Syndiotactic/isotactic acrylic polymers

Much literature precedent supports the assignment of tacticity in methyl acrylate polymers using NMR techniques [40,41]. In the H-NMR spectrum, the shift of the methylene protons is sensitive to dyad stereochemistry. For example, in an isotactic (meso) dyad 28, the methylene protons are chemically non-equivalent and appear as two separate sets of signals, whereas in a syndiotactic (racemic) dyad 29, the methylene protons are equivalent. The H-NMR spectrum of 27 showed multiplets at 1.89 and 1.5 ppm due to the two diastereotopic methylene protons of the isotactic dyad. The rest of the spectrum is consistent with the structure of the n=4 tetrad 27. A racemic dyad structure would have been expeeted to give resonances of intermediate shift to that of the two resonances observed for the telomer 27. This evidence strongly implies that 27 has the allisotactic configuration shown in Scheme 8-12. 

Comparisons of isotactic, syndiotactic and atactic polymers also cause problems of explanation. For many polymers the type of tacticity appears to have little effect on Tg. Exceptions are provided by the methacrylate polymers and, in contrast to other acrylates, polyisopropyl acrylate. 

Table III summarizes a variety of observations on the alkaline hydrolysis of several acrylic polymers. To be noted is that the difference in the rates of hydrolysis of isotactic and syndiotactic poly(methyl methacrylate) is sufficiently great that mixtures of such polymers may be separated on the basis of this behavior [18].

Tactic isomers of acrylic polymers can be further classified on the basis of the repetition of the configuration of die asymmetric C atom. Where repetition is consecutive, the isomer is said to be isotactic. Where the configuration of successive asynunetric C atoms alternates, the polymer is said to be syndiotactic. 

Unlike most crystalline polymers, PVDF exhibits thermodynamic compatibiUty with other polymers (133). Blends of PVDF and poly(methyl methacrylate) (PMMA) are compatible over a wide range of blend composition (134,135). SoHd-state nmr studies showed that isotactic PMMA is more miscible with PVDF than atactic and syndiotactic PMMA (136). MiscibiUty of PVDF and poly(alkyl acrylates) depends on a specific interaction between PVDF and oxygen within the acrylate and the effect of this interaction is diminished as the hydrocarbon content of the ester is increased (137). Strong dipolar interactions are important to achieve miscibility with poly(vinyhdene fluoride) (138). PVDF blends are the object of many papers and patents specific blends of PVDF and acryflc copolymers have seen large commercial use. 

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. 

TP he free radical polymerization of vinyl and acryl monomers normally does not lead to a true atactic polymer (I). A true atactic polymer is defined in this context as a polymer consisting of 50 isotactic and syndiotactic diads each, 25 iso- and syndiotactic triads each, and 50 heteroactic triads, etc. Furthermore all diads, triads, tetrads, etc., must be distributed at random. 

As expected, the monomers that produce isotactic polymers in nonpolar solvents generally give syndiotactic polymers with the same initiators in polar solvents. Linear esters of acrylates always produce amorphous polymers. Polymers of x, -unsaturated cyclic ketones obtained in the presence of Grignard reagents were also amorphous [62]. 

Poly(methacrylic acid) and poly(acrylic acid) (PAA) were prepared by AIBN initiated polymerization of the freshly distilled monomer in deoxygenated methyl ethyl ketone at 60°C. The incorporation of 9,10-dimethylanthracene (9,10-DMA) end-groups in the polymer was achieved by the addition of the chain transfer agent (1% by weight) to the polymerization mixture. Unreacted 9,10-DMA was separated from the polymer by gel permeation chromatography using a column packed with Sephadex LH-20 and methanol as the eluent. Analysis of the PMA sample by NMR indicates that the polymer produced under these conditions consists of 57% syndiotactic, 33% heterotactic and 10% isotactic triads (15). Solution concentrations were 0.02 M in repeating units of the polymer. 

Poly(methyl methacrylate) is an amorphous polymer composed of linear chains. The bulky nature of the pendant group (-O-CO-Me), and the absence of complete stereoregularity makes PMMA an amorphous polymer. Isotactic and syndiotactic PMMA may be produced by anionic polymerization of metiyl methacrylate at low temperatures. However, these forms of PMMA are not available commercially. Modified PMMA can be obtained by copolymerizing methyl methacrylate with monomers such as acrylates, acrylonitrile, and butadiene. 

The Tg of pdymethacrylates increases as the bulkiness of side chain increases and its flexibility decreases. Tg also correlates with the tacticity of poly(oe-substituted acrylate) and is lower for isotactic pdymer than for syndiotactic one. Good correlation was obtdned between the dyad tacticity and Tg. Therefore, the Tg value of 100 % isotactic or 100 % Syndiotactic polymer could be determined by the extrapolation of observed values even if it has not been prepared. The Tg for purely syndiotactic pdy(methyl methacrylate) thus obtained is... 

Polymerizations of polar monomers, like acrylic and methacrylic esters with alkyllithium initiators, yield the greatest amount of steric control. Almost all isotactic poly(methyl methacrylate) foims at low temperatures. Addition of Lewis bases such as ethers or amines reduces the degree of isotactic placement. Depending upon the temperature, atactic or syndiotactic polymers form. Also, butyllithium in heptane yields an isotactic poly(A, A -dibutylacrylamide) at room temperature. ... 

Solvents influence the rate of free-radical homopolymerization of acrylic acid and its copolymerization with other monomers. Hydrogen-bonding solvents slow down the reaction rates. Due to the electron-withdrawing nature of the ester groups, acrylic and methacrylic ester polymerize by anionic but not by cationic mechanisms. Lithium alkyls are very effective initiators of a-methyl methacrylate polymerization yielding stereospecific polymers.Isotactic poly(methyl methacrylate) forms in hydrocarbon solvents. Block copolymers of isotactic and syndiotactic poly(methyl methacrylate) form in solvents of medium polarity. Syndiotactic polymers form in polar solvents, like ethylene glycol dimethyl ether, or pyridine. This solvent influence is related to Lewis basicity in the following order ... 

Dielectric relaxation spectra of poly(methyl acrylate) (IfA) and poly(t-butyl acrylate) (tBA) were measured at temperatures above and below Tg, and both a- and 3-relaxation processes were observed. As for the 3-relaxation process, in order to clarify the quantitative relationship between the relaxation mechanism and the polymer structure, the effective dipole moment(Pg) was estimated by a method according to the 2-state transition theory. In the estimation, the average local configuration of the main chain was assumed to be in isotactic form or syndiotactic form. Since samples used were atactic polymers, the authors assume that Pg(atact) = Xi Pe(i) + (1 - X ) Pe(s)> where X denotes the tacticity, i represents isotactic form, and s, sytidiotactic form, respectively. And, the activation energy for the atactic form sample is examined in a similar way. From the results, it can be concluded that the 3-relaxation of samples is attributed to the restricted rotation of the side chain, especially, to the rotation of the first bond-axis connecting the side chain and main chain. 

Chemical Properties. The chemical-resistance properties of methacrylic ester polymers are even higher than those of the acrylic esters. Methacrylic esters imdergo a lower degree of hydrolysis in either acidic or alkaline media than acrylics. Both acrylics and methacrylics easily outperform vinyl acetate-containing polymers which are well known to be susceptible to hydrolysis of the side-chain ester. There are marked differences in the chemical-resistance properties of different forms of PMMA. The syndiotactic (alternating) form of PMMA is the most chemically inert. The rate of hydrolysis for syndiotactic PMMA is lower than that for isotactic (26) radical polymerizations generate large portions of syndiotactic PMMA and benefit in terms of stability. 

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