Polymers, tactic

Examples (4), (5), (6), (8), (10), (11) and (12) are tactic polymers. A stereoregular polymer is always a tactic polymer, but a tactic polymer is not always stereoregular because a tactic polymer need not have all sites of stereoisomerism defined. 

Note Structure-based names of tactic polymers are formed before the application of adjectives designating tacticity thus, syndiotactic poly(methylmethylene) is preferred to syndiotactic poly(l,2-dimethylethane-l,2-diyl) because a shorter repeating unit is identified, in conformity with the rules in Ref. 3. 

In addition to isotactic, syndiotactic and atactic polymers (and other well-defined types of tactic polymers), there exists the whole range of possible arrangements between the completely ordered and the eompletely random distributions of configurational base units,... 

Polymerization in which a tactic polymer is formed. However, polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific. For example, the polymerization of a chiral monomer, e.g., R)-propylene oxide ((i )-methyloxirane), with retention of configuration is not considered to be a stereospecific reaction however, selective polymerization, with retention, of one of the enantiomers present in a mixture of R)- and (S)-propylene oxide molecules is so classified. 

Tactic polymer that contains two sites of defined stereoisomerism in the main chain of the configurational base unit. 

Tactic polymer in which the main-chain double bonds of the configurational base units are entirely in the Z configuration. 

Note 4 Some stereospecific polymerizations produce tactic polymers [3] that contain a mixture of pairs of enantiomeric polymer molecules in equal amounts. For example, in the case of a polymerization leading to an isotactic polymer the product consists of... 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems... 

Besides ethylene, higher olefins (propylene, 1-butylene), dienes, and a number of other monomers can be polymerized with organometallic mixed catalysts the polymerization frequently proceeds stereospecifically, leading to tactic polymers (see Examples 3-30 to 3-32). 

It has also been found that the polymer formed from the beginning of the reaction is already prevaiUngly isotactic. This means that, from the start of the reaction, there exist a certain number of active centers on the solid a-TiCU surface which immediately yield iso tactic polymer consequently, it can be excluded that, at least for the active centers present on the initial free surface of a-titanium trichloride, there is an initial activation process, whose rate is slow enough to be observed even when operating at low temperature (30°). 

ZIEGLER-NATTA POLYMERIZATION. Polymerization of vinyl monomers under mild conditions using aluminum alkyls and TiCL lor other transition element halide) catalyst to give a stereoregulated, or tactic, polymer. These polymers, in which the stereochemistry of the chain is not random have very useful physical properties. 

Three broad types of tacticity may be distinguished in polymers made by ROMP (i) fully tactic polymers which may be divided into the first four groups c/r, c/m, t/m, t/r listed in Table 7, and a fifth group in which the polymer has intermediate cis content but in which only c/r and t/m structures are found (ii) completely atactic polymers, which may be of any cis content and (iii) polymers of intermediate tacticity. 

TABLE 7. Some limiting cases of tactic polymers made by ROMP... 

A cistactic polymer is a tactic polymer in which the main chain double bonds of the configurational base units are entirely in the cis arrangement. A transtactic polymer is a tactic polymer in which the main chain double bonds of the configurational base units are entirely in the trans arrangement [134]. Geometrical cis-trans isomerism also arises from different configurations of substituents on a cyclic structure. 

It should be mentioned, in connection with definitions of stereoregular and tactic polymers, that a stereoregular polymer is always a tactic polymer, but... 

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