. Sfluparek and Mleziva 43) observed that the reaction rate of the benzoate anion was about twice as high as that of the bromide anion in the copolymerization of epichlorohydrine with phthalic anhydride. On the other hand, Luston and Manasek56) did not detect any effect of the anion size on the copolymerization rate initiated by tetramethylammonium and tetrabutylammonium halides. 

From the analogy with epoxide determination, we can assume that, despite the less polar medium in copolymerization, the rate of reaction (11) is high at elevated temperatures and that the reaction proceeds irreversibly. Therefore, pyridinium perchlorate, an ammonium salt with protonized nitrogen, does not initiate copolymerization but addition to the epoxy group occurs 56). 

Table 4. Reaction rates and dielectric constants of solvents and reaction mixtures for the copolymerization of ethylene glycol carbonate (0.1 mol) with phthalic anhydride (0.1 mol) in, 100 ml of solvent initiated with KC1 (0.001 mol-%) at 200 °C. 541 (Reproduced by courtesy of Hiithig and...

Fischer 39 40> did not observe any effect of allyl alcohol on the copolymerization of allyl glycidyl ether with phthalic anhydride. Reaction rates were identical and indpendent of proton donor concentration. This finding indicates that the presence of... 

Clear indications of the induction period and of an increase in the reaction rate after copolymerization has started were found for isothermal runs by DSC measurements by Peyser and Bascom 941 even for melt copolymerization. According to the copolymerization mechanism, the induction period is interpreted as a gradual increase in the concentration of active centres45,52 and is identical with the time for reaching the maximum on the conductivity curves57). An induction period has also been established by other measurements 39,40>73.90.95), where it is often considered as an imprecision in the determination of the monomer concentration, mixing effect, temperature establishement, or it is not considered at all. 

Tanaka and Kakiuchi52 > analyzed the conversion curves for the copolymerization of substituted phenylglycidyl ethers with hexahydrophthalic anhydride and found a first-order dependence with respect to the initiator at the maximum reaction rate (Eq. (80)), and second-order kinetics for the region of initiation (Eq. (81)). 

Condensation of TEOS could be controlled by the reaction rate and/or the diffusion of water, while copolymerization could be controlled solely by the diffusion rate of PDMS. Proposed structural models of ormosils based on the reaction mechanisms before gelation are shown in Figure 14. The TEOS/PDMS ratio of the ormosils was 1/0.082. Immediately after mixing, the self-condensation of TEOS(I) was predominant over copolymerization between PDMS and TEOS. As the reaction time increased, copolymerization between PDMS and TEOS(II) was promoted. At this time, the PDMS chains were broken into shorter chains and/or cyclic D4C tetramers. As copolymerization and condensation reactions of TEOS proceeded, the solution gelled (III). After gelation, syneresis (IV) occurred and nonbridging PDMS chains and cyclic D4C tetramers were released from the gel. 

The previous concepts may be illustrated with the experimental determination of the evolution of reaction rate, measured by DSC at T = 60°C, for the copolymerization of methyl methacrylate (MMA) with variable amounts of ethylene glycol dimethacrylate (EGDMA), a vinyl-divinyl system (Sun et al., 1997). The reaction was initiated with 2,5-dimethyl-2,5-bis(2-ethylhexanoyl)peroxy hexane. 

In copolymerizing acrylonitrile with another monomer, conditions must be controlled in such a way that the reaction produces a polymer having the desired chain structure and length. The reaction takes place in the presence of substances capable of producing free radicals. In addition, certain trace metals that have been found to increase reaction rates offer a means of controlling chain length. When polymerization is carried... 

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