Copolymer uniform

Solution Polymerization. In solution polymerization, a solvent for the monomer is often used to obtain very uniform copolymers. Polymerization rates ate normally slower than those for suspension or emulsion PVC. Eor example, vinyl chloride, vinyl acetate, and sometimes maleic acid are polymerized in a solvent where the resulting polymer is insoluble in the solvent. This makes a uniform copolymer, free of suspending agents, that is used in solution coatings (99). 

AH iagredients in the polymerization recipe are not always added at the beginning of the process. For example, better latex stabHity can sometimes be achieved by starting with only part of the emulsifier, saving the rest for later addition. Sometimes a portion of the modifier is held out for late addition to aHow higher final conversion without premature consumption of aH of it. OccasionaHy, if a low acrylonitrile product is the objective, part of the acrylonitrile monomer wiH be saved for late addition so that a chemically more uniform copolymer is produced, which can sometimes enhance properties in critical appHcations. 

Table 10 The Size of the Uniform Copolymer Latices Prepared by Direct Soapless Emulsion Copolymerization [113,114]...

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

The soapless seeded emulsion copolymerization method was used for producing uniform microspheres prepared by the copolymerization of styrene with polar, functional monomers [115-117]. In this series, polysty-rene-polymethacrylic acid (PS/PMAAc), poly sty rene-polymethylmethacrylate-polymethacrylic acid (PS/ PMMA/PMAAc), polystyrene-polyhydroxyethylmeth-acrylate (PS/PHEMA), and polystyrene-polyacrylic acid (PS/PAAc) uniform copolymer microspheres were synthesized by applying a multistage soapless emulsion polymerization process. The composition and the average size of the uniform copolymer latices prepared by multistage soapless emulsion copolymerization are given in Table 11. 

Compositionally uniform copolymers of tributyltin methacrylate (TBTM) and methyl methacrylate (MMA) are produced in a free running batch process by virtue of the monomer reactivity ratios for this combination of monomers (r (TBTM) = 0.96, r (MMA) = 1.0 at 80°C). Compositional ly homogeneous terpolymers were synthesised by keeping constant the instantaneous ratio of the three monomers in the reactor through the addition of the more reactive monomer (or monomers) at an appropriate rate. This procedure has been used by Guyot et al 6 in the preparation of butadiene-acrylonitrile emulsion copolymers and by Johnson et al (7) in the solution copolymerisation of styrene with methyl acrylate. 

Solution Polymerization. In solution polymerization, a solvent for the monomer is often used to obtain very uniform copolymers, Polymerization rates are normally slower than those for suspension or emulsion PVC,... 

Another important result was the finding that inhomogeneous binary mixtures of chemically uniform copolymers did not homogenize upon adding a third copolymer capable of forming clear films with each of the two mixture components. As Molau (4) already found in the styrene-acrylonitrile system there is no homogenization. ... 

In practice, there are effective means for nearly eliminating the spread of chemical compositions (14). The essential condition for the formation of a chemically uniform copolymer is that the monomers must be maintained in the same ratio throughout polymerization. There are two procedures for maintaining this condition. First, it can be achieved by making a continuous addition of either the more reactive monomer alone or certain mixtures of the comonomers as the reaction proceeds. The second method of producing chemically homogeneous copolymers is to proportion a mixture of monomers whose composition is the same as that of the copolymer being formed. 

Research (Fontenot and Schork 1993a, b) indicates that miniemulsion polymerization can provide benefits over the current process technology of conventional emulsion polymerization. Among these are a process which is much more robust to contamination and operating errors, a more uniform copolymer composition when used for copolymerization, and a final product which is far more shear-stable than the product of conventional emulsion polymerization. 

As expected, a single sharp transition is observed characteristic of a reasonably uniform copolymer. Figure 8 shows the dynamic mechanical spectrum for a two-stage process in which the first stage feed was ethyl acrylate and the second stage feed was styrene. This time, two well-defined transitions are observed characteristic of the hard and soft homopolymers. 

