Polyolefins tactic

FIGURE 6.3 The crystallinity and composition continuum for ethylene- and propylene-dominated polyolefins. Note the dispersion for the propylene-dominated polyolefins due to much-greater prevalence of blends and the presence of tacticity derived changes in crystallinity. 

Like polypropylene, PVC has the problem of stereospecificity. The carbon atom to which the chlorine atom is attached is asymmetrical. (See Figure 23-8.) As a result, PVC molecules can be iso tactic, syndiotactic, and atactic. Commercial PVC is only 5—10% crystalline—low percent isotactic. It is more dense, 1.3 to 1..8 g/cc, than the polyolefins, (fe Figure 23—9.)... 

A major breakthrough in polymer production occurred with the discovery of metallocene catalysts [1]. We are now able to make polyolefins with a controlled level of branching (and tacticity). The simplest object is a statistically branched polymer, with a certain overall degree of polymerisation X, and a certain distance (monomer units) between successive branch points, which we shall call b. The basic goal of characterisation is to measure X and b from a minimum number of experiments in dilute solutions. 

By contrast, other polymers such as polyolefins and PVA form immiscible blends with HKL. In fact, phase separation is observed in fibers produced from the thermal spinning of HKL and PP. The fiber spinning properties of several PP samples having various melt viscosities and tacticities were examined. Most of the HKL/PP blends show poor or bad fiber spinnability. However, excellent fiber spinning was achieved with PP samples having a thermal viscosity comparable to the HKL. 

In the past decade, our group at Penn State has been focusing on a functionalization approach by the combination of metallocene catalysts and reactive comonomers. The chemistry takes the advantage of metallocene catalyst with a tunable single active site to prepare polyolefin copolymers with narrow molecular weight and composition distributions, high catalyst activities, and predictable tacticities and copolymer compositions. 

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. 

Table 6.8. Melting temperatures (Tm, second and third editions of Ref. [30]), in degrees Kelvin (K). Wide ranges of Tm values are reported for many polymers because of factors such as differences in types, sizes and quality of crystalline regions, difficulties in discerning Tm in some polymers of very low maximum percent crystallinity, and differences in tacticity. In fact, for many of the polymers for which only one Tm value is listed, a measured Tm value has only been reported once in the literature. The list is ordered by polymer type (polyolefins, polydienes, polystyrenes, polyxylylenes, polyethers, polyacrylates, polymethacrylates, various polyvinyls, polysulfides, polyesters, polyamides, polycarbonates, and others). 

These homopolymers have structure units as shown in Fig. 2.1, where the asterisk indicates asymmetric carbon atoms. Thus, polypropylene (PP), poly (butene-1) (PB1), and poly(4-methylpentene-l) (P4MP1) have different tactic forms. The most important commercial polyolefins are polyethylene, polyisobutene, and the isotactic forms, that is, iPP, iPBl, and iP4MPl. Polyisobutene was first polymerized by the IG Farbenindustries (BASF) in the late 1920s. Polyethylene was first polymerized by ICI in the late 1930s in a branched form (1). Linear polyethylene... 

There is a history of investigations of blends of polyolefins. Many of these blends were not produced on purpose but were the results of incompletely understood polymerization processes. Examples of these are the various smdies of elastomeric polypropylenes, which are mixtures of polypropylenes of varying tacticity levels (67-71). These materials often have interesting technological properties. It is of more concern to consider the results of specially prepared blends of known characterized polymers. 

The major application significance, and resulting economic importance, of the polyolefins were not completely satisfactory. The main characteristics of this material included a molar mass distribution that was very wide, in addition to which its tacticity was not uniform. These aspects affected its properties and processing and restricted its applicability. The cause of these difficulties lay in the fact that the Z/N catalysts comprised solid bodies on whose surface large numbers of chains grew at varying rates, resulting in wide molar mass distribution. 

The use of statistical models to interpret (and to rationalize) NMR tacticity and sequence data is well established (97,98). In this volume the enantiomorphic-site model has been used by Segre et al. in their studies of polypropylene at high fields (55). A two-site model has been en loyed by Shimozawa et al. to observe the effects of internal donors m propylene polymerization (56). Other models for polyolefins have been reported in the literature, e.g., the multi-site model (99), the dual catalytic-site/chain-end model (100), the perturbed model (101), the consecutive two-site model (102), the four-component model (103), and the chain end model (104). 

Crystalline polyolefins as polypropylene (PP) with high tacticity and copolymers of propylene with ethylene and/or higher a-olefins containing sufficiently long isotactic PP blocks can only be dissolved under conditions (solvent/temperature) which cause complete melting of the crystalline domains. Therefore, SEC of PP and crystalline copolymers of propylene must be carried out at elevated temperatures which requires the special equipment of high-temperature SEC (HT-SEC) which is commercially available from several sources, e.g. Millipore-Waters Corp. (Milford, MA, USA) and Polymer Laboratories Ltd (Church Stretton, Shropshire, UK). 

The final part of this book features tactic polymerizations of functional and nonolefinic (ring-opening) monomers— materials for which many aspects of polymer stereochemistry and microstructure control are very different than for simple polyolefins. Acrylate, epoxide, and lactide polymerizations are addressed in this part, along with tactic olefin/carbon monoxide co- and terpoly-mers. These final chapters provide an expanded view of polymer microstructures and stereocontrol strategies, such as enantiomer-selective polymerization, that may be less familiar to the polyolefin-minded chemist and serve to enhance the reader s overall understanding of stereoregular polymers and polymerization. 

Polyolefins, PO. First impact modification of PO, by addition of elastomers, was patented independently by Bayer A.-G. and Standard Oil Co. in 1937. The isotactic polypropylene, PP, was commercialized in 1957, and its first blends (with polyisobutylene, PIB, and polyethylene, PE) were patented in 1958. In 1960, du Pont started manufacturing ethylene-propylene, EPR, and three years later ethylene-propylene-diene, EPDM, copolymers [Gresham and Hunt, I960]. The first patent on impact modification of PP by addition of EPR dates from 1960. Direct reactor blending of PE/PP/EPR resulting in a thermoplastic polyolefin, R-TPO, dates from 1979. The newest (introduced in 1992) single-site metallocene catalysts generate polymers with controlled tacticity, co-monomer sequences, molecular... 

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