Conversion polymer composition

Once the primary variables were obtained, numerous secondary variables were also calculated such as overall conversion, monomer A and B conversions, polymer composition from the moles of A and B in the copolymer, and number average molecular weight. The latter was obtained by dividing the mass of monomers A and B in the polymer by the moles of polymer. 

One final point should be made. The observation of significant solvent effects on kp in homopolymerization and on reactivity ratios in copolymerization (Section 8.3.1) calls into question the methods for reactivity ratio measurement which rely on evaluation of the polymer composition for various monomer feed ratios (Section 7.3.2). If solvent effects arc significant, it would seem to follow that reactivity ratios in bulk copolymerization should be a function of the feed composition.138 Moreover, since the reaction medium alters with conversion, the reactivity ratios may also vary with conversion. Thus the two most common sources of data used in reactivity ratio determination (i.e. low conversion composition measurements and composition conversion measurements) are potentially flawed. A corollary of this statement also provides one explanation for any failure of reactivity ratios to predict copolymer composition at high conversion. The effect of solvents on radical copolymerization remains an area in need of further research. 

Terminal model reactivity ratios may be estimated from the initial monomer feed composition and the dyad concentrations in low conversion polymers using the following relationships (eqs. 45, 46). 

The polymeric material produced in a single stirred-tank reactor will, except for stochastic variations, be of uniform composition. This polymer composition can be significantly different from the composition in the monomer feed mixture unless the conversion is high. If several tanks are connected in series the composition of the polymer produced in each reactor can be quite different. Since most particles are formed in the first reactor this change in composition in the following reactors can yield polymer particles in which composition varies with radius within the particles. 

Solution The conversion is low so that the polymer composition is given by Equation 13.41 with the monomer concentrations at the initial values. There are five data and only two unknowns, so that nonlinear regression is appropriate. The sum-of-squares to be minimized is... 

Figure 8. Overall conversion vs. time, and polymer composition, styrene concentration in the particles, and MMA concentration in the particles vs. overall conversion for the data of Nomura and Fujita (12.). Initial weight ratio (MMA/Total monomer) = 0.5.

Gel Permeation Chromatography (CPC) is often the source of molecular wei t averages used in polymerization kinetic modelling Q.,2). Kinetic models also r uire measurement of molecular weight distribution, conversion to polymer, composition of monomers in a copolymerization rea tion mixture, copolymer composition distribution, and sequence length distribution. The GPC chromatogram often reflects these properties (3,. ... 

In the model under consideration all macromolecules of fixed length Z have the same composition X(Z). However, owing to the substantial polydispersity of the products of free-radical copolymerization for length (even for polymer specimens obtained under low conversions), their composition distribution ... 

As a necessary preliminary to the study of how compositional heterogeneity affects the properties of the polymers, compositionally heterogeneous and homogeneous co- and terpolymers had to be synthesised. It Is common in copolymerisations for the relative reactivity of the co-monomers to be different (5) so that during polymerisations carried out to high conversion In a free-running batch reactor, the Initially formed polymer Is richer In the more reactive monomer, whereas, at the end of the reaction the polymer produced contains a greater proportion of the less reactive monomer. In such circumstances, compositional... 

Johnson PM, Reynolds TB, Stansbury JW, Bowman CN (2005) High throughput kinetic analysis of photopolymer conversion using composition and exposure time gradients. Polymer 46 3300-3306... 

As can be seen from Fig. 3.4, it is very rare for the polymer composition to correspond to that of the monomer mixture. For this reason the composition of the monomer mixture, and hence also that of the resulting polymer, generally changes as the copolymerization proceeds. Therefore, for the determination of the reactivity ratios one must work at the lowest possible conversion. In practical situations where, for various reasons, one is forced to polymerize to higher conversions, this leads to a chemical non-uniformity of the copolymers in addition to the usual non-uniformity of molecular weights. 

Since the polymer composition does not vary with polymerization, at least up to a conversion of 15-20, it is unlikely that the similarity between the feed and product ratios results from the generation of a mixture of polymers of... 

