Copolymer-composition distribution

More recent inventions are the metallocene catalysts based on zirconium. They offer more uniform catalyst activity and can give a relatively narrow molecular weight distribution. More importantly, they offer better control over structure and copolymer composition distributions. 

In a batch reactor, the relative monomer concentrations will change with time because the two monomers react at different rates. For polymerizations with a short chain life, the change in monomer concentration results in a copolymer composition distribution where polymer molecules formed early in the batch will have a different composition from molecules formed late in the batch. For living polymers, the drift in monomer composition causes a corresponding change down the growing chain. This phenomenon can be used advantageously to produce tapered block copolymers. 

Example 13.7 A 50/50 (molar) mixture of st5Tene and acrylonitrile is batch polymerized by free-radical kinetics until 80% molar conversion of the monomers is achieved. Determine the copolymer composition distribution. 

FIGURE 13.6 Copolymer composition distribution resulting from the batch polymerization of styrene and acrylonitrile. 

The properties of a polymer depend not only on its gross chemical composition but also on its molecular weight distribution, copolymer composition distribution, branch length distribution, and so on. The same monomer(s) can be converted to widely differing polymers depending on the polymerization mechanism and reactor type. This is an example of product by process, and no single product is best for all applications. Thus, there are several commercial varieties each of polyethylene, polystyrene, and polyvinyl chloride that are made by distinctly different processes. 

Continuous-flow stirred tank reactors are widely used for free-radical polymerizations. They have two main advantages the solvent or monomer can be boiled to remove the heat of polymerization, and fairly narrow molecular weight and copolymer composition distributions can be achieved. Stirred tanks or... 

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 analysis of homopolymers the critical interpretation problems are calibration of retention time for molecular weight and allowance for the imperfect re >lution of the GPC. In copolymer analysis these interpretation problems remain but are ven added dimensions by the simultaneous presence of molecular weight distribution, copolymer composition distribution and monomer sequence length distribution. Since, the GPC usu y separates on the basis of "molecular size" in solution and not on the basB of any one of these particular properties, this means that at any retention time there can be distributions of all three. The usual GPC chromatogram then represents a r onse to the concentration of some avera of e h of these properties at each retention time. 

Figure 17. Theoretical normalized copolymer composition distributions (ti 0.56, Tf = 0.40 letters correspond to sample in Table V)...

Difficulty in estimating molecular weight and copolymer composition distributions, which affect product performance. 

Definition of a Complex Polymer. A simple polymer is one vrtiich has at most one broad molecular property distribution (e.g., a broad molecular weight distribution). A complex polymer is one which has two or more broad molecular property distributions (e.g., a broad molecular weight distribution and a broad copolymer composition distribution) ( ). Properties such as molecular weight and composition, Aiich can be in so much variety in a polymer that they must be described as a distribution, are here termed "distributed properties". It is the presence of simultaneous breadth (i.e., variety) in more than one distributed property which is the defining characteristic of a "complex" polymer and the source of analysis difficulties. 

Figure 2 Property distributions in a linear copolymer composition distribution, molecular weight distribution and sequence length distribution of poly styrene-co-n-butyl methacrylate). (Styrene units are represented by "A and n-butyl methacrylate units by B".)...

The diode array UV/vis spectrophotometer was used to both Identify the polymer exiting and to obtain a quantitative analysis of the copolymer composition distribution. Figure 9 (6) shows the result of summing many individual fraction analyses to see the total copolymer composition distribution. The result had the correct average composition but not the skewed shape expected from theory. Part of the difficulty was the relatively small number of cross fractionations done. 

Figure 9 Copolymer composition distribution for Whole polymer as sum of distributions obtained from individual cross fractionations. (Reproduced from Ref. 6. Copyright 1983,...

Extensive work on copolymer composition distribution has been carried out using SEC apparatus equipped with an additional detector sensitive to only one kind of constituting unit. The advantage of this approach is the avoidance of extra experimental work. Its problems are connected with the evaluation of data 16 The method will not be dealt with within these pages since it is a broad field in itself and has been reviewed recently 17 20>. 

Vinyl acetate-butyl acrylate copolymers (0-100% butyl acrylate) were prepared by both batch and starved semi-continuous polymerization using sodium lauryl sulfate emulsifier, potassium persulfate initiator, and sodium bicarbonate buffer. This copolymer system was selected, not only because of its industrial importance, but also because of its copolymerization reactivity ratios, which predict a critical dependence of copolymer compositional distribution on the technique of polymerization. The butyl acrylate is so much more reactive than the vinyl acetate that batch polymerization of any monomer ratio would be expected to give a butyl acrylate-rich copolymer until the butyl acrylate is exhausted and polyvinyl acetate thereafter. 

The results showed that all batch polymerizations gave a two-peaked copolymer compositional distribution, a butyl acrylate-rich fraction, which varied according to the monomer ratio, and polyvinyl acetate. All starved semi-continuous polymerizations gave a single-peaked copolymer compositional distribution which corresponded to the monomer ratio. The latex particle sizes and type and concentration of surface groups were correlated with the conditions of polymerization. The stability of the latex to added electrolyte showed that particles were stabilized by both electrostatic and steric stabilization with the steric stabilization groups provided by surface hydrolysis of vinyl acetate units in the polymer chain. The extent of this surface hydrolysis was greater for the starved semi-continuous sample than for the batch sample. 

The calculations of the statistical characteristics of such polymers within the framework of the kinetic models different from the terminal one do not present any difficulties at all. So in the case of the penultimate model, Harwood [193-194] worked out a special computer program for calculating the dependencies of the sequences probabilities on conversion. Within the framework of this model, Eq. (5.2) can be integrated in terms of the elementary functions as it was done earlier [177] in order to calculate copolymer composition distribution in the case of the simplified (r 2 = Fj) penultimate model. In the framework of the latter the possibility of the existence of systems with two azeotropes was proved for the first time and the regions of the reactivity ratios of such systems [6] were determined. In a general version of the penultimate model (2.3-24) the azeotropic compositions x = 1/(1 + 0 ) are determined [6] by the positive roots 0 =0 of the following... 

A single CSTR or a train of CSTRs may offer several advantages over batch reactors both with regard to production rate and polymer quality. The copolymer composition distribution obtained in a CSTR is generally narrower than that for a batch reactor. To obtain fairly narrow copolymer composition distributions without appreciable loss in productivity, one has the flexibility to increase the number of CSTRs in series to approach plug flow. 

Composition Drift. Determinations of copolymer composition distribution by SEC with dual UV and RI detectors were developed by several researchers (1, 20-22). To exemplify that this methodology may only be capable of producing average composition data across a chromatogram as a function of retention volume V, results for samples of the diblock copolymer PS-PT are presented. The block lengths in samples of PS-PT were chosen such that the SEC peak for copolymer was well resolved from the PS prepolymer peak. 

Copolymer properties are known to be a function of the molecular weight distribution (MWD), the copolymer composition distribution (CCD) and in some cases the sequence length distribution (SLD). The optimal design, operation and control of reactors to produce high quality copolymers with efficient production rates requires ... 

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