The first-order rate constants for the cyclization reactions are needed and, ideally, should be determined experimentally. Therefore, rate constants for the cyclization reactions for n were obtained through trial and error runs of the computer model. This procedure assumed that the cyclic oligomers were produced solely by intramolecular condensations of the Na SC6H4 nCl intermediates. Rate constant values for the cyclization reactions initially were assumed, and the computer model was allowed to predict reaction outcomes.
The process was repeated varying the rate constant values until the predicted cyclic oligomer yields accurately matched those measured experimentally. The first-order rate constant values that provided accurate predictions are listed in Table 3. Another class of reactions required for a computer modeling effort are side reactions that occur during the PPS polymerization. Aryl chlorides are consumed in the presence of SMAB in a process that competes with the polymer growth steps. Rate constants for these side reactions were determined by following the disappearances of DCB and bis 4-chlorophenyl sulfide in the presence of SMAB.
The results are reported in Table 6. This observation is believed to be a function of changing the character of the reaction medium as the reagent concentrations were a Model sulfur reagent for p-Na SC6H4 nCl. Figure 1. Commercial polymerizations are conducted at high concentrations, conditions where the solvation of salts and intermediates are clearly not optimum.
Computer Modeling. On the basis of the step polymerization concept and the rate constant data from Tables 2, 3, and 5, a computer model of the polymerization was constructed that considers all possible combinations of polycondensation reactions. Because an infinite set of differential equations is necessary for normal modeling, the mathematics was simplified by lumping,29 i.
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Numerical integration of the system of differential equations for Fahey et al. Figure 2. Figure 3. Tracking of monomers, initial oligomers, cyclic oligomers, molecular weight, and polymer yields under various reaction conditions can be simulated. PPS computer model predictions of the disappearance of DCB monomer and the appearance of initial intermediates were compared to concentrations experimentally observed in a PPS polymerization that was deliberately designed to be slow in rate to allow sampling and analysis of the reaction mixture during the early stages of the polymerization.
Experimentally determined concentrations of DCB and several oligomers are shown at various time intervals in Figures 2 and 3. Computer-predicted concentrations for these compounds are shown as solid curves. The correlations in these figures demonstrate that the model reasonably predicts monomer disappearance and oligomer concentrations early in the reaction.
Development of weight average molecular weight Mw during polymerization for PPS synthesis from 1. Figure 5. Figure 6. The inset is an expansion of a portion of the plot. As the polymerization continues, Mn begins to increase more rapidly than Mw. The former predicts significant yields of dichloro oligomers well beyond the pentamer, while the latter does not. The contributions of higher mers are extremely significant when considered on a total product weight basis. Second, the experimental data, which are valid only up to the dichloro pentamer due to solubility limitations, are in quite good agreement with the PPS model prediction.
Accordingly, the measured yields are systematically slightly lower than the PPS model prediction. Although the insolubility of the dichloro hexamer and higher mers prevented their reliable yield determinations, mers up to the dichloro heptadecamer were detected by laser desorption-Fourier transform mass spectrometry analysis of the solid product mixture. The unusual features displayed in PPS polymerizations are accounted for in terms of a step-growth polymerization with unequal rate constants for the key polymer growth steps.
The polymer growth steps proceed by ionic SNAr reactions with the rate constant for each reaction dependent upon the electronic influences of substituents. These reactivity differences are manifested in both the sulfur nucleophiles and the aryl chloride electrophiles. Computer simulations of the polymerization based upon these reactivity differences provide good correlations with experimental data for monomer consumption, transient oligomer concentrations, and molecular weights of the final polymer.
Other examples where step-growth polymerization with unequal step reactivity have been recognized are poly phenylene ether sulfone and a poly arylene sulfone syntheses. Experimental Section Most commercially available compounds were used as received. NMP and thiophenol were distilled before use. Sodium benzenethiolate and sodium 4-chlorobenzenethiolate were prepared by dissolving equimolar amounts of NaOH and the appropriate thiophenol in methanol under argon and evaporating the solution to dryness.
