Monomer Purity

Monomer purity is vital in ADMET polymerization primarily for two reasons (i) any mono-ene impurity will function as an end-capping agent in the polymerization, and (ii) some impurities may promote unwanted side reactions. Cationic addition was an issue when classical catalysts were used, but with the introduction of well-defined catalysts, however, this became less of a problem. Still, the elimination of unwanted side reactions is of paramount importance in ADMET polymerization. 

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Numerous methods for the deterrnination of monomer purity, including procedures for the deterrnination of saponification equivalent and bromine number, specific gravity, refractive index, and color, are available from manufacturers (68—70). Concentrations of minor components are deterrnined by iodimetry or colorimetry for HQ or MEHQ, by the Kad-Eisher method for water, and by turbidity measurements for trace amounts of polymer. 

High molecular weight polymers or gums are made from cyclotrisdoxane monomer and base catalyst. In order to achieve a good peroxide-curable gum, vinyl groups are added at 0.1 to 0.6% by copolymerization with methylvinylcyclosiloxanes. Gum polymers have a degree of polymerization (DP) of about 5000 and are useful for manufacture of fluorosiUcone mbber. In order to achieve the gum state, the polymerization must be conducted in a kineticaHy controlled manner because of the rapid depolymerization rate of fluorosiUcone. The expected thermodynamic end point of such a process is the conversion of cyclotrisdoxane to polymer and then rapid reversion of the polymer to cyclotetrasdoxane [429-67 ]. Careful control of the monomer purity, reaction time, reaction temperature, and method for quenching the base catalyst are essential for rehable gum production. 

Like other step-growth polymerization methods, factors such as the monomer purity, ratio of the monomers, conversion, temperature, and concentration will greatly influence the transition metal coupling polymerization. These factors have to be taken into account when higher molecular weight polymers need to be prepared.33... 

The curing reaction can be carried out thermally or with the addition of a catalyst. The thermal cure is strongly influenced by impurities associated with the synthesis. The greater the degree of monomer purity, the more slowly the thermal cure proceeds. If the monomer is sufficiently purified, the cure rate can be predictably controlled by the addition of catalysts. As with the aromatic cyanate esters, the fluoromethylene cyanate esters can be cured by the addition of active hydrogen compounds and transition metal complexes. Addition of 1.5 wt% of the fluorinated diol precursor serves as a suitable catalyst.9 The acetylacetonate transition metal salts, which work well for the aromatic cyanate esters,1 are also good catalysts. 

Keeping in mind these factors and the fact that for polymerizations of this type the absence of side reactions, monomer purity and solvent dryness are extremely critical, poly(amic acid) synthesis should be readily accomplished. As it turns out, poly(amic acid) formation is actually a very complex situation and involves some very unique features. As illustrated in Scheme 7, five additional potential reaction pathways besides the propagation reaction are possible. To... 

In conclusion, crystallization polymerization is cationic in nature, and is not an easy method for obtaining the polyacetalic polymer in fact, the yield was rather low (about 15%), and high monomer purity and a careful control of the experimental techniques are required. 

The catalyst, Ru(PMe3)4(GeMe3)2, is active at levels as low as 0.01 mol%, depending on monomer purity. As the catalyst is prepared from the reaction of HGeMe3 with Ru(PMe3)4Me2, this more readily obtainable ruthenium complex can be used directly in the polymerization as a convenient catalyst precursor. The properties of the polygermanes prepared with either complex appear to be identical. 

Monomer purity can be expected to have a significant effect on the properties of the gel synthesized because it has been shown by numerous investigators that copolymerizing NIP A Am monomer with even small amounts of different monomers can alter the transition temperature, swelling degree and sharpness of... 

