Poly Polypropylene, syndiotactic

Being acquainted with the structure of poly(a-olefin)s, one may reasonably explain some of the differences in their physicochemical properties. For example, isotactic polypropylene, the chains of which in the helical conformation can be closely packed, has rather a high density (0.92-0.94 g/cm3) and melting point (175°C) and is insoluble in low-boiling aliphatic hydrocarbons at boiling point. Syndiotactic polypropylene, consisting of chains in the form of binary helices, which cannot be packed so closely as in the previous case, has a density of 0.89-0.91 g/cm3 and a melting point of 135°C, which is 40 k lower from that of isotactic polypropylene syndiotactic polypropylene is also moderately soluble in... 

Poly(2,6-dimethyl-l,4-phenylene ether) 298 18.1 Polypropylene, syndiotactic 298 17.6... 

Polypropylene, isotactic Polypropylene, syndiotactic Poly (tetrafluoroethy lene)... 

FIGURE 7 16 Poly mers of propene The mam chain IS shown in a zigzag conformation Every other carbon bears a methyl sub stituent and is a chirality center (a) All the methyl groups are on the same side of the carbon chain in isotactic polypropylene (b) Methyl groups alternate from one side to the other in syndiotactic polypropy lene (c) The spatial orienta tion of the methyl groups IS random in atactic polypropylene... 

Polymers that incorporate steric centers into their backbones can display various types of tacticity. The three principal types of tacticity are isotactic, syndiotactic, and atactic, as illustrated in Fig. 1.8 for polypropylene. Other polymers that display tacticity include polystyrene and poly a-olefins,... 

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

Recently, a similar analysis of the conformational energy has been performed also for various new syndiotactic polymers.27,47 The conformational energy maps of syndiotactic polypropylene (sPP),48 polystyrene (sPS),49 poly butene (sPB),25 and poly(4-methyl-l-pentene) (sP4MP)26 are reported in Figure 2.12. A line repetition group s(M/N)2 for the polymer chain, and, hence, a succession of the torsion angles. .. 0i, 0i, 02, 02,..., has been... 

Figure 2.12 Maps of conformational energy of various syndiotactic polymers as function of backbone torsion angles 0 and 0227 (a) syndiotactic polystyrene, (b) polypropylene, (c) poly (1-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i 0i 0202 - - -[s(M/N)2 symmetry] has been assumed. Isoenergetic curves are reported every 5 kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero. Values of energies corresponding to minima (x) are also indicated. Experimental conformations observed for different polymorphic forms of polymers are indicated by triangles. (Reproduced with permission from Ref. 27. Copyright 1992 by the Socicta Chimica Italiana.)...

There are three principal stereochemical types of poly(l-alkene)s, illustrated in Scheme 8.38 for polypropylene. In isotactic polypropylene 80 (i-PP) all methyl substituents have the same relative orientation (m). The scheme shows the stereochemistry with the usual Fischer projection underneath. In syndiotactic PP (81, s-PP) every second CHMe unit has the opposite stereochemistry to the first, while in atactic PP (82, a-PP) the orientation of the methyl substituents is random. In some polymers there is partial order, i. e. only every second monomer orientation is random (83, hemi-isotactic PP). 

In all of the above discussions we have treated only the monomers with carbon-carbon double-bonds. It is probable that polymers of non-oleftnic monomers such as polypropylene oxide (103, 104, 105) poly-ethylidene (106, 107, 108, 109) and polyaldehydes (110, 111) polymerize to isotactic structures by the same mechanism. The same correlation of ionicity of the catalysts with the isotactic structures and syndiotactic structures should also be possible. 

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

The affect of polymer stereoregularity in the chains on the PAL data has also been studied. Hamielec et al [56] found what appears to be an increased lifetime (hole size) with increased randomness of the chain configuration in a series of polyvinlychloride (PVC) polymers, despite the large degree of scatter in the sample (probably due to the fact that a series of commercially available products were used.). They however found little correlation with tacticity in polypropylene. More recently a PAL study on a series of very well characterized polystyrene and poly(p-methlystyrene) samples of differing tacticity [57] was performed. In addition to finding that the polystyrene samples have smaller free volume holes than the poly(p-methylstyrene) samples, they found that the syndiotactic samples had broader hole distributions than the attactic samples. 

Iodine readily reacts with polymers bound to coordination centres, yielding macromolecules containing iodide end groups. The C—I bond can react with silver salts of superacids yielding carbocations essentially in the sense of the last steps of eqn. (32). Doi et al. [251, 252] have used this reaction for the synthesis of the copolymer poly(propene)-Mocfc-poly(tetramethylene oxide) with blocks of syndiotactic polypropylene. 

Vanadium-containing coordination centres producing syndiotactic polypropylene at 195 K can be transformed to radical centres simply by raising the temperature to 298 K [252]. In this way, Japanese authors have prepared the copolymer poly(propene)-Wocfc-poly(methylmethacrylate). The radical end is probably formed by homolytic splitting of the C—V bond, and it can be stabilized by the V ion. The authors state that, in this way, two-component blocks of polypropylene with various polymers propagating by the radical mechanism can be prepared. 

Rather recently, we have studied the soUd-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],andamylose [31] with solid-state high-resolution NMR with supplementary use of other methods, such as X-ray dif-... 

Figure 8 Segments of isotactic (a), syndiotactic (b), atactic (c), and hemiisotactic polypropylene (d) chains. Segments of erythro-6 soXaci c (e), f/ reo-diisotactic (f), and disyndiotactic (g) poly-diolefin chains. The modified Fischer projection is shown. For parts, (a)-(c) a zigzag representation is also reported.

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

The tacticity dependence of the C-T values was also observed for the methine carbons of polypropylene and poly(1-butene). Smaller Tx values were observed for the syndiotactic polymers as compared with the corresponding values for the isotactic polymers. The difference is small but significant and tends to be larger when the comparison is done on the correlation times determined with the log x distribution model.318... 

C-7 1 values of isotactic-rich poly(alkyl vinyl ether)s (alkyl = CH3, C2H5, W0-C3H7, W0-C4H9, /-C4H9) were measured in toluene at 110°C and 100 MHz.319 The T values for syndiotactic sequences are consistently larger than those for isotactic sequences, in contrast to the cases of polymethacrylates and polypropylenes mentioned above. The NOE values for the polymers of methyl and /-butyl vinyl ethers are close to the theoretical maximum. These results indicate that the syndiotactic sequences have higher mobility than the isotactic sequences. 

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