Stereoregular polymer

The ehemistry of water-soluble polymers ean take various forms. These polymers ean be anionie, cationie, or nonionie. Their polymer baekbone ean eontain hydrophilie and hydrophobie pendant groups. Branehing and polymer stereoregularity also play a role in the physieal behavior of these materials. 

As a consequence of the dependence of the polymer stereoregularity on monomer concentration and polymerization temperature, the mechanism shown in Fig. 6 was established. 

The unconventional applications of SEC usually produce estimated values of various characteristics, which are valuable for further analyses. These embrace assessment of theta conditions for given polymer (mixed solvent-eluent composition and temperature Section 16.2.2), second virial coefficients A2 [109], coefficients of preferential solvation of macromolecules in mixed solvents (eluents) [40], as well as estimation of pore size distribution within porous bodies (inverse SEC) [136-140] and rates of diffusion of macromolecules within porous bodies. Some semiquantitative information on polymer samples can be obtained from the SEC results indirectly, for example, the assessment of the polymer stereoregularity from the stability of macromolecular aggregates (PVC [140]), of the segment lengths in polymer crystallites after their controlled partial degradation [141], and of the enthalpic interactions between unlike polymers in solution (in eluent) [142], as well as between polymer and column packing [123,143]. 

After the Natta s discovery of highly stereospecific polymerization processes, the interest in the preparation and properties of optically active polymers has greatly increased. In fact, the use of asymmetric catalysts or monomers to obtain optically active polymers may supply interesting informations on the mechanism of steric control in stereo-specific polymerization furthermore optical activity is an useful tool to study the polymer stereoregularity and the chain conformations of polymers in the molten state or in solution. 

An appreciable optical activity, depending on the degree of polymer stereoregularity, may be expected and, as will be shown below, has been actually observed in vinyl polymers derived from racemic monomers, when one of the two antipodes is preferentially polymerized (104). 

Steric Defects in Stereospecific Chain growth Reactions and Analysis of Polymer Stereoregularity... 

A low degree of tacticity is obtained because these monomer-orienting forces are quite weak. Thus, polymer stereoregularity is achieved only with certain suitable monomers and at low temperatures. Increased tacticity can be achieved in some cases by using monomer-orienting forces other than the catalyst or the polymer end groups, but these have rather limited utility (canal complexes, solid state polymerizations, etc.). Simple polymerization systems fall outside the scope of this review and are not discussed further. 

Simple anionic polymerizations are considered to be those in which the growing anion is essentially a free propagating ion and the cation only maintains electroneutrality. It is possible that no system actually exists in which the cation does not in any way associate with, polarize or sterically influence the monomer prior to the addition of the anion. Practically, however, one can define simple anionic polymerizations as those in which polymer stereoregularity parallels that of free radical-polymerizations. The polymer microstructure will tend to be... 

Polymer stereoregularity obtained with styrene and butadiene is particularly significant. In the polymerization of styrene with the three cocatalyst-dependent systems... 

In coordinated polymerizations with alkyl metal and Ziegler-type catalysts, vacant p- or d-orbitals of a metal component coordinate with the jr-electrons of olefins, diolefins and non-polar monomers. When the polymer chain end is fixed in position and partially stabilized by its metal-containing gegen-ion, repetitive insertion of the polarized and oriented monomer between the chain end and gegen-ion yields stereoregular polymers. Of the various factors which affect polymer stereoregularity, the most important appears to be the gegen-ion structure and its ability to coordinate and orient the monomer. 

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. 

The affect of polymer stereoregularity in the chains on the PAL data has also been studied. Hamielec et al [56] found what appears to be an increased lifetime (hole size) with increased randomness of the chain configuration in a series of polyvinlychloride (PVC) polymers, despite the large degree of scatter in the sample (probably due to the fact that a series of commercially available products were used.). They however found little correlation with tacticity in polypropylene. More recently a PAL study on a series of very well characterized polystyrene and poly(p-methlystyrene) samples of differing tacticity [57] was performed. In addition to finding that the polystyrene samples have smaller free volume holes than the poly(p-methylstyrene) samples, they found that the syndiotactic samples had broader hole distributions than the attactic samples. 

