Poly tacticity dependence

The tacticity dependence of the C-T values was also observed for the methine carbons of polypropylene and poly(1-butene). Smaller Tx values were observed for the syndiotactic polymers as compared with the corresponding values for the isotactic polymers. The difference is small but significant and tends to be larger when the comparison is done on the correlation times determined with the log x distribution model.318... 

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. 

Seki and Tirrell [436] studied the pH-dependent complexation of poly(acrylic acid) derivatives with phospholipid vesicle membranes. These authors found that polyfacrylic acid), poly(methacrylic arid) and poly(ethacrylic acid) modify the properties of a phospholipid vesicle membrane. At or below a critical pH the polymers complex with the membrane, resulting in broadening of the melting transition. The value of the critical pH depends on the chemical structure and tacticity of the polymer and increases with polymer hydro-phobicity from approximately 4.6 for poly(acrylic acid) to approximately 8 for poly(ethacrylic acid). Subsequent photophysical and calorimetric experiments [437] and kinetic studies [398] support the hypothesis that these transitions are caused by pH dependent adsorption of hydrophobic polymeric carboxylic acids... 

Comparison of the data shown in Figure 10 with d values from the diffraction pattern in Figure 11 indicate (Figure 11a) that the mesomorphous component (a narrow reflex), d, is independent of poly(decamethylcyclohexasiloxane) chain tacticity. Hence, irrespective of trans-tactic polymer abi-lity to crystallization, the mesophase temperature range changes insignificantly. At lower tempera-ture of -73°C only, a bending on the temperature dependence curve, d(T), of the mesophase peak occurs. 

Using the methods of amorphous halo separation from the narrow reflex, Figure lib shows d(T) dependences for poly(decamethylcyclohexasiloxane) of different tacticity. Figure lib indicates that tacticity variation affects the values and shape of d(T) dependence of amorphous halo. At 20°C, the difference of studied samples in d is insignificant, but at 197 -... 

Table 1 Dependence on Tacticity of the Glass Transition Temperature Tg of Poly(methylmethacrylate)...

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-... 

Significant interaction in mixed monolayers containing poly (methyl methacrylate) (as evidenced by non-additivity of film areas) was noted in certain cases by Wu and Huntsberger (20). However, there have been a number of reports (21, 22) which suggest that the monolayer characteristics of this polymer depend appreciably on both molecular structure details, such as tacticity, and experimental procedure, notably compression rate. Accordingly, a more detailed examination of this effect is beyond the scope of the present work. 

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. 

Poly(acrylic acid) (PAA) has several vibrational modes in the 900-1350-cm region whose activity depends on the tacticity of the polymer. Figure 4... 

Figure 4-24. Dependence of the reciprocal molar optical rotaton,, with tacticity content of isotactic, x, and syndiotactic, Xs, diads for poly (S)-4-methyl-l-hexene. Optical purity of initial monomer was 93% (according to measurements by P. Pino et al).

Over the past several decades, polylactide - i.e. poly(lactic acid) (PLA) - and its copolymers have attracted significant attention in environmental, biomedical, and pharmaceutical applications as well as alternatives to petro-based polymers [1-18], Plant-derived carbohydrates such as glucose, which is derived from corn, are most frequently used as raw materials of PLA. Among their applications as alternatives to petro-based polymers, packaging applications are the primary ones. Poly(lactic acid)s can be synthesized either by direct polycondensation of lactic acid (lUPAC name 2-hydroxypropanoic acid) or by ring-opening polymerization (ROP) of lactide (LA) (lUPAC name 3,6-dimethyl-l,4-dioxane-2,5-dione). Lactic acid is optically active and has two enantiomeric forms, that is, L- and D- (S- and R-). Lactide is a cyclic dimer of lactic acid that has three possible stereoisomers (i) L-lactide (LLA), which is composed of two L-lactic acids, (ii) D-lactide (DLA), which is composed of two D-lactic acids, and (iii) meso-lactide (MLA), which is composed of an L-lactic acid and a D-lactic acid. Due to the two enantiomeric forms of lactic acids, their homopolymers are stereoisomeric and their crystallizability, physical properties, and processability depend on their tacticity, optical purity, and molecular weight the latter two are dominant factors. 

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. ... 

PAA can be prepared using bulk polymerization, aqueous polymerization, nonaqueous polymerization, inverse phase emulsion and suspension polymerization. The precise structure of the resulting PAA chain is dependent upon many factors including the polymerization process and conditions. The tacticity of poly(methacrylic acid), PMA, has been studied using NMR spectroscopy (6). For polymerization of methacrylic acid in methyl ethyl ketone at 60 C gives a polymer with 57% syndiotactic triads. Polymerization at low temperatures gives a more syndiotactic product (6) as does polymerization at high pH (7). 

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