Polymers isotactic

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

The successive repeat units in strucutres [VI]-[VIII] are of two different kinds. If they were labeled Mj and M2, we would find that, as far as microstructure is concerned, isotactic polymers are formally the same as homopolymers, syndiotactic polymers are formally the same as alternating copolymers, and atactic polymers are formally the same as random copolymers. The analog of block copolymers, stereoblock polymers, also exist. Instead of using Mj and M2 to differentiate between the two kinds of repeat units, we shall use the letters D and L as we did in Chap. I. 

The peaks centered at 5 = 1.84 ppm-a singlet in the syndiotactic and a quartet in the isotactic polymers-are thus identified with these protons. This provides an unambiguous identification of the predominant stereoregularity of these samples. 

Working from the methylene assignments, we see that the peak at 5 = 1.22 ppm in the isotactic polymer arises from the methyl in the center of an... 

Polyacetaldehyde, a mbbery polymer with an acetal stmcture, was first discovered in 1936 (49,50). More recentiy, it has been shown that a white, nontacky, and highly elastic polymer can be formed by cationic polymerization using BF in Hquid ethylene (51). At temperatures below —75° C using anionic initiators, such as metal alkyls in a hydrocarbon solvent, a crystalline, isotactic polymer is obtained (52). This polymer also has an acetal [poly(oxymethylene)] stmcture. Molecular weights in the range of 800,000—3,000,000 have been reported. Polyacetaldehyde is unstable and depolymerizes in a few days to acetaldehyde. The methods used for stabilizing polyformaldehyde have not been successful with poly acetaldehyde and the polymer has no practical significance (see Acetalresins). 

Crystallinity of polypropylene is usually determined by x-ray diffraction (21). Isotactic polymer consists of heHcal molecules, with three monomer units pet chain unit, resulting in a spacing between units of identical conformation of 0.65 nm (Fig. 2a). These molecules interact with others, or different... 

Syndiotactic polypropylene has an ultimate melting point of 174°C, and extrapolated heat of fusion of 105 J/g (25.1 cal/g) both lower than those of isotactic polymer. The heat of fusion of the polymer produced using a metallocene catalyst is reported as 79 J/g (19 cal/g) (41). 

The synthesis of isotactic polymers of higher a-olefins was discovered in 1955, simultaneously with the synthesis of isotactic PP (1,2) syndiotactic polymers of higher a-olefins were first prepared in 1990 (3,4). The first commercial production of isotactic poly(l-butene) [9003-29-6] (PB) and poly(4-methyl-l-pentene) [9016-80-2] (PMP) started in 1965 (5). 

Zirconocene complexes containing two indenyl or tetrahydroindenyl groups bridged with short links such as —CH2—CH2— or —Si(CH3)— can produce isotactic polymers of higher a-olefins (32). To synthesize syndiotactic PO, bridged zirconocene complexes with rings of two different types are required, one example of which is isopropyl(cyclopentadienyl)(l-fluorenyl)zirconocene. These complexes are used for the synthesis of syndiotactic PB... 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

The syndiotactic polymer configuration is not obtained in pure form from polymerizations carried out above 20°C and, thus has not been a serious concern to most propylene polymerization catalyst designers. Eor most commercial appHcations of polypropylene, a resin with 96+% isotacticity is desired. Carbon-13 nmr can be used to estimate the isotactic fraction in a polypropylene sample. Another common analytical method is to dissolve the sample in boiling xylene and measure the amount of isotactic polymer that precipitates on cooling. 

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. 

Triethyl aluminum, complexed with another electron donor, typically ethyl -anisate [94-30-4J, was used as cocatalyst with the FT-1 catalyst and acted to reduce and stabilize the active titanium-containing catalytic site. The early versions of the FT-1 catalyst required extremely high molar ratios (>400 1) of aluminum to titanium to obtain satisfactory activity and selectivity to isotactic polymer. This resulted in excessively high aluminum residues in the polymer. Later versions of the FT-1 catalyst attained much higher activity. 

