Isotactic polymers various

Figure 2.10 Maps of conformational energy of various isotactic polymers as function of backbone torsion angles 0i and 02 (a) Isotactic polystyrene, (b) polypropylene, (c) poly(l-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i020i02 [s(M/N) symmetry] has been assumed. Isoenergetic curves are reported every 10 (a,c,d) or 5 (b) kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero.

Data concerning the chain conformations of isotactic polymers are reported in Table 2.1. In all the observed cases the torsion angles do not deviate more than 20° from the staggered (60° and 180°) values and the number of monomeric units per turn MIN ranges between 3 and 4. Chains of 3-substituted polyolefins, like poly(3-methyl-l-butene), assume a 4/1 helical conformation (T G )4,45,46 while 4-substituted polyolefins, like poly(4-methyl-1-pentene), have less distorted helices with 7/2 symmetry (T G )3.5-39 When the substituent on the side group is far from the chain atoms, as in poly(5-methyl-1-hexene), the polymer crystallizes again with a threefold helical conformation (Table 2.1). Models of the chain conformations found for the polymorphic forms of various isotactic polymers are reported in Figure 2.11. 

Hie study of effects of the catalyst components also help clarify the ionic factors in the steric control of isotactic polyvinylethers. Dall Asta and Bassi (15) studied the polymerization of butylvinylether with various alkylaluminum halides. They found that diethylaluminum chloride and ethylaluminum dichloride were the most effective catalysts for the production of isotactic polymer. Ethylaluminum dibromide and ethoxyaluminum dichloride were of questionable effectiveness, while diethylaluminum fluoride was completely ineffective. 

A wide variety of homogeneous and hetrogeneous systems are effective for polymerizing various monomers to isotactic structures. Therefore the question must be raised as to whether macro-surface theories have any required validity for steric control but whether some other factor is important. Fig. 10 summarizes the monomers polymerized to isotactic polymers and shows that all propagate with an asymmetric center at the end of the chain. 

The polymerisation of styrene, which is an exceptionally versatile monomer, in the presence of various Ziegler-Natta and related coordination catalysts produces high molecular weight polystyrenes, both highly isotactic polymers [1-4] and highly syndiotactic polymers [5-10]. 

The first report on the coordination polymerisation of epoxide, leading to a stereoregular (isotactic) polymer, concerned the polymerisation of propylene oxide in the presence of a ferric chloride-propylene oxide catalyst the respective patent appeared in 1955 [13]. In this catalyst, which is referred to as the Pruitt Baggett adduct of the general formula Cl(C3H60)vFe(Cl)(0C3H6),CI, two substituents of the alcoholate type formed by the addition of propylene oxide to Fe Cl bonds and one chlorine atom at the iron atom are present [14]. A few years later, various types of catalyst effective for stereoselective polymerisation of propylene oxide were found and developed aluminium isopropoxide-zinc chloride [15], dialkylzinc-water [16], dialkylzinc alcohol [16], trialkylalumi-nium water [17] and trialkylaluminium-water acetylacetone [18] and trialkyla-luminium lanthanide triacetylacetonate H20 [19]. Other important catalysts for the stereoselective polymerisation of propylene oxide, such as bimetallic /1-oxoalkoxides of the [(R0)2A10]2Zn type, were obtained by condensation of zinc acetate with aluminium isopropoxide in a 1 2 molar ratio of reactants [20-22]. 

The dependence of the protonation degree on the neutralization degree has been determined for various acids from spectroscopic and conductivity measurements 75). It was found that the protonation behaviour is only slightly dependent on the nature of the counterions, and on the nature of the macromoiecular chain (the isotactic polymers are less protonated, but the difference is very small) while a large amount... 

