Monomer feed control

Via Controlled Self-Assemblies of Amphiphilic Block Copolymers under Hydrophobic Monomer Feed Control... 

Random copolymers which show variations between chains and along the chain. This is likely to occur with solution polymerisation where the reactivity ratios differ widely and where no monomer feed control is exercised. 

Both propylene and isobutylene ate comonomers that are incorporated along the chain, resulting in additional short-chain branching. One important factor in controlling polymer crystallinity is the choice of chain-transfer agent. Ethane and methane, for example, are inefficient agents whose presence in the monomer feed stream must be considered in reaction control. 

The refining of oil produces the monomer feed streams for tackifiers as byproducts of the process. Tackifiers are low molecular weight polymers, typically 300-5000 g/mol, most often 500-1000 g/mol. Generally molecular weights are well below Mg and thus, within a given class of resins, softening points are controlled primarily by molecular weight (see Fig. 1). 

Values of 0 required to fit the rate of copolymerization by the chemical control model were typically in the range 5-50 though values <1 are also known. In the case of S-MMA copolymerization, the model requires 0 to be in the range 5-14 depending on the monomer feed ratio. This "chemical control" model generally fell from favor wfith the recognition that chain diffusion should be the rate determining step in termination. 

In eq. 68, is defined as in the chemical control model but this expression is cast in terms of the monomer feed composition rather than the radical chain end population. 

The influence of changes in these other variables on MWD in a homopolymerization has not yet been tested, but whatever perturbations are introduced to the feed in a radical polymerization in a laboratory-scale CSTR, they are unlikely to introduce dramatic changes in the MWD of the product because of the extremely short life-time of the active propagating chains in relation to the hold-up time of the reactor. This small change in MWD could be advantageous in a radically initiated copolymerization where perturbations in monomer feeds could give control over polymer compositions independent of the MWD. This postulate is being explored currently. 

Non-cross-linked polystyrene is readily prepared from inexpensive materials using standard conditions and the functional group content of the polymer easily controlled by the stoichiometry of each monomer present in the monomer feed. As with PEG, the functional group content can be readily quantified using simple NMR analysis. The polymer has remarkable solubility properties that are extremely useful to organic chemists. It is soluble in THE, dichloromethane, chloro-... 

All polymerisations were carried out in nitrogen purged xylene solutions in a thermostatically controlled one litre glass reactor. Semi-batch processes were carried out in a similar reactor which was provided with calibrated peristaltic pumps (computer controlled when necessary) for delivering the monomer feeds. Typically, experiments were carried out at 80°C with monomer concentrations which gave solids contents in the range 10 - 60% at 100% conversion. 

No in-process contact water is currently used by the latex rubber industry. No raw material recycling is practised because of poor control of monomer feeds and the buildup of impurities in the water. 

As noted earlier, copolymerization can be controlled by regulating the monomer feed in accordance with, for example, Equations 7.17 and 7.19. 

In the commercial use of copolymerization it is usually desirable to obtain a copolymer with as narrow a distribution of compositions as possible, since polymer properties (and therefore utilization) are often highly dependent on copolymer composition [Athey, 1978]. Two approaches are simultaneously used to minimize heterogeneity in the copolymer composition. One is the choice of comonomers. Choosing a pair of monomers whose copolymerization behavior is such that F is not too different from f is highly desirable as long as that copolymer has the desired properties. The other approach is to maintain the feed composition approximately constant by the batchwise or continuous addition of the more reactive monomer. The control necessary in maintaining f constant depends on the extent to which the copolymer composition differs from the feed. 

Artificial control of the monomer concentrations is possible by changing the monomer feed methods, which includes multishot, stage feed (19), and continuous feed. A multishot emulsion polymerization is expected to form multilayered particles if the monomers are chosen properly. When the layers are sufficiently thin, the particles exhibit unique thermal and mechanical properties. The stage feed system is shown in Figure 11.1.6. It makes it possible to produce particles having gradient composition of different monomer units. 

