Syndiotactic poly polymers

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). 

The synthesis of isotactic and syndiotactic polymers has been achieved for a number of polymers. For example poly (methyl methacrylate) can be prepared in either isotactic or syndiotactic configurations depending on the details of the polymerisation conditions. 

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.)...

Mesomorphic forms characterized by conformationally ordered polymer chains packed in lattices with different kinds of lateral disorder have been described for various isotactic and syndiotactic polymers. For instance, for iPP,706 sPP,201 sPS,202 syndiotactic poly(p-methylstyrene) (sPPMS),203 and syndiotactic poly(m -methylstyrene),204 mesomorphic forms have been found. In all of these cases the X-ray fiber diffraction patterns show diffraction confined in well-defined layer lines, indicating order in the conformation of the chains, but broad reflections and diffuse haloes on the equator and on the other layer lines, indicating the presence of disorder in the arrangement of the chain axes as well as the absence of long-range lateral correlations between the chains. 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

Polyacetaldehyde [IUPAC Poly(oxy[methylmethylene])], like the systems described previously, contains stereocenters that are achirotopic. Both the isotactic and syndiotactic polymers are achiral and do not possess optical activity. 

Optically active polymers are rarely encountered. Most syndiotactic polymers are optically inactive since they are achiral. Most isotactic polymers, such as polypropene and poly(methyl methacrylate), are also inactive (Sec. 8-la-l). Optically active polymers have been obtained in some situations and these are discussed below. 

If the successive vertical lines are located alternately on the opposite sides of the horizontal line, above and below it, in the adapted Fischer projection, or if the successive alkyl substituents appear alternately in front of and behind the plane of the extended polymer backbone in the flat zigzag projection (Figure 3.1), then a syndiotactic poly(a-olefin) structure occurs. These two representations show that the syndiotactic polymer contains neighbouring tertiary carbon atoms of opposite relative configurations. The syndiotactic poly (a-olefin) chain is characterised by the appearance of stereodiads of the opposed relative... 

Polymerisation of racemic 4-methyl-1-hexene with the Me2C(Cp)(Flu) ZrCl2— [Al(Me)0]x catalyst was found obviously to produce syndiotactic polymer, which appeared to be a random copolymer of two enantiomers, poly[(7 ,5)-4-methyl-1 -hexene] [436,437],... 

Figure 4.3 Stereoisomerism of poly(l-oxo-2-phenyltrimethylene) -[—CH(Ph)—CH2— Isotactic and syndiotactic polymers...

Syndiotactic l,2-poly(4-methyl-l,3-pentadiene) has been formed by polymerisation with homogeneous catalysts, e.g. TiBz4—[Al(Me)0]x and CpTiCl3—[Al (Me)0]x [41,43]. The coordination of the monomer as an s-trans-t/2 ligand rather than an s-cis-r A ligand at the Ti atom has been postulated to be involved in the polymerisation. The s-cis-r A monomer coordination is less favoured for steric reasons in the case of 4-methyl-1,3-pentadiene. A possible scheme for the formation of the 1,2-syndiotactic polymer from this monomer is presented in Figure 5.7 [41,43],... 

However, cz s-isotactic and cz s-syndiotactic polymers have been obtained predominantly from common cycloolefins such as cyclopentene the 1,3-insertion isomerisation-polymerisation of cycloalkenes such as cyclopentene farely yields polymers with a trans structure (usually in an amount not exceeding 3%) [15,19,20], Figure 6.2 shows both cw-isomers of poly(l,3-cyclopentylene). 

Because atactic polymer has no ordered structure and shows only slight intramolecular interactions, the interactions between atactic polymers is the strongest (Fig. 10 a). The isotactic polymers may be stabilized by assuming the helix conformation reported for isotactic poly(methyl methacrylate)401. Nucleic add bases are situated outside the polymer chain so that they can form the complex, although the interaction is not so strong. On the other hand, the syndiotactic polymer may have a rod-like conformation that is supported by the low solubility of the polymer and by NMR spectra321. Tlierefoie, it is well understood that the complex formation ability of the syndiotactic polymers is very low. 

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. 

An elegant alternative to living polymerization for the preparation of block polymers is to use functionalized Grignard initiators. The polymerization of methyl methacrylate to isotactic (in toluene at — 78"C) or syndiotactic polymers (in THF at — llO C) can be initiated by o-, m-, and p-vinylbenzylmagnesium chloride. The polymers had a low polydispersity and contained one vinylbenzyl group at the chain end, by H-NMR. The poly(methylmethacrylate) macromers thus obtained were polymerized or copolymerized with styrene to give graft and block polymers of controlled architecture [50,51]. 

Syndiotactic polymers, as we have seen above, are stereoregular and so are crystallizable. They, however, do not have the same mechanical properties as isotactic polymers, because the different configurations affect the crystal structures of the polymers. Most highly stereoregular polymers of commercial importance are isotactic, and relatively few syndiotactic polymers are made. Atactic polymers, on the other hand, are usually completely amorphous unless the side group is so small or so polar as to permit some crystallinity. Thus, while atactic poly(vinyl acetate) has never been crystallized, poly(vinyl alcohol), which is derived from it and is also atactic, has been found to crystallize. 

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... 

Matsuzaki et al. [101] have found that isotactic poly-t-butylmethacrylate decomposes more rapidly into isobutene and polyanhydride than the corresponding syndiotactic polymer. This is in agreement with the above mechanism. 

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