When uniform copolymers are desired, semibatch polymerization with continuous addition of the more reactive monomer is required. There is also a special, happenchance case where the monomer and polymer compositions are equal ... 

Producing a fine porous powder which is easy to blend with compounding ingredients. A smaller amount is polymerized in emulsion and spray-dried for plastisols and organosols. And an even smaller amount is copolymerized with vinyl acetate in organic solution, to produce a uniform copolymer which precipitates at a constant composition and molecular weight. 

Manufacture. Propylene oxide is copolymerized with allyl glycidyl ether in an aliphatic, aromatic, or chlorinated hydrocarbon solution using Vandenberg-type catalysts. A complete conversion and a uniform copolymer is obtained containing about 6% of AGE. 

The copolymer composition may drift during the course of an emulsion copolymerization because of differences in monomer reactivity ratios or water solubilities. Various techniques have been developed to produce a uniform copolymer composition. The feed composition may be continuously or periodically enriched in a particular monomer, to compensate for its lower reactivity. A much more common procedure involves pumping the monomers into the reactor at such a rate that the extent of conversion is always very high [>about 90%]. This way, the polymer composition is always that of the last increment of the monomer feed. 

Semibatch operations are commonly employed in copolymerizations to maintain a more uniform copolymer o>mposition. The monomers are fed at rates equsl to their consumption rates in the reactor. 

As seen from Figure 2.3.2, the composition of the polymer and that of the feed is usually different. As the reaction progresses, only by maintaining a constant composition of the feed is it possible to obtain a uniform copolymer. For rA and re less than unity, the variation of Fa as a function of fa has a point where (Fa)c = (fA)c. where the composition of the copolymer is identical to the concentration of the feed. The copolymerization at this concentration is known as azeotropic copolymerization. This is achieved when fA has the value given by rel. (2.3.24) ... 

Introduction of metallocenes in state-of-the-art technologies gives access to new copolymers of ethylene and 1-olefins such as propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene with narrow molecular mass distributions and uniform copolymer compositions. On this basis it is possible to synthesize polyolefins with well-balanced properties. These metallocene/methylalumoxane... 

In copolymerization, the more reactive monomer may be added to the reactor over time to produce a more uniform copolymer con sition distribution. If copolymer composition is measured or estimated on-line, the reactive monomer can be added in a closed-loop fashion [56]. If a portion of all monomers is added to the reactor over time, the copolymer composition will r proximate the... 

Incompatible and ideal copolymerizations behave very differently in high-conversion batch polymerizations. This is illustrated in Figure 12.4 for the AN-VA and AN-MA monomer pairs discussed above. In the AN-VA system, the AN is consumed more rapidly than the VA. Therefore, the unreacted monomer mixture becomes richer in VA as conversion increases. This results in a drift in copolymer composition with conversion. The AN-MA system, in contrast, shows little or no drift, and consequently a more uniform copolymer... 

At first only little styrene is converted, and it is not until the monomer mixture is largely depeleted of 1,3-butadiene that the principal amount of styrene polymerizes forming a block consisting mainly of styrene. In THF, also, the content of vinyl double bonds increases and thus the glass transition temperature is raised [475]. A uniform copolymer composition can be achieved by a continuous process or by programmed addition of butadiene to the reaction mixture [476,477]. Copolymers with a high trans-1,4 content are obtained with barium compounds [478]. In contrast, the coordinative copolymerization yields products with a high cis-1,4 content. 

Commercially, only a very small proportion of the total production of PVC is carried out in solution. The cost of the solvent and of solvent recovery makes the process unattractive. Its primary application is in certain processes such as the copolymerization of vinyl chloride with vinyl acetate (10-25% vinyl acetate) or the terpolymerization of vinyl chloride-vinyl acetate and certain maleates. The resultant polymer solutions are valuable coating materials [1]. The solution processes are said to produce highly uniform copolymers with narrow molecular weight distributions [1,167]. 

---