Microstructure, polypropylene

In order to identify differences in the polymer microstructures, polypropylenes containing similar amounts of isotactic mmmm pentads but prepared with either MAO (A26-A54, Table 9.2) or borate (B29-B51, Table 9.2) cocatalyst have been subjected to further analysis. No comparable differences were found in the samples NMR spectra or in their isotactic block lengths however, WAXS and differential scanning calorimetry (DSC) experiments revealed specific particularities corresponding to each activation method. 

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

Baum, R. Elastomeric Polypropylene Oscillating Catalyst Controls Microstructure, Chemical and Engineering News, Jan., 16, 1995, pp. 6-7. 

Figure 19.3 Crosshatched microstructure of alpha crystalline isotactrc polypropylene...

Figure 8 Schemes of (A) head-tail , (B) head-head and (C) tail-tail microstructural arrangements in polypropylene.

Chien JCW, Babu GN, Newmark RA, Cheng HH, Llinas GH (1992) Microstructure of elastomeric polypropylenes obtained with nonsymmetric ansa-titanocene catalysts. Macromolecules 25 7400-7402... 

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

In principle, therefore, the microstructure of such a polymer could be described by a sequence of m and r diads. However, the limits of NMR resolution is such that the microstructure can only be determined over a limited section of the polymer, usually no longer than 5-7 monomer units. NMR spectra on comparatively insoluble polymers such as polyethylene and polypropylene are usually recorded in 1,2,4-trichlorobenzene or tetrachloroethane-d2 at elevated temperatures (110-150 °C). 

The application of this technique to talc-reinforced polypropylene has shown that the microstructure of platelets and the resulting physical properties of the moulded composites are markedly affected [171)]. With two live feeds located at either end of the mould cavity, the talc platelets exhibited strong talc platelet alignment throughout the thickness of the moulding in the direction of the ap-... 

Microstructure-property correlations in dynamically vulcanized thermoplastic elastomers based on polypropylene (PP)/EPDM have shown that clay was nearly exfoliated and randomly distributed into the continuous polypropylene phase [23]. SEM photomicrographs revealed that the size of rubber particles increased with clay incorporation. Also, the clay layers act as nucleating agents, resulting in higher crystallization temperature and reduced degree of crystallinity. 

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

Table 10. Microstructure of polypropylene oxide-a-d prepared by ZnEt2—H20 (1 0.7) catalyst system...

Another example is illustrated in the relationship between the specific rotation and the microstructure of polypropylene oxide reported by Price. Optically active propylene oxide and racemic propylene oxide-a-d were polymerized under otherwise identical conditions by the freeze-dried ZnEt2-H20 (1 0.7) catalyst system containing varying amounts of ZnEt2. A linear relationship was observed between specific rotation of the former polymer and the tail-to-tail dyad content of the latter (Fig. 14). This result proves quantitatively that the decrease in the specific rotation of polymer prepared by several catalysts is due to the presence of head-to-head and tail-to-tail linkages, and also provides supporting evidence for our microstructure analysis. 

Oguni.N, Lee,K., Tani,H. Microstructure analysis of polypropylene oxide by 13C-nmr spectroscopy. Macromolecules, in press. 

In addition, borane-containing POs can be prepared by copolymerization of olefin with borane monomers or by hydroboration of polyolefins including unsaturated groups, such as olefin-divinylbenzene copolymer and olefin-diene copolymers. Many kinds of graft copolymers, such as poly-elhylene-gra/f-poly( vinyl alcohol), PE-g-PMMA, polypropylcnc-gra/f-poly-(maleicanhydride-co-styrene), polypropylene-gra/f-poly(methacrylic acid), polypropylene-gra/f-poly(vinyl alcohol), polypropylene-gra/f-polycaprolac-tone (PP-g-PCL), polypropylcnc-gra/f-poly(methyl methacrylate) (PP-g-PMMA), poly( ethylene-co-propylene)-gra/f-poly(methyl methacrylate) (EPR-g-PMMA), and poly(ethylene-co-propylene)-gra/f-poly(maleic anhydride-costyrene), have been synthesized by such a method resulting in controllable composition and molecular microstructures [63-66]. 

The range of polypropylene microstructures available by procatalyst ligand (and/or metal) variation in the metallocene-methylaluminoxane system is illustrated in Table 3.1 [22,23,101,105,107,112,113,124,127,132,137], The ligands shown in Table 3.1, representative of the particular symmetry and class of the catalyst, are given as examples only. There are a variety of other metallocenes that have been successfully used to obtain polypropylenes of various stereostructures. 

Table 3.2 Non-bridged bent metallocene procatalysts and the polypropylene microstructures...

Therefore, according to Razavi et al. [127,143], chain migratory insertions and chain stationary insertions would follow each other alternately, probably involving a pen-penultimate mechanism dictated by the periodicity of the helices of the growing chain [413]. If such a coordination site switching mechanism intervenes periodically, the resulting polypropylene chain would expose a syndioisoblock microstructure. 

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