Tacticity solvent dependence

Early studies of the free radical polymerization of methyl methacrylate did not show a solvent influence (18, 22, 23, 24) and consequently no solvent dependent influence of the conversion on the tacticity (23). A solvent dependence on stereocontrol in methyl methacrylate polymerization was however found by Watanabe and Sono (25) as early as 1962. Apparently, their paper has been overlooked. A literature search and a recalculation of most of the published data showed solvent influences on stereocontrol to be the rule and not the exception (6). Later experimental data on methyl methacrylate in about 50 solvents (7) and in 14 solvents (8) confirmed the earlier findings of Watanabe and Sono (25). 

The difference in f3C-Tj between polymers with different tacticities but the same chemical structure results not only from differences in the chain segmental motion between stereoregular polymers or sequences but also from differences in preferred conformations between the stereoregular polymers or sequences which lead to different average distances for the interaction of a carbon with a proton of a neighbouring monomeric unit.298 The solvent dependence of l3C-T for stereoregular PMMAs has been explained by the solvent-dependent conformation of the polymer chain.299-301 In the case of H-Tj the mechanism of relaxation is rather complicated and the observed T cannot be directly related to the segmental mobility of the polymer chains. However, the H-T] values of poly (alkyl methacrylate )s were found to be parallel with the values, i.e. the Tx of the protons in the isotactic poly-... 

Many of the properties of a polymer depend upon the presence or absence of crystallites. The factors that determine whether crystallinity occurs are known (see Chapter 2) and depend on the chemical structure of the polymer chain, e.g., chain mobility, tacticity, regularity and side-chain volume. Although polymers may satisfy the above requirements, other factors determine the morphology and size of crystallites. These include the rate of cooling from the melt to solid, stress and orientation applied during processing, impurities (catalyst and solvent residues), latent crystallites which have not melted (this is called self-nucleation). 

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. 

Schrbder et al.81 studied the effect of solvent on the tacticity of poly(meth-acrylic acid). Unlike the methyl ester, the structure of poly(methacrylic acid) prepared at 60 °C was found to depend on the solvent, changing from 70% syndi-otactic in xylene to 91-92% syndiotactic in polar solvents such as tetrahydrofuran and hexamethylphosphoric triamide. 

In analytical practice, the logarithm of sample molar masses, or molar volumes, is plotted versus retention volumes in calibration dependences of low molecular substances while values or effective hydrodynamic volumes, are used as size parameters in gel chromatography of macromolecules [12,13]. is often called universal calibration parameter because in ideal gel chromatography of randomly coiled macromolecules, it enables the transfer of data from one polymer to another regardless of both the physical (linearity, branching, tacticity, etc.) and the chemical (composition) structure of macromolecules [12]. The hydrodynamic volume of a particular polymer is proportional to the product of its molar mass and limiting viscosity number [ij], in the solvent that is used as mobile phase [ij]Mm. 

Polymerization of 1,3-pentadiene can potentially result in five different insertions of the monomers. These are 1,4-cw,, A-trans, 1,2-ds,, 2-trans, and 3,4. In addition, there are potentially 3-cw-l,4 and 2>-trans- A structures (isotactic, syndiotactic, and atactic). Formations of trans-, A-isotactic, c/s-1,4-isotactic, and cw-l,4-syndiotactic polymers are possible with Ziegler-Natta cata-lysts. " Amorphous polymers also form that are predominantly cw-l,4 or rra/w-1,4, but lack tactic order. Stereospecificity in poly( 1,3-pentadiene) is strongly dependent upon the solvent used... 

There are some important structural aspects of the polymer which are necessary to take into account in the analysis of the polymer behavior in mixture solvents, such as its polarity, chemical structure, microtacticity, molecular weight. The analysis of these properties shows that they are determinant factors in preferential adsorption phenomena involved. It has been pointed out that the effect of tacticity, and particularly the molecular weight, is a complex problem. In the case of poly(2-vinylpyridine), when the polar solvent is preferentially adsorbed, preferential solvation is independent of molecular weight but when the non-polar solvent is adsorbed, there is a dependence on the molecular weight. ... 

The halogenated polymers are slightly less tractable, but they are are in any case known to give shifts that depend somewhat upon solvent, especially in the methylene region. However, the spectrum of poly(methylmethacrylate) (PMMA) cannot be predicted by any reasonable conformational analysis [34]. In this case the shift of the methyl carbon in the M-trans chain should be insensitive to the stereochemistry at its neighbouring centres, for the PMMA chain is known from other measurements to have this predominantly a -trans stereochemistry. Yet the shifts of its carbons show a very large dependence upon tacticity, in both the solution and crystalline states. 

Even polymers with the same chemical composition can show differences in the intrinsic viscosity depending on the chemical structure. Polymers with different tacticities (for example the poly(propylene) in Fig. 5.13) show an expansion of the coil in theta-solvents from atactic to iso- and syndiotactic structures. The different coil expansions are caused by different short-range interactions between the side groups [47] (see The influence of the tacticity of a polymer in Chap. 6). In good solvents, the coil expansion is dominated by the solvation of the polymer chain. In this case, there are no observable differences in the intrinsic viscosity of poly(propylenes) with different tacticities. The same observation is made for poly(styrene) in different good solvents in Fig. 6.14. 

Figure 6.14 shows the dependence of the intrinsic viscosity from the molar mass for linear poly(styrene) (PS) in the good solvents trichlorobenzene (TCB) and toluene. Whereas the chemical composition of the polymers in this example is not changed, the tacticity (the stereo chemical position of the side group) differs between the used samples. 

Effects of solvents on the tacticity of the generated polymeric material have been reported (155). When fluoroalcohols were used as solvents, the stereochemistry of the polymerizations of some vinyl esters was affected to var3dng extents, depending on the structure of the acyl group of the monomer (156-159). 

As far as the stereoregularity is concerned, studies of various types of initiation show that methacrylates could be polymerized to give as well as isotaetie, syndiotaetie atactic polymers. Numerous physical properties are tacticity dependent for example, the rate of water absorption is higher for syndiotaetie than for isotactic polymer [97], the transition temperatures of liquid crystalline methacrylic polymers can be specifically influenced [160-162], and the miscibility of polymer blends is changed [163-165]. In general, the stereoregularity depends on the solvent used, the initiator, and the reaction temperature. Reviews have provided an overview concerning analysis, properties and reactivities of polymers with respect to their tacticity [97,166,167]. 

The photodegradation of poly(alkylacrylate)s and poly(methacrylate)s under UV irradiation (248 nm) in solution was studied for the first time by TR EPR by Harbron et Well-resolved spectra of oxo-acyl radicals from the ester side chain and of main-chain polymeric alkyl radicals were used to show the side-chain cleavage via the Norrish I process. The methacrylate spectra are strongly influenced by the stereoregularity of different polymer tacticity, the temperature and the solvent. The relations of these dependences on the conformational motion in the polymer chain are discussed. 

It was found that stereoregularity of the template plays an important role in the process. In a set of experiments, it was shown that the template effect is dependent on the tacticity of the template and also on the solvent used. 

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