Polymers, stereoblock

The successive repeat units in strucutres [VI]-[VIII] are of two different kinds. If they were labeled Mj and M2, we would find that, as far as microstructure is concerned, isotactic polymers are formally the same as homopolymers, syndiotactic polymers are formally the same as alternating copolymers, and atactic polymers are formally the same as random copolymers. The analog of block copolymers, stereoblock polymers, also exist. Instead of using Mj and M2 to differentiate between the two kinds of repeat units, we shall use the letters D and L as we did in Chap. I. 

Optical properties of the blends are somewhat dependent on the molecular weight of the polystyrene, presence of additives such as lubricant in the polystyrene, ratio of polystyrene to SBS, processing conditions and mixing effectiveness of the extruder. It is stated that the optical properties of the sheets are similar whether linear or radial type stereoblock polymers are used. 

According to analogous molecular mechanics analyses,38 this stereoselectivity mechanism would also operate for catalytic systems with oscillating stereocontrol, leading to atactic-isotactic stereoblock polymers,39,40 like those based on two unbridged 2-phenyl-indenyl ligands.40... 

Unbridged metallocenes rarely achieve highly stereoselective polymerizations because free rotation of the r 5-ligands results in achiral environments at the active sites. An exception occurs when there is an appreciable barrier to free rotation of the r 5-ligands. Fluxional (con-formationally dynamic) metallocenes are initiators that can exist in different conformations during propagation. Stereoblock copolymers are possible when the conformations differ in stereoselectivity and each conformation has a sufficient lifetime for monomer insertion to occur prior to conversion to the other conformation(s). Isotactic-atactic stereoblock polymers would result if one conformation were isoselective and the other, aselective. An isotactic-atactic stereoblock polymer has potential utility as a thermoplastic elastomer in which the isotactic crystalline blocks act as physical crosslinks. 

Bis(2-arylindene)zirconium dichlorides have been studied for the purpose of synthesizing isotactic-atactic stereoblock polymers [Busico et al., 2001 Lin et al., 2000 Lin and Way-mouth, 2002 Nele et al., 2000], Without the phenyl substituents, bisindenylzirconium dichloride yields atactic polypropene because there is rapid rotation of the r 5-ligands. The 2-phenyl substituents in bis(2-arylindene)zirconium dichloride interfere with each other suf-ficently that rotation is slowed to produce isotactic-atactic stereoblock polypropene. Three conformational isomers (conformers) are possible in this metallocene (Eq. 8-54). There is... 

Another way of preparing stereoblock polymers is the use of binary catalyst systems, where the exchange of growing alkyl chains with different tacticity might be expected.The alkyl chains are presumably carried by alkylaluminums and transferred between zirconocenes, and the products are a mixture of homopolymers from each catalyst and a stereoblock polymer containing both segments. 

The atactic amorphous portion (9-16% of the total) contained in the obtained polypropylene has been separated by treating the raw polymer with n-heptane at room temperature. When operating in such a way, we have not separated the stereoblock fraction (extractable in boiling n-hep-tane) from the isotactic (not extractable in boiling n-heptane) fraction of polymer. The results reported in this paper are generally referred to the crystalline fraction, named non-atactic, which contains also some stereoblock polymers (at the considered polymerization temperatures, the latter generally correspond only to 5-7 % of the total) (9)... 

The data are related to the polymers insoluble in n-heptane at room temperature and include the stereoblock polymers soluble in boiling n-heptane (6 7% of the whole polymer) and the isotactic polymer. 

This fraction contains the isotactic and stereoblock polymers. 

These data were calculated from the preceding ones, taking into account the percentage and the molecular weight of the stereoblock polymers contained in the fraction extractable in boiling n-heptane. 

The same methods employed with raw polymers were used for fractions of isotactic polypropylene (ether extraction residue), containing also some stereoblock polymers 32). 

It will be seen that an excess of this, as indicated by mol percent (IOOXthf) on the open circles, led to syndiotactic-like polymers. Except for a few samples having very low isotactic content, all polymers lie to the right of the Bernoullian line and have a stereoblock character. While toluene solutions of t-butylmagnesium bromide and chloride give polymers which were isotactic within the sensitivity of our assay (i = 1 00+0.01), toluene solutions of di-t-butylmagnesium gave stereoblock polymer with i = 0 33. 

COLEMAN and Fox (18) have pointed out that the non Bernoullian sequence distribution observed in some of these systems can be formed without the hypothesis of penultimate effects. All that is required is that two or more types of active species be present which do not rapidly interconvert. Each can add monomer at its own rate and with its own characteristic regulating effect. No penultimate effect is necessary but the sequence distribution will be non-Bernoullian. This type of mechanism is particularly attractive in the explanation of stereoblock polymer formation in the lithium alkyl systems in toluene with small amounts of ether present. The presence of at least two species of active centres has been inferred from an examination of polymer fractions obtained from butyllithium initiated polymerizations (19) in toluene. The change in molecular weight distribution with time suggests the presence of two... 

Figure 29-9 Proton-decoupled 13C spectra of different polypropene samples taken in CHCI2CHCI2 solution at 150° at 15.9 MHz. The upper spectrum is of a highly isotactic polypropene, which shows only the faintest indication of lack of stereoregularity. The middle spectrum is of atactic polypropene, which shows a variety of chemical shifts for the CH3 groups as expected from the different steric interactions generated by random configurations of the methyl groups. The lower spectrum is of a sample of so-called stereoblock polymer, which is very largely isotactic. The 13C spectrum of syndiotactic polypropene looks exactly like that of the isotactic polymer, except that the CH3— peak is about 1 ppm upfield of the position of the isotactic CH3 peak and the CH2 peak is about 1 ppm downfield of the isotactic CH2 peak.

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