The polymerization of a mixture of more than one monomer leads to copolymers if two monomers are involved and to terpolymers in the case of three monomers. At low conversions, the composition of the polymer that forms from just two monomers depends on the reactivity of the free radical formed from one monomer toward the other monomer or the free radical chain of the second monomer as well as toward its own monomer and its free radical chain. As the process continues, the monomer composition changes continually and the nature of the monomer distribution in the polymer chains changes. It is beyond the scope of this laboratory manual to discuss the complexity of reactivity ratios in copolymerization. It should be pointed out that the formation of terpolymers is even more complex from the theoretical standpoint. This does not mean that such terpolymers cannot be prepared and applied to practical situations. In fact, Experiment 5 is an example of the preparation of a terpolymer latex that has been suggested for use as an exterior protective coating. 

Procedures. A standard recipe for the latex preparation is shown below (St + M2) 20 g, (water + buffer) 160 g, and initiator 5 mmole/1. The weight fraction of M2 in monomer charge (f) was varied from 0.01 to 0.50. Polymerizations were carried out at 55°C or 70°C and pH 2.5 or 9.0 under nitrogen. Samples were withdrawn from the reaction mixture at various time intervals and the polymer was precipitated in an excess of acetone. The conversion and polymer composition were determined by gravimetric means and by elemental analysis, respectively. The M2 fraction in instantaneously-formed copolymer ( Fi ) was calculated from eq. 1 ... 

The dependence of the conversion and polymer-composition on the reaction time is shown in Figure 1 for the copolymerization of St with AA at f = 0.4, pH 9.0 and 70°C. The polymerization course was found to consist of three stages At first AA polymerizes preferentially but the preference decreases rapidly with increasing conversion (0 - 1.0 hr) St polymerizes exclusively (1.0 - 1.5 hrs) and again AA polymerizes preferentially (1.5 hrs - end). A similar copolymerization mode was observed in an aqueous copolym-... 

Some polymer-composition vs. conversion curves were obtained for the copolymerizations with different f s (Figure 2), and all of them seem to intersect the ordinate at 1.0. From the initial slope of the curves and the monomer ratio in the aqueous phase the monomer reactivity ratio was calculated, but the calculation resulted in a negative r2. Therefore, it was concluded that the copolymerization could not be regarded as a homogeneous one even just after the beginning of the reaction. The first stage was considered to be a transitional stage to establish the particle formation. 

Figure 4. Dependence of polymer composition on conversion in copolymerization...

G. Yu, AJ. Heeger, Charge separation and photovoltaic conversion in polymer composites with internal donor/acceptor heterojunctions, J. Appl. Phys. 78 (1995) 4510 1515. 

Numerous reports are available [19,229-248] on the development and analysis of the different procedures of estimating the reactivity ratio from the experimental data obtained over a wide range of conversions. These procedures employ different modifications of the integrated form of the copolymerization equation. For example, intersection [19,229,231,235], (KT) [236,240], (YBR) [235], and other [242] linear least-squares procedures have been developed for the treatment of initial polymer composition data. Naturally, the application of the non-linear procedures allows one to obtain more accurate estimates of the reactivity ratios. However, majority of the calculation procedures suffers from the fact that the measurement errors of the independent variable (the monomer feed composition) are not considered. This simplification can lead in certain cases to significant errors in the estimated kinetic parameters [239]. Special methods [238, 239, 241, 247] were developed to avoid these difficulties. One of them called error-in-variables method (EVM) [239, 241, 247] seems to be the best. EVM implies a statistical approach to the general problem of estimating parameters in mathematical models when the errors in all measured variables are taken into account. Though this method requires more information than do ordinary non-linear least-squares procedures, it provides more reliable estimates of rt and r2 as well as their confidence limits. 

FIGURE 6-22 Polymer composition as a function of conversion for the VC/VDC system (initial concentrations / =/2 = 0.5). 

As shown in Table 6-2, the calculated number of A and B sequences at specific values of copolymer composition (corresponding to specific values of conversion) show that initially the vast majority of VC segments are present as isolated units, while there is a broad distribution of VDC sequence lengths. At high degrees of conversion, as you might expect, this situation is reversed. At 60% conversion the polymer composition is nearly equimolar and the distribution of VC and VDC units is almost the same, with 50% of the units present as monomers, 25% as... 

Preparation by Sequential Polymerisation. Two-polymer composite latex particles may be prepared using either emulsion or dispersion polymerisation techniques. A dispersion (latex) of particles of a first polymer may be prepared in the usual manner after complete conversion of monomer to polymer, a different monomer or monomer mixture is added and polymerised to provide the second polymer. 

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. 

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