Polymers atactic polypropylene

Study the chain dynamics in the amorphous polymer atactic polypropylene. " The arrest of the structural relaxation above the glass-transition temperature Tg was investigated using ID and 2D deuteron NMR spectra. 

Considering again two adjacent carbons in the main chain of the polymer, six conformations are now possible because of the presence of an asymmetrically substituted carbon atom, as shown in Fig. 2.7. Forms 1 and 6 can be neglected for steric reasons so four different conformations are still possible for the polymer. Atactic polypropylene (see Stereoisomerism) has two trans forms (2 and 5) in the fully extended state (III)... 

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. 

Modified Bitumen Membranes. These membranes were developed in Europe during the late 1950s and have been used in the United States since the late 1970s. There are two basic types of modified asphalts and two types of reinforcement used in the membranes. The two polymeric modifiers used are atactic polypropylene (APP) and styrene—butadiene—styrene (SBS). APP is a thermoplastic polymer, whereas SBS is an elastomer (see Elastomers, thermoplastic elastomers). These modified asphalts have very different physical properties that affect the reinforcements used. 

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

The regular syndiotactic and isotactic structures are capable of crystallisation whereas the atactic polymer carmot normally do so. In the case of polypropylene the isotactic material is a crystalline fibre-forming material. It is also an important thermoplastic which can withstand boiling water for prolonged periods. Atactic polypropylene is a dead amorphous material. Polystyrene as commonly encountered is atactic and glass-like but the syndiotactic material... 

For many years atactic polypropylene was an unwanted by-product but today it finds use in a number of markets and is specially made for these purposes rather than being a by-product. In Europe the main use has been in conjuction with bitumen as coating compounds for roofing materials, for sealing strips where it confers improved aging properties and in road construction where it improves the stability of asphalt surfaces. Less important in Europe but more important in USA is its use for paper laminating for which low-viscosity polymers are used, often in conjunction with other resins. Limestone/atactic... 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

A new generation coordination catalysts are metallocenes. The chiral form of metallocene produces isotactic polypropylene, whereas the achiral form produces atactic polypropylene. As the ligands rotate, the catalyst produces alternating blocks of isotactic and atactic polymer much like a miniature sewing machine which switches back and forth between two different kinds of stitches. 

Polypropylene (PP) is a major thermoplastic polymer. Although polypropylene did not take its position among the large volume polymers until fairly recently, it is currently the third largest thermoplastic after PVC. The delay in polypropylene development may be attributed to technical reasons related to its polymerization. Polypropylene produced by free radical initiation is mainly the atactic form. Due to its low crystallinity, it is not suitable for thermoplastic or fiber use. The turning point in polypropylene production was the development of a Ziegler-type catalyst by Natta to produce the stereoregular form (isotactic). 

Problem 31.5 Polymers such as polypropylene contain a large number of chirality centers. Would you therefore expect samples of isotactic, syndiotactic, or atactic polypropylene to rotate plane-polarized light Explain. 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

During solution polymerization the monomer, catalyst, and diluent are introduced to a reactor maintained at a temperature between 175 and 250 °C. The resulting polymer forms a viscous solution in the solvent, which is pumped out of the reactor. If necessary, the solution can be filtered to remove the catalyst residue. Solvent is removed from the solution and recycled, leaving behind a mixture of isotactic and a small amount of atactic polypropylene. 

The polymerizations occur at much lower pressures, and the polymers that are produced are much higher melting than atactic polypropylene, i) Isotactic polypropylenemelts at 175 °C. 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

Polymers are very good glass formers, with a few notable exceptions. For some polymers, such as atactic polypropylene or random copolymers like... 

For syndiotactic polypropylene the symmetric Bernoulli trial, expressed in m and r dyads, is quite adequate for the representation of experimental data, and agrees with the stereochemical control being exerted by the growing chain end (145, 409). In its turn, atactic polypropylene is considered as a mixture of the products of two superposed processes, of the type discussed for isotactic and syndiotactic polymers, and is described by a simplified two-state model (145). 

Atomistic simulation of an atactic polypropylene/graphite interface has shown that the local structure of the polymer in the vicinity of the surface is different in many ways from that of the corresponding bulk. Near the solid surface the density profile of the polymer displays a local maximum, the backbone bonds of the polymer chains develop considerable parallel orientation to the surface [52]. This parallel orientation due to adsorption can be one of the reasons for the transcrystallinity observed in the case of many anisotropic filler particles. 

Fio. 32. Relation between the reciprocal of the intrinsic viscosity raised to 1.35 (proportional to the reciprocal of polymerization degree) of the non-atactic polypropylene fraction and the number of mol. of —CjHs groups found in the polymer. (Tests performed with >[Pg.42]

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