The thiols and their salts, NaSH, and Na2S are susceptible to air oxidation, so they were handled under argon or nitrogen. It was not isolated from its NMP solutions but was used directly in the kinetic experiments. Molecular weight analyses were performed on a specially constructed high temperature gel permeation chromatography GPC instrument, utilizing a Waters Model A HPLC pump, connected to a Tracor Model flame ionization detector FID with a modified application nozzle.
Reliable Mn values are not obtained in this analysis because the sensitivity of the FID drops sharply below a molecular weight of and because of an increase in the tailing of the signal as the columns age. Extractable PPS oligomer analyses by nonaqueous reverse-phase high performance liquid chromatography HPLC was accomplished on a Hewlett-Packard liquid chromatograph equipped with a photodiodearray UV detector.
Laser desorption-Fourier transform mass spectrometry LD-FTMS was performed on a Nicolet Fourier transform mass spectrometer interfaced with a Tachisto pulsed carbon dioxide laser. Details of this equipment and its operation are reported elsewhere. Rate studies and polymerizations were performed in Autoclave Engineers mL and 1-L autoclaves constructed of stainless steel or titanium metallurgies. Pressurized vessels connected in series were used to add coreactants and additional solvent to the autoclave to initiate reactions. Rate Studies. A typical pseudo-first-order kinetic experiment is described.
A mL autoclave was charged with 2. The apparatus was assembled and flushed with argon, and the temperature of the autoclave was increased to a few degrees higher than the desired run temperature. A deaerated, heated solution of 0. Immediately thereafter, 15 g of deaerated heated NMP was added from the second vessel through the first, and the autoclave pressure was increased to psig with argon.
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About 15 s elapsed during these operations and the temperature would dip several degrees below the set point. At preselected intervals, 3-mL samples were withdrawn through the sampling tube discarding a mL forerun under argon. Later, the pH of each sample was adjusted to 4. The three ether extracts were combined and analyzed by GC.
Polyesters, polyamides and polyolefins are usually spun from melts, provided the Tm is not too high. Polyacrylates suffer thermal degradation and are therefore spun from solution in a volatile solvent. Cold-drawing is an important physical treatment that improves the strength and appearance of these polymer fibers. At temperatures above T g , a thicker than desired fiber can be forcibly stretched to many times its length; and in so doing the polymer chains become untangled, and tend to align in a parallel fashion. This cold-drawing procedure organizes randomly oriented crystalline domains, and also aligns amorphous domains so they become more crystalline.
In these cases, the physically oriented morphology is stabilized and retained in the final product. This contrasts with elastomeric polymers, for which the stretched or aligned morphology is unstable relative to the amorphous random coil morphology. This cold-drawing treatment may also be used to treat polymer films e. Step-growth polymerization is also used for preparing a class of adhesives and amorphous solids called epoxy resins. LDPE is insoluble in water, but softens and swells on exposure to hydrocarbon solvents. It swells to more than double its size in nonpolar organic solvents like toluene, eventually dissolving, but is impermeable to water.
The C 5 H 8 monomer isoprene is a volatile liquid b. Cotton absorbs water readily, but is unaffected by immersion in toluene or most other organic solvents. Cellulose fibers may be bent and twisted, but do not stretch much before breaking.
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The monomer of cellulose is the C 6 H 12 O 6 aldohexose D-glucose. To account for the differences noted here we need to consider the nature of the aggregate macromolecular structure, or morphology , of each substance.
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Because polymer molecules are so large, they generally pack together in a non-uniform fashion, with ordered or crystalline-like regions mixed together with disordered or amorphous domains. In some cases the entire solid may be amorphous, composed entirely of coiled and tangled macromolecular chains. Crystallinity occurs when linear polymer chains are structurally oriented in a uniform three-dimensional matrix. In the diagram on the right, crystalline domains are colored blue.
Increased crystallinity is associated with an increase in rigidity, tensile strength and opacity due to light scattering. Amorphous polymers are usually less rigid, weaker and more easily deformed. They are often transparent. As noted earlier, HDPE is composed of very long unbranched hydrocarbon chains.
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