Tacticity syndiotactic polymers

In practice, there is no such thing as a pure isotactic or syndiotactic polymer. Once again, we find that polymers comprise a statistical distribution of chemical structures. Polymers that contain steric centers inevitably incorporate a certain number of steric defects that prevent us from obtaining 100% isotacticity or syndiotacticity. Polymer manufacturers vary the catalyst type and reaction conditions to control the tacticity level and the resulting properties. 

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. 

Stereoselective polymerizations yielding isotactic and syndiotactic polymers are termed isoselective and syndioselective polymerizations, respectively. The polymer structures are termed stereoregular polymers. The terms isotactic and syndiotactic are placed before the name of a polymer to indicate the respective tactic structures, such as isotactic polypro-pene and syndiotactic polypropene. The absence of these terms denotes the atactic structure polypropene means atactic polypropene. The prefixes it- and st- together with the formula of the polymer, have been suggested for the same purpose it-[CH2CH(CH3)] and st-[CH2 CH(CH3)] [IUPAC, 1966],... 

The second major topic in the field of olefin polymerization is that of the tacticity of the polymer [28]. If the olefin being polymerized is less symmetrical than ethylene, stereogenic centers will appear at the polymer, and the arrangement of these stereocenters can produce highly organized isotactic or syndiotactic polymers, as depicted in Fig. 5 for the case of propene polymerization. The alternative is an atactic polymer where the distribution of stereocenters is random. 

Two examples clearly illustrate the relationship between molecular structures of the metallocene catalysts on the one hand, and the tacticity of the resultant polymers on the other. As shown in Fig. 6.9, complexes 6.32, 6.33, and 6.34 have very similar structures. In 6.33 and 6.34 the cyclopentadiene ring of 6.32 has been substituted with a methyl and a f-butyl group, respectively. The effect of this substitution on the tacticity of the polypropylene is remarkable. As already mentioned, 6.32, which has Cs symmetry, gives a syndiotactic polymer. In 6.33 the symmetry is lost and the chirality of the catalyst is reflected in the hemi-isotacticity of the polymer, where every alternate methyl has a random orientation. In other words, the insertion of every alternate propylene molecule is stereospecific and has an isotactic relationship. In 6.34 the more bulky t-butyl group ensures that every propylene molecule inserts in a stereospecific manner and the resultant polymer is fully isotactic. 

Propene and the higher 1-alkenes can be polymerized to chains with the required degree of tacticity from almost atactic up to very highly tactic structures. However, a syndiotactic polymer can only be obtained from propene, mostly on soluble catalysts. The main factors determining controlled tactic addition are complexation, cis or trans addition, and primary or secondary addition. Most authors agree on the point that the interaction of the alkene molecule with the transition metal atom of the active centre leads to complex formation immediately before monomer insertion into the metal—polymer bond. The assumed existence of the complex is based on indirect experimental evidence and on theoretical considerations. 

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. 

Each carbon within the polymer chain may be considered as a chiral center. As a result, a parameter referred to as the tacticity may be defined that describes the stereoregularity of adjacent carbon centers (Figure 5.6). A polymer that contains substituents on the same side of the polymer chain is referred to as isotactic, and often exhibits some degree of crystallinity. In contrast, if adjacent substituents are arranged on alternating sides of the polymer chain, a syndiotactic polymer is formed. Unless a... 

Figure 5.6. The tacticity of polymer chains. Illustrated are (a) isotactic, (b) syndiotactic, and (c) atactic polymers.

Zambelli et al. reported on the mechanism of styrene polymerization [36]. They showed that the main chain of the syndiotactic polymer has a statistically trans-trans conformation. It was established then the double-bond opening mechanism in the syndiospecific polymerization of styrene involves a cis opening. The details in the control of the monomer coordination for this polymerization mechanism were examined by Newman and Malanga using detailed, 3C NMR. It was shown through the analysis of tacticity error (rmrr) that the tacticity in the polymer is chain-end controlled and that the last monomer added directs the orientation and coordination of the incoming monomer unit prior to insertion [37]. 

Sundararajan has studied silane polymers including polymethylphenyl-silylene)243 and polysilapropylene. " " Different tacticities and conformations of polymethylphenylsilylene were examined and the relative stabilities of each were reported. The most stable configuration was for the syndiotactic polymer with an all-trans conformation. Helix parameters, a priori bond probability, and characteristic ratios were calculated for this polymer. Polysilapropylene was studied in a similar manner. MM2 energies of the minimum for each of the different rotational isomers were included. As with the first study, contour plots of both energy and helix parameters versus (j) and < ) + 1 angles were given. 

Numerous initiator systems have been used for the tactic polymerization of methyl methacrylate (7, 8, 14). Reaction conditions determine the structure of each polymer, and each structure might vary considerably from one polymer to another. Nearly pure isotactic and syndiotactic polymers are known, in addition to all transitions between these limits and also stereoblock copolymers. In the amorphous state, the differenes in the individual structures are clearly shown by their ultrared (J) and nuclear resonance spectra (2,11). 

Solvent Polarity Effects. To investigate the effect of solvent polarity on these systems, a series of polymerization reactions was performed using mixtures of 2 solvents with different dielectric constants (e) methylene chloride (e=14.8) and hexane (e-2.0). As seen in Table V, the solvent polarity had a great effect on the tacticity of the resultant polymers. Predominately syndiotactic polymers were formed in the more polar solutions as previously found ( ), and the level of isotacticity increased as the solutions became less polar. These results support the proposal that isotactic sequences come from backside attack on... 

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