Chemical Composition Distributions CCDs

Beyers et aV° in the Polymer Research Division of BASF-AG used in-line transflectance NIR to monitor methyl methacrylate (MMA) and iV,7V-dimethylacrylamide (DMAAm) monomers in a copolymerization reaction. The work in this paper is of interest as it illustrates an example of calibration development done off-line with a very limited number of prepared calibration samples. The value of the measurement is to control the end properties of the products resulting from the copolymerization reaction. The end properties are related to many parameters including the intramolecular chemical composition distribution (CCD). The... 

Figure 9.11 Theoretical chemical composition distribution (CCD) profiles for PMMA-g-PDMS copolymers with various graft molecular weights at constant composition (Podesva et al., 1987).

The reaction where vinylic monomers polymerize through coordination at the metallic center of some catalytic species is called coordination polymerization. Although the first catalytic system based on this kind of coordination chemistry was reported by Phillips Petroleum Co., most of the literature concerning coordination polymerization refers to the Ziegler-Natta catalysts because of their versatility in controlling chemical composition distribution (CCD) and of the wider variety of monomers they can polymerize [1]. 

In this section some of the methods used to analyse for the chemical composition (CQ and chemical composition distribution (CCD) of binary and ternary copolymers (henceforth termed copolymers and terpolymers respectively) are discussed. Some practical examples from the literature are given to illustrate the methods. Further, the determination of the chemical composition of copolymers as function of their molar mass as well as the three-dimensicxial combination of the MMD and CCD, namely MMCCD, are treated. Examples of both emulsion polymers and polymers produced by solution and bulk polymerization are given, because with respect to the determination of molar mass and chemical composition emulsion (co)polymers do not require an essentially different approach. 

Contrary to the usual organic compounds, polymers are far from being homogeneuos maferials (i.e., polymer chains do not possess the same molar mass and chemical structure). As matter of fact, many synthetic polymers are heterogeneous in several respects. Homopolymers may exhibit both molar-mass distribution (MMD) and end-groups (EG) distribution. Copolymers may also show chemical composition distribution (CCD) and functionality distribution (FTD) in addition to the MMD. Therefore, different kinds of heterogeneity need to be investigated in order to proceed to the structural and molecular characterization of polymeric materials. 

These metallocene/aluminoxane catalysts are considered "single site" catalysts and therefore produce PE with veiy narrow polydispersily index (close to Stockmayer s distribution where the polydispeisity index is 2) and narrow chemical composition distribution, CCD. They ate usefiil in producing HDPE, and LLDPE. However, one drawback to the metallocene/aluminoxane catalysts is that significant quantities of aluminoxane by-products must be removed after production, due to the high Al/Zr ratios required for optimum performance. The primary role of the aluminoxane is in formation of the active site species. (The initial stracture in the catalytic cycle shown in Figure 5.) The series of complexation and fast alkylation... 

In ordinary batch copolymerization there is usually a considerable drift in monomer composition because of different reactivities of the two monomers (based on the values of the reactivity ratios). This leads to a copolymer with a broad chemical composition distribution (CCD). In many cases (depending on the specific final product application) a composition drift as low as 3-5% cannot be tolerated, for example, copolymers for optical applications on the other hand, during production of GRIN (gradient index) lenses, a controlled traj ectory of copolymer composition is required. This is partly circumvented in semibatch operation where the composition drift can be minimized (i.e., copolymer composition can be kept constant ) by feeding a mixture of the monomers to the reactor with the same rate by which each of them is consumed in the reactor. 

It is well known that most copolymers have both molecular mass and composition distributions and that copolymer properties are affected by both composition and molecular mass distributions. Therefore, we must know average values of molecular mass and composition, and their distributions. These two distributions are inherently independent of each other. However, it is not easy to determine the molecular mass distribution (MMD) independently of the composition, or inversely, to determine the chemical composition distribution (CCD) independently of the molecular mass, even by modern techniques. 

The company Polymer Char (Valencia, Spain) was created for developing fully automated PO characterization instruments. The first device, commerciahzed and patented in 1994, was the CRYSTAF, crystallization analysis fractionation, for the fast measurement of the chemical composition distribution (CCD) in PE, PP, copolymers, and blends. Next came the SEC (with a quadruple detector system) and then SEC/a-TREF and p-TREF instruments. The first commercial, fully automated cross-fractionating SEC/TREF apparatus for microstructure characterization of POs was described by Ortin et al. (2007). The instrument yields a bivariate distribution CCD by TREE fractionation and then SEC fraction analysis in a single run. A schematic diagram of this new cross-fractionation instrument is shown in Fig. 18.7. 

Exxon Qiem. (Stehling et al. 1995) disclosed blends of linear ethylene interpolymer that comprised m-LLDPE and VLDPE (Appendix, Table 18.12). The polymers prepared using metallocene catalyst had narrow MWD, = 1-3, and a narrow chemical composition distribution (CCD), expressed by its index, CDBl > 50 %, and measured by TREE. The components could have the same MW but different BC, the same BC but different MW, or BC that increase with MW. The blends had either > 3 or CDBI < 50 % or both these... 

The complexity of molecular weight distributions (MWDs), chemical composition distributions (CCDs) and isotacticity distributions (IDs) of homo- and copolymers of propylene made with Ziegler-Natta catalysts constitutes a challenging problem for polymer quality control. These distributions affect the final mechanical and rheological properties of polypropylene (PP) and ultimately determine its applications. This issue becomes very complex with PP and copolymers of propylene and a-olefins made with heterogeneous Ziegler-Natta catalysts because polymers with broad and sometimes multimodal MWDs, CCDs and IDs can be produced. The major objective of propylene pol)onerization models is to predict these distributions and ultimately correlate them to mechanical and rheological properties [1]. 

Keywords polymerization kinetics, polymerization reactors, mathematical modelling, molecular weight distribution (MWD), chemical composition distribution (CCD), Ziegler-Natta catalysts, metallocenes, microstructure, isotacticity distribution, mass transfer resistances, heat transfer resistances, effects of multiple site types. 

More importantly, while it is very difficult to control the nature of the site t)rpes on conventional heterogeneous Ziegler-Natta catalysts, metallocene catalysts can be designed to synthesize PP with different chain microstructures. PP chains with atactic, isotactic, isotactic-stereoblock, atactic-stereoblock and hemiisotactic configurations can be produced with metallocene catalysts (Figure 2). It is also possible to s)mthesize PP chains that have optical activity by using only one of the enantiomeric forms of the catalyst. Additionally, copolymers of propylene, ethylene and higher a-olefins made with metallocene catalysts have random (or near random) comonomer incorporation and narrow chemical composition distributions (CCD). 

Liquid adsorption chromatography (LAV) and GPC have been used to determine chemical composition distribution (CCD) and MWD of styrene-methyacrylate copolymers (230). 

For ethylene/1-olefin copolymers, chain crystaUizabihty is mainly controlled by the fraction of noncrystalhzable comonomer imits in the chain. Consequently, the differential Crystaf profile shown in Fig. 1, together with an appropriate cahbration curve, can be used to estimate the copolymer chemical composition distribution (CCD), also called the short-chain branch distribution. The CCD of a copolymer describes the distribution of the... 

Fig. 2 Chemical composition distribution (CCD) of a typical Ziegler-Natta linear low-density polyethylene, reflecting the composition heterogeneity of these copolymers...

In order to evaluate the results, van Doremaele analysed the copolymers formed by means of high performance liquid chromatography (HPLC) providing detailed microstmctural information (viz., chemical composition distribution, CCD) of the copolymers. 

---