Detailed modifications in the polymerisation procedure have led to continuing developments in the materials available. For example in the 1990s greater understanding of the crystalline nature of isotactic polymers gave rise to developments of enhanced flexural modulus (up to 2300 MPa). Greater control of molecular weight distribution has led to broad MWD polymers produced by use of twin-reactors, and very narrow MWD polymers by use of metallocenes (see below). There is current interest in the production of polymers with a bimodal MWD (for explanations see the Appendix to Chapter 4). 

Substituents on the a-carbon atom restrict chain flexibility but, being relatively small, lead to a significantly higher Tg than with polyethylene. Differences in the Tg s of commercial polymers (approx. 104°C), syndiotactic polymers (approx. 115°C) and anionically prepared isotactic polymers (45°C) are generally ascribed to the differences in intermolecular dipole forces acting through the polar groups. 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

The brittleness of isotactic polystyrenes has hindered their commercial development. Quoted Izod impact strengths are only 20% that of conventional amorphous polymer. Impact strength double that of the amorphous material has, however, been claimed when isotactic polymer is blended with a synthetic rubber or a polyolefin. 

Whereas the isotactic polymer has not been commercialised Dow were scheduled to bring on stream plant with a nameplate capacity of 37 000 t.p.a. in 1999 to produce a syndiotactic polystyrene under the trade name Questra. The particular features of this material are ... 

Isotactic polymer (Section 7.15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain. 

Stereoregular polymer (Section 7.15) Polymer containing chirality centers according to a regular repeating pattern. Syndiotactic and isotactic polymers are stereoregular. 

The properties of commercial polypropylene vary widely according to the percentage of crystalline isotactic polymer and the degree of polymerization. Polypropylenes with a 99% isotactic index are currently produced. 

An alternative method for generating enriched 1,2-diols from meso-epoxides consists of asymmetric copolymerization with carbon dioxide. Nozaki demonstrated that a zinc complex formed in situ from diethylzinc and diphenylprolinol catalyzed the copolymerization with cyclohexene oxide in high yield. Alkaline hydrolysis of the isotactic polymer then liberated the trans diol in 94% yield and 70% ee (Scheme 7.20) [40]. Coates later found that other zinc complexes such as 12 are also effective in forming isotactic polymers [41-42]. 

A polymorphic behavior involving packing of chains having completely different conformations has been found also for isotactic polymers. For instance, isotactic polystyrene, under suitable experimental conditions, can produce crystalline gels in which the chains assume a nearly fully extended conformation [11,12], very close to a truns-planar, rather than the classical conformation of three-fold helix [13]. The two possible conformations proposed for the two crystalline forms of i-PS are shown in Fig. 2. 

Polymerization of t-butyl methacrylate initiated by lithium compounds in toluene yields 100% isotactic polymers 64,65), and significantly, of a nearly uniform molecular-weight, while the isotactic polymethyl methacrylate formed under these conditions has a bimodal distribution. Significantly, the propagation of the lithium pairs of the t-Bu ester carbanion, is faster in toluene than in THF. In hydrocarbon solvents the monomers seem to interact strongly with the Li+ cations in the transition state of the addition, while the conventional direct monomer interaction with carbanions, that requires partial dissociation of ion-pair in the transition state of propagation, governs the addition in ethereal solvents. 

Okamoto and his colleagues60) described the interesting polymerization of tri-phenylmethyl methacrylate. The bulkiness of this group affects the reactivity and the mode of placement of this monomer. The anionic polymerization yields a highly isotactic polymer, whether the reaction proceeds in toluene or in THF. In fact, even radical polymerization of this monomer yields polymers of relatively high isotacticity. Anionic polymerization of triphenylmethyl methacrylate initiated by optically active initiators e.g. PhN(CH2Ph)Li, or the sparteine-BuLi complex, produces an optically active polymer 60). Its optical activity is attributed to the chirality of the helix structure maintained in solution. 

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