If interactions of an electrostatic nature have some effect on the relative stability of trans and gauche rotational isomers, it seems reasonable to expect an effect of solvent, though perhaps small, on the factor a. Such effects on the conformations of small molecules are well known [see Mizushima (14,188) and Wada (259)]. The relatively high value of a for poly(methyl methacrylate), as discussed in paragraph (iv) above, may be due to electrostatic interactions, and it is therefore appropriate to search for a solvent effect here. No such effect was found by Marchal and Lapp (175) in their measurements of the apparent dipole moment of the polymer in various solvents as compared with that of the model monomeric compound, methyl isobutyrate. They concluded that to within experimental error (about 3%) the unperturbed dimensions were independent of the solvent. More recently a very small solvent effect on the dipole moment of the isotactic polymer has been reported by Salovey (223a). 

Currently this technology is of minor commercial significance, but stereoregular forms of numerous polyacrylates have been prepared and characterized These include poly(/-butyl acrylate) (138—141), poly(isopropyl acrylate) (142), and poly(isobutyl acrylate) (143,144). Carefully controlled reaction conditions are usually required to obtain polymers with some measurable degree of crystallinity. In nonpolar solvents the anionic polymerization of acrylates generally yields isotactic polymer, whereas in polar solvents syndiotactie polymerization is favored. The physical and chemical properties of the various forms are often quite different. A general review covers these and other aspects of the anionic polymerization of acrylates (145). 

Proton NMR spectroscopy has been used to characterize the tacticity of various vinyl polymers in solution. In the case of isotactic polymers, there are two magnetically non-equivalent protons (Figure 7-34) and, as we discussed earlier in this chapter, this can result in the appearance of four bands (the chemical shift difference is of the same order of magnitude as the coupling constant, so the simple rules for mnltiplicities don t apply and we get what we called an AB pattern). On the other hand, in syndiotactic polymers the two methylene protons are equivalent and we observe only one line. Let s look at this in more detail, using poly(methyl methacrylate) (PMMA), as an example, because bands due to various tactic sequences are particularly well resolved in the spectrum of this material. 

FIGURE 6.4 A diagrammatic representation of various other ordered helical structures adopted by isotactic polymers. (From Natta, G. and Corradird, V., Rubber Chem. TechnoL, 33,703,1960. With permission.)... 

Figure 4-8. Schematic representation of the different kinds of helices occurring in various isotactic polymers 4-CH2—CHR->7i. (1) 3i (II) (111, IV) 4i (after G. Natta, P. Corradini, and...

Isotactic polymers with two chain atoms per monomeric unit thus tend to occur in more or less ideal TG conformations. In addition, the low energy difference for slight deviations from the ideal torsion angle can lead to various helix types. Rapid crystallization of it-poly (butene-1) produces a 4i helix, for example, which, as a high-energy form, changes into a 3i helix on annealing (see also Chapter 10). 

In the present paper, differences between isotactic PMEPL and the stereocomplex are examined by solid state nuclear magnetic resonance (NMR) spectroscopy. These studies reveal new polymorphic behaviour of the isotactic polymer and differences in crystal structure which depend on tacticity. Crystal structures of the various polymorphs were also determined by electron and x-ray diffraction studies. 

A stereospecific anionic polymerization of acetaldehyde was originally reported in 1960 [343, 344]. Two alkali metal compounds [341] and an organozinc [342] one were used as the initiators. Trialkylaluminum and triarylaluminum in heptane also yield crystalline, isotactic polymers from acetaldehyde, heptaldehyde, and propionaldehyde at -80°C [343]. Aluminum oxide, activated by diethylzinc, yields stereoblock crystalline polymers from various aldehydes [342, 344], Lithium alkoxide formed polyacetaldehyde is insoluble in common solvents. It melts at 165°C [341],... 

For the various pol)oners that we have prepared and which are reported in Table 2, molecular weights larger than 20 kg/mole can often be measured. However, the isotactic polymers are insoluble in most organic solvents and their molecular weights difficult to measure. The only appropriate solvents for the isotactic polymers (and not for all of them) are hexafluoroiso-propanol, trifluoroacetic acid, and trifluoroethanol. 

The polymerization of 4-MPD to give 1,2-isotactic polymer has been performed with various heterogeneous titanium catalysts The product in this case contains a portion (20-30%) of soluble, amorphous 1,4 polymer. The insoluble polymer material is highly crystalline with a Tm of... 

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