The small reactivity ratio for AN indicates that a growing AN radical is reluctant to react with an AN monomer, but rather will react with a styrene monomer. On the other hand, even when a growing styrene radical reacts rather with an AN monomer, the tendency is not as marked. In the limiting case, if both monomer reactivity rations are going to zero, this effects the formation of strictly alternating polymers. The composition of the polymer can be controlled by the ratio of monomers in the monomer feed. In particular, since one of the monomers will be consumed faster that the other in a discontinuous process, the monomer feed can be adjusted accordingly in the course of polymerization. Also in a continuous process, in a cascade of reaction vessels, monomer can be fed into certain stages. 

SAN polymers have a natural tendency to assume a yellowish cast when conventionally manufactured (17). This arises from the residual oxygen in the monomer feed. However, when the level of oxygen is below 2 ppm then this problem can be controlled. 

For compounds bearing acrylate or methacrylate groups, the copolymer compositions were almost the same as the monomer feed compositions, and the molecular weights were nearly identical to that of poly(methyl methacrylate) (PMMA) synthesized as a control under the same reaction conditions. In addition, the dye-bearing repeat units were present uniformly in all molecular weights, as seen by comparing GPC molecular weight distribution curves determined by differential refractometry and by visible absorbance detection at the X of the... 

In contrast, diallylamino-substituted dyes copolymerized poorly with MMA, despite the reported polymerizability of other aliphatic (24-25) and aromatic (26) diallylamines. The concentration of dye-bearing repeat units m the polymer was far below the monomer feed concentrations. The possibility that these diallyl dye-monomers (or some unidentified impurity in them) acted as inhibitors of polymerization can be ruled out because the dye was found to be present uniformly in all molecular weight fractions and the molecular weights of the copolymers were again nearly identical to control samples of PMMA. Therefore, only a small amount of the diallyl amino substituted chromophores can be covalently incorporated into the crosslinked matrix because of the apparently unfavorable reactivity ratio. The balance of the chromophore remains simply dissolved in the mixture. 

When a large amount of anionic emulsifier was initially present in the reactor, many particles were formed. This number decreased during the latex preparation because not enough emulsifier was added with the monomer feed emulsion to keep the surface of the particles covered. Controlled agglomeration of polymer particles occurred to a stage where... 

For the liquid-phase reactor shown in Fig. 4.37, monomer feed is introduced and the effluent stream controls the level (residence time). Heat is removed via cooling water. We want to remove the water to push the equilibrium to the right and increase conversion. Due to its volatility, it would be natural to remove the water vapor from the reactor to control pressure. 

More industrial polyethylene copolymers were modeled using the same method of ADMET polymerization followed by hydrogenation using catalyst residue. Copolymers of ethylene-styrene, ethylene-vinyl chloride, and ethylene-acrylate were prepared to examine the effect of incorporation of available vinyl monomer feed stocks into polyethylene [81]. Previously prepared ADMET model copolymers include ethylene-co-carbon monoxide, ethylene-co-carbon dioxide, and ethylene-co-vinyl alcohol [82,83]. In most cases,these copolymers are unattainable by traditional chain polymerization chemistry, but a recent report has revealed a highly active Ni catalyst that can successfully copolymerize ethylene with some functionalized monomers [84]. Although catalyst advances are proving more and more useful in novel polymer synthesis, poor structure control and reactivity ratio considerations are still problematic in chain polymerization chemistry. 

Compositional control for other than azeotropic compositions can be achieved with both batch and semibatch emulsion processes. Continuous addition of the faster reacting monomer, styrene, can be practiced for batch systems, with the feed rate adjusted by computer through gas chromatographic monitoring during the course of the reaction (54). A calorimetric method to control the monomer feed rate has also been described (8). For semibatch processes, adding the monomers at a rate that is slower than copolymerization can achieve equilibrium. It has been found that constant composition in the emulsion can be achieved after ca 20% of the monomers have been charged (55). 

---