Poly methacrylates Tacticity

Table 8.3 Effect of Amines on Tacticity of Poly(methacrylic acid) at 60 DC90...

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... 

Polymerization. Poly (methyl methacrylate) was obtained commercially. The polymers of other methacrylates and their copolymers were prepared in toluene with 2,2 -azobisisobutyronitrile (AIBN) at 60 °C. All the polymers prepared free radically were syndiotactic or atactic. Isotactic poly(a,a-dimethylbenzyl methacrylate) was obtained using C6H5MgBr as the initiator in toluene at 0°C. Poly(methacrylic acid) was prepared in water using potassium persulfate at as the initiator 60 °C. The molecular weights, glass transition temperatures and tacticities of the polymethacrylates are summarized in Table I. 

Microstructure. Interest in PVP microstructure and the potential for tacticity has been reviewed (39,40). PVP generated by free radicals has been shown to be atactic except when polymerization is conducted in water. In this case some syndiotacticity is observed (40). In the presence of syndiotactic templates of poly(methacrylic acid) (or poly(MAA)), VP will apparently polymerize with syndiotactic microstructure, although proof is lacking (41—45). The reverse, polymerization of MAA in the presence of PVP, affords, as expected, atactic poly(MAA) (46,47). 

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. 

Very little research has been done on the relations between glass transition temperatures and tacticity. Atactic and isotactic poly(styrenes) almost always have the same glass transition temperatures, and this is also the case for at- and it-poly(methacrylate). The glass transition temperature of it-poly(methyl methacrylate) (42° C), on the other hand, is distinctly lower than that of the atactic product (103°C). 

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). 

Poly(methacrylic acid) in the nonionized form in solution has a compact conformation and low intrinsic viscosity. Upon ionization to the polyelectrolyte form, chain expansion occurs and viscosity increases. Unlike PAA, PMAA shows inverse solubility-temperature behavior. The presence of chain-stiffening methyl groups and their added hydrophobicity are responsible for the phase and viscosity behavior. Tacticity also plays an important role. 

Poly(acrylates) and poly(methacrylates) are commercially important polymers with a myriad of uses, including paper and textile coatings, adhesives, caulks and sealants, plasticizers, paint and ink additives, and optical components for computer displays. Since they are derived from monosubsti-tuted and unsymmetrical 1,1-disubstituted vinyl monomers, poly(acrylate) and poly(methacrylate) products with a spectrum of tacticities and thereby mechanical properties are potentially accessible. To date, however, industrially produced materials are generated using free-radical polymerization technology, which offers limited scope for tacticity control. Therefore, there has been much interest in the development of metal-catalyzed routes to these polymers where the coordination environment of the metal offers the potential to influence tacticity. 

Challa and co-workers " observed that rates of polymerization of )V-vinyl-pyrrolidone were increased by ih presence of poly(methacrylic acid) (PMAA), the enhancement becoming more pronounced with increasing chain length and syndiotacticity of the latter. The mechanism was considered to be similar to that in PMMA-MMA systems [case (ii). Scheme 2] but no information as to tacticities of the daughter polymers could be obtained. Strongest complexes were supposed to be formed between poly()V-vinylpyTrolidone) and syndiotactic PMAA. 

The first attempts to synthesize poly(acrylic acid) (PAA) or poly(methacrylic acid) (PMAA) were the hydrolysis of poly(acid derivatives) such as esters, acylchlorides, nitriles, or amides. The hydrolysis has to be quantitative otherwise, one obtains a copolymer of acid and derivative [472]. On the other hand, hydrolysis in boiling alkaline solution can diminish the molar masses [473]. One possibility is to polymerize methyl methacrylic ester and to hydrolyze the resulting PMMA in acetic acid by the addition of a small amount of -toluenesulfonic acid as a catalyst. The solution is kept at 120 °C for 18 h and the methylacetate fonned is removed by distillation [474]. The degree of hydrolysis depends strongly on the tacticity of the original polymer. Syndiotactic PMMA is hydrolyzed slowly, but isotactic polymer is hydrolyzed very rapidly [475]. The polymers examined had molar masses up to 125,000. 

The more drastic requirements for hydrolyzing poly(methacrylates) are illustrated in Procedure 2-3. The tacticity of the polymer seems to have a profound effect on the ease of alkaline hydrolysis. Thus, for example. 

The polymer is methylated with diazomethane in benzene, and the tacticity of the resulting poly(methyl methacrylate) is studied by NMR spectroscopy. The polymer exhibited 85% sindiotactic triads. Poly(methacrylic acid) prepared at 60°C in methyl ethyl ketone solution with AIBN was 57% syndiotactic. 

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. 

A dependence of Ij /i upon tacticity in fluid solutions of poly(l-and 2-naphthyl methacrylate)s has also been reported [31]. The implications of these observations are less apparent, since it has been suggested [12,30] that excimer formation in such poly (methacrylate)s is dominated by long-range interactions. 

MorceUet M. Effect of tacticity on the association of poly(methacrylic acid) with divalent metal ions. J Polym Sci Polym Lett Ed. 1985 23 99-102. 

Figure 7.10 Nuclear magnetic resonance spectra of three poly(methyl methacrylate samples. Curves are labeled according to the preominant tacticity of samples. [From D. W. McCall and W. P. Slichter, in Newer Methods of Polymer Characterization, B. Ke (Ed.), Interscience, New York, 1964, used with permission.]...

In this stage of the investigation, poly(methyl methacrylates) (PMMAs) were selected as the polymeric probes of intermediate polarity. Polymers of medium broad molar mass distribution and of low tacticity (14) were a gift of Dr. W. Wunderlich of Rohm Co., Darmstadt, Germany. Their molar masses ranged from 1.6 X 10" to 6.13 X 10 g-mol. For some comparative tests, narrow polystyrene standards from Pressure Co. (Pittsburgh, PA) were used. 

Table 8.2 Effect of Solvent on Tacticity of Poly(alkyl methacrylate) at -40 °C94...

Iso tactic poly(methyl methacrylate) (it-PMMA) can form a stereocomplex with st-PMMA. Recent X-ray studies 179) of this material indicate that the two polymer chains probably interact to form a double helical structure. The it-PMMA chain forms the inner helix and is surrounded by the st-PMMA helical chain which winds around it. If subsequent work confirms this model, this material would constitute a most unusual inclusion compound involving only one monomeric substance. 

The tacticity or distribution of asymmetric units in a polymer chain can be directly determined using NMR spectroscopy and infrared (IR) spectroscopy and has been studied for a variety of polymers. Figure 5(a) and 5(b) show the proton NMR spectra [26,27] and IR spectra [28,29], respectively, for the two stereoisomers of poly(methyl methacrylate) (PMMA), syndiotactic and isotactic PMMA. These two structures in a polymer like PMMA give rise to different signatures in both the techniques. In the case of the NMR spectra [26,27], the... 

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

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