Stereoregular syndiotactic polymers

Actually, the fixation of diad configuration in the polymer chain proceeds only when the next monomer molecule is attached. Until this moment, the end unit can still undergo isomerization becoming either planar or its mirror antipode. In the absence of isomerization, the formation of a stereoregular syndiotactic polymer should be expected. In the case when an end unit has the time (before the attack of the following monomer molecule) to adopt a planar configuration, the distribution of diads in the polymer is described by Bernoulli s statistics (binomial distribution) [42]. If the state to two end units of... 

The level of stereochemical defects in highly stereoregular syndiotactic polymers can be roughly estimated from only two pentads since mm - rmmr and m > l/2[xmrx]. 

Atactic polymer (Section 7 15) Polymer characterized by ran dom stereochemistry at its chirality centers An atactic polymer unlike an isotactic or a syndiotactic polymer is not a stereoregular polymer... 

Syndiotactic polymer (Section 7 15) Stereoregular polymer in which the configuration of successive chirality centers alternates along the chain... 

Inspired by the design of metallocence catalysts, there has been research into the use of C2-symmetric Ni a-diimine catalysts for the preparation of polyolefins with stereoregularity. Such catalysts were shown to afford higher degrees of isotacticity in polypropylenes as compared to the standard C2v-symmetric catalysts, which afford mostly syndiotactic polymer [106], Coates and coworkers have studied... 

As is well known, the most simple head-to-tail stereoregular vinyl polymers were called isotactic (22-24) and syndiotactic (25) by Natta. The first compounds to be recognized as such were polypropylene and 1,2-polybutadiene, respectively (26). Ideal isotactic vinyl polymers (4, 5, Scheme 1) have all the substituents on the same side of the chain while in syndiotactic polymers (6, 7) the substituents regularly alternate between the two sides of the chain (27). 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

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],... 

Isotactic and syndiotactic polymers are collectively referred to as stereoregular polymers. They are always arranged in a head to tail (a, b) arrangement. Karl Ziegler and Giulio Natta, the co-winners of the 1963, Nobel Prize in Chemistry developed... 

The structures of the two stereoregular polymers assuming an imaginary planar, fully extended backbone are given in Scheme 13.6. In this representation all methyl groups in the isotactic polymer lie on the same side of the backbone plane, while in the syndiotactic polymer they alternate. 

Lithium and magnesium alkyl catalysts yield metal-polymer bonds with appreciable covalent character and their cations coordinate strongly with nucleophiles. Therefore, these catalysts will initiate simple anionic polymerization only under the most favorable conditions, e. g., in basic solvents and with monomers which produce resonance stabilized polymer anions. As examples of stereoregular anionic polymerization, a-methyl-methacrylate yields syndiotactic polymer with an alkyl lithium catalyst in 1,2-dimethoxyethane at — 60° C. (211, 212) or with a Grignard catalyst at -40° C. (213). 

Stereochemistry also affects the crystallinity of a polymer. Stereoregular isotactic and syndiotactic polymers are generally more crystalline than atactic polymers. By careful choice of catalysts, we can make a linear polymer with either isotactic or syndiotactic stereochemistry. 

Fig. 12.21. Diagrams showing (a) irregular atactic, (b) stereoregular isotactic, and (c) stereoregular syndiotactic configurations in polypropylene polymer. (Source Ahmed, M., "Polypropylene Fibers-Science andTechnology," Textile Science and Technology, 5, 16, Elsevier Science Publishers SV, Academic Publishing Div., New York, Amsterdam (1982).) Copyright M. Ahmed. By permission.

The cis-1,4 syndiotactic polypentadiene has been obtained by soluble catalysts prepared from an aluminum alkyl chloride and a cobalt compound. Only the trans isomer of pentadiene has been polymerized by these catalysts. However, mixtures of the cis and trans isomers can be used, the cis isomer remaining seemingly unaltered. The presence of the cis isomer was found to have no appreciable influence on the stereospecificity when its percentage in the mixture of the two isomers is lower than about 30%. For higher percentages a decrease in the stereoregularity of the syndiotactic polymer obtained was observed. 

---