Copolymers, ethylene-propylene polymerization mechanism

Since the discovery of olefin polymerization using the Ziegler-Natta eatalyst, polyolefin has become one of the most important polymers produeed industrially. In particular, polyethylene, polypropylene and ethylene-propylene copolymers have been widely used as commercial products. High resolution solution NMR has become the most powerful analytieal method used to investigate the microstructures of these polymers. It is well known that the tacticity and comonomer sequence distribution are important factors for determining the mechanical properties of these copolymers. Furthermore, information on polymer microstructures from the analysis of solution NMR has added to an understanding of the mechanism of polymerization. 

Chapter 22 presents the different types of polymers produced from ethylene, propylene, and copolymers of these olefins with other monomers, along with some basic principles that apply to polyolefin synthesis and characterization. The basic features of the mechanism of the polymerization are presented first to provide a framework for the description of different catalysts and materials. After presenting different types of catalysts that have been used for the polymerization of these monomers, a more detailed description of several features of the mechanism is presented. Finally, an overview of ethylene oligomerization, as well as diene polymerization and oligomerization, is presented. The primary journal literature and patent literature on olefin polymerization is immense. Fortunately, many reviews of olefin polymerization have been published. The coverage of olefin polymerization and oligomerization in this chapter is selective, and the reader is directed to review articles for more comprehensive coverage of these topics. " ... 

This unified volume explains the mechanistic basics of tactic polymerizations, beginning with an extensive survey of the most important classes of metallocene and post-metallocene catalysts used to make polypropylenes. It also focuses on tactic stereoblock and ethylene/propylene copolymers and catalyst active site models, followed by chapters discussing the structure of more stereochemically complex polymers and polymerizations that proceed via non-vinyl-addition mechanisms. Individual chapters thoroughly describe tactic polymerizations of a-olefins, styrene, dienes, acetylenes, lactides, epoxides, acrylates, and cyclic monomers, as well as cyclopolymerizations and ditactic structures, olefin/CO copolymers, and metathesis polyalkenamers. 

Although NMR spectroscopy is a powerful method for studying the structure of different vinyl copolymers i0l its usefulness for the olehn copolymers is rather limit The main reason is undoubtedly the strong overlap between the resonances of the main-chain protons and those of the pendant groups. Unfortunately, this situation is unlikely to be improved by increasing the spectrometer frequency. One of tte possible solutions to this problem is copolymerization with fully or partially deuterated monomers. The only exceptions to this unfortunate situation are ethylene-propylene copolymers and some styrene copolymers, and here important information about the copolymer structures was obtained, including data relevant to the mechanism of stereosp dfic polymerization (see Section IV.A.1 b). 

Another widely used approach is the in situ polymerization of an intractable polymer such as polypyrrole onto a polymer matrix with some degree of processibil-ity. Bjorklund [30] reported the formation of polypyrrole on methylcellulose and studied the kinetics of the in situ polymerization. Likewise, Gregory et al. [31] reported that conductive fabrics can be prepared by the in situ polymerization of either pyrrole or aniline onto textile substrates. The fabrics obtained by this process maintain the mechanical properties of the substrate and have reasonable surface conductivities. In situ polymerization of acetylene within swollen matrices such as polyethylene, polybutadiene, block copolymers of styrene and diene, and ethylene-propylene-diene terpolymers have also been investigated [32,33]. For example, when a stretched polyacetylene-polybutadiene composite prepared by this approach was iodine-doped, it had a conductivity of around 575 S/cm and excellent environmental stability due to the encapsulation of the ICP [34]. Likewise, composites of polypyrrole and polythiophene prepared by in situ polymerization in matrices such as poly(vinyl chloride), poly(vinyl alcohol), poly(vinylidine chloride-( o-trifluoroethylene), and brominated poly(vi-nyl carbazole) have also been reported. The conductivity of these composites can reach up to 60 S/cm when they are doped with appropriate species [10]. 

The commercial production of LLDPE relies on copolymetiza-tion with a-olefins such as 1-butene, 1-hexene, 1-oaene, and, to a smaller degree, 4-methyl-l-pentene. Prop ylene is not employed for LLDPE because the relatively small and sparse methyl groups of ethylene/propylene copolymers are accommodated in the crystal and are far inferior to butyl and hexyl groups (from 1-hexene and 1-octene, respertively) for the improvement of mechanical properties. Very long a-olefins, such as 1 -hexadecene, have been investigated for creating specialized LLDPE that shares properties with commercial LDPE made via free radical polymerization. ... 

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