With the exception of [64], the majority of copolymerizations has been carried out with non-recrystallized DADMAC. Although, there is no evidence that the monomer purity markedly influences the reactivity ratios of Table 5, a general influence on the rate of polymerization should be taken into account. The majority of analytical methods require removal of the monomers before the copolymer composition can be determined. For this reason, HPLC has been shown to provide estimates of reactivity ratios with more narrow confidence intervals [70]. Due to the differences between rx and r2, particularly at higher DADMAC contents in the monomer feed, it is quite challenging to maintain a low conversion of AAM and a constant monomer feed composition. 

The acrylic acid used in this study was normally distilled once prior to each use. However, in an attempt to increase grafting yields, the effect of redistilling was examined. Somewhat higher yields were obtained with the redistilled acrylic acid. For example, 47.5% grafting was obtained by redistilling as opposed to 39.2% for single distillation. Further distillations were not performed due to the susceptibility of uninhibited acrylic acid to spontaneous polymerization. The effect of monomer purity will be examined more extensively in future work. 

Impurities already present in monomeric feeds of the polymerization plants, although it may be generally stated that monomer purity is recognized as a critical variable by commercial polymer producers... 

The Effect of Monomer Purity on Protein Adsorption onto Poly-(HEMA). The importance of relatively minor contamination of the monomers used in formulating hydrogels to be used in biomedical applications has not been recognized widely as yet, although Bruck has referred to this problem in connection with the soft contact lens (26). Protein adsorption studies performed with hydrogels made with monomers of typical commercial quality illustrate this potential problem. 

The data reproducibility was excellent as long as a single batch of control resin was used for the studies so that variables such as monomer purity and polymer molecular weight were controlled. In addition films were cast in a clean room to minimize dust incorporation which can result in large errors in film thickness measurements. Figure 3 shows the typical example and the worst case for dielectric constant data variability on these films. 

A Hewlett-Packard 5610A Gas Chromatograph (gc), with 10% FFAP on Chromosorb W (AW-DMCS) column using N2 carrier gas was used for monomer purity studies. Acid numbers were determined by nonaqueous titration of the copolymers in acetone, using 0.1N sodium hydroxide titrant, phenolphthalein indicator and 0.1N sulfuric acid back titrant. Elemental analyses were performed by Huffman Laboratories, Inc., Wheatridge, Colorado. 

In most applications, and especially in the field of fibers, monomer purity is an essential factor in polymer quality. This is why dimethyl terephthalate is used in preference to the acid, which is more difficult to purify. Terephthafic add (dJJ — 1.510)u> does not melt but sublimes at 300°C under 101-3 kPa absolute. It is also practically insoluble in water and in most organic solvents. However, dimethyl terephthalate = 1J283)[Pg.287]

However, in practice, deviations from this relationship often occur and usually depend on the purity of the monomer. With increasing monomer purity, the value of n increasingly approaches 0.5 even for processes that without doubt occur by the ionic mechanism. This fact, which seems surprising at first, is due to the specific features of radiation-induced initiation. As already mentioned, the initiating particles are monomer ions (M+ or M ) without counterions. Apart from the formation of free ions in radiolysis, secondary processes of the capture of a thermal electron by monomer or solvent molecules also occur, which leads to a free solvated anion. Since the contents of ions of the same sign are equal, their neutralization is expressed by bimolecular termination and leads to n = 0.5. Then we have R + R — P... 

The polymerization rate is proportional to the dose rate to the power of unity. The monomer purity plays an important role in ionic polymerization, and the exponent of the dose rate is profoundly affected by it, being 0.5 for super-pure monomers. 

The massive effect of this relatively small (about 0.7%) concentration of MAA on polymer properties (swelling, viscosity, etc.) indicates that monomer purity should be a driving concern in the synthesis of soluble hydrogel polymers, and that EWC measurements in unbuffered solution should be treated with caution. On the other hand, the incorporation of trace quantities of acidic and basic monomers into this type of polymer may prove a useful method of behavioral modification. 

The purity of the monomer can be determined by GC or cryometry 10). The refractive index can also be used to determine the purity. For polymerization grade monomer (purity over 99.8% according to GC) n20 = 1.48583-1.48586 n). 

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