The use of electron donors is of particular importance for propylene polymerization, where a severe control on polymer stereoregularity is required. 

Furthermore the development, during the 1970 s, of high yield catalysts for polypropylene made it possible to achieve also a better control of polymer stereoregularity. All this was the result of a careful choice, case by case, of an appropriate combination among carrier, transition metal compound, metal alkyl and possible third component. 

Polymer Tacticity. Our initial results on the polymerization of several different p-substituted-a-methylstyrene monomers indicated that there was some relationship between polymer stereoregularity and both the type of initiator and substituent in these monomers ( ). However, our recent investigations with a much wider variety of monomers, catalysts and cocatalysts revealed that the classical approach to analyzing substituent effects in organic reactions, the use of the Hammett pa relationship, gave no simple and self-consistent relationship between tacticity and the a (or a ) constant for the para-substituent. These results are summarized in the data in Table I for the cationic polymerization of a-methylstyrene and a series of five p-substituted-a-methylstyrene monomers initiated with two different Friedel-Crafts catalysts, TiCl and SnCl, either alone or with a cocatalyst benzyl chloride (BC) or t-butyl chloride (TBC), in methylene chloride at -78°C. Where a cocatalyst was used, the initiator was presumably a carbonium ion formed by the following reaction ... 

A good reason to look at history is to select those parts we should like to repeat. One part of science history that both polymer chemists and businessmen would surely like to see emulated is the unprecedented, explosive burst of creativity, invention, and successful development that occurred in the 1950s and gave the world a new class of polymers (stereoregular), a family of new plastics (linear and stereoregular polyolefins), a family of new synthetic rubbers, including the first duplication of a natural high polymer... 

Polymerization by coordination is one of the most versatile methods to produce a variety of polymers. Stereoregularity is one of the outstanding characteristics of the coordination polymerization that relies on the use of catalytic systems based on organometallic or coordination complexes of special structures and symmetries to make highly stereospecific polymers. 

Polymer stereoregularity is important since the morphology of the macromolecule depends on crystallization and the degree of crystallinity determines, in the end, several physical properties such as density, mechanical properties, as well as thermal properties. Atactic polymers have difficulties in crystalling they are amorphous or show a low degree of crystallinity, while the degree of crystallinity in isotactic and syndiotactic polymers is high. 

NMR is the method of choice to determine polymer stereoregularity, whereas MS is notoriously not sensitive to this property. In the case of homopolymers, the application of NMR analysis to determining the stereosequences is straightforward. 

Polymer stereoregularity was observed to have a significant effect on TOF-SIMS spectra, the best example of which is poly(methyl methacrylate) (PMMA). Figure 8.29 shows a comparison of the spectra of isotactic and atactic PMM A. 

It was shown subsequently that the effect of polymer stereoregularity is related to the double helical structure of PMMA. Thus, while the simple statistical model is effective for interpreting TOF-SIMS spectra of polymers having isolated chains, it is not for a polymer having the structure of isotactic PMMA. The crystal structure of iso-PMMA has acrylate groups rotated toward the center of the double helix in close proximity to each other. Also, LB films of iso-PMMA show the double helical structure. Therefore there are many possibilities for cross-chain reactions in iso-PMMA films deposited on a surface. A combinatorial spreadsheet analysis was done to determine... 

Syndiotactic polymer Stereoregular polymer in which the configuration of successive chirality centers alternates along the chain. 

The position of the counterion is assumed to be at the side of the carbon cation and away from the penultimate unit. The stability of such conformations should be very dependent on the temperature of the polymerization and on the size of the substituents. Experimental evidence confirms this. Thus, it is known that the stereoregular polymers, whether isotactic or syndiotactic, form only at low temperatures in homogeneous polymerizations (as stated earlier). This suggests that the fixation of the conformations of the growing chain ends is very important in enhancing polymer stereoregularity. 

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