Polyfmethyl syndiotactic

Figure 2. 200 MHz H NMR spectrum of isotactic (a) and syndiotactic (jb) polyfmethyl methac-lylate).

Fig. 33a-d. Solvent effect on the formation of the stereocomplex of isotactic polyfmethyl methacrylate) (iso-PMMA) with syndiotactic PMMA (synd-PMMA)300. (a) In chloroform, (b) in benzene, (c) in dimethylformamide, (d) in acetone... 

Draw a three-dimensional representation of segments of the following polymer (a) Syndiotactic polyacrylonitrile (h) Atactic polyfmethyl methacrylate)... 

It should be noted that the steric effects of the pendant groups considered above are simply additional contributions to the main chain effects. Similarly cis-trans isomerism in polydienes and tacticity variations in certain a-methyl substituted polymers alter chain flexibility and hence affect Tg. Well-known examples of cis-trans variations are polybutadiene cis Tg= — 108°C) and trans(T = — 18°C) or polyisoprene cis Tg = —73°C) and trans T = —53°C). An example of tacticity variation is polyfmethyl methacrylate) for which the isotactic, atactic, and syndiotactic stereostructures are associated with Tg values of 45, 105, and 115°C, respectively. 

The appearance of a maximum in the chain length dependence of the characteristic ratio was first found by Yoon and Flory [60] in the RIS chain computations on atactic and syndiotactic polyfmethyl methacrylate)(PMMA). Yamakawa and Fujii [63] showed that their formula for the HW chain with <7o can be fitted accurately to the computed results of Yoon and Flory over dm almost entire range of chain length if the basic parameters of the HW chain are suitably chosen. A similar agreement in characteristic ratio between the HW (<7o = 0) and RIS chains was obtained for other flexible polymers. The choice of relation between the RIS chain model and the discrete chain whose continuous limit is the HW chain. 

Bolig, A. D. Chen, E. Y.-X. Isotactic-fc-syndiotactic stereoblock polyfmethyl methacrylate) by chiral metallocene/Lewis acid hybrid catalysts. J. Am. Chem. Soc. 2002,124, 5612-5613. 

Until 2003, Chen s [28], Qu s [29-31], and Hu s [32] groups independently reported nanocomposites with polymeric matrices for the first time the. In Hsueh and Chen s work, exfoUated polyimide/LDH was prepared by in situ polymerization of a mixture of aminobenzoate-modified Mg-Al LDH and polyamic acid (polyimide precursor) in N,N-dimethylactamide [28]. In other work, Chen and Qu successfully synthesized exfoliated polyethylene-g-maleic anhydride (PE-g-MA)/LDH nanocomposites by refluxing in a nonpolar xylene solution of PE-g-MA [29,30]. Then, Li et al. prepared polyfmethyl methacrylate) (PMMA)/MgAl LDH by exfoliation/adsorption with acetone as cosolvent [32]. Since then, polymer/LDH nanocomposites have attracted extensive interest. The wide variety of polymers used for nanocomposite preparation include polyethylene (PE) [29, 30, 33 9], polystyrene (PS) [48, 50-58], poly(propylene carbonate) [59], poly(3-hydroxybutyrate) [60-62], poly(vinyl chloride) [63], syndiotactic polystyrene [64], polyurethane [65], poly[(3-hydroxybutyrate)-co-(3-hydroxyvalerate)] [66], polypropylene (PP) [48, 67-70], nylon 6 [9,71,72], ethylene vinyl acetate copolymer (EVA) [73-77], poly(L-lactide) [78], poly(ethylene terephthalate) [79, 80], poly(caprolactone) [81], poly(p-dioxanone) [82], poly(vinyl alcohol) [83], PMMA [32,47, 48, 57, 84-93], poly(2-hydroxyethyl methacrylate) [94], poly(styrene-co-methyl methacrylate) [95], polyimide [28], and epoxy [96-98]. These nanocomposites often exhibit enhanced mechanical, thermal, optical, and electrical properties and flame retardancy. Among them, the thermal properties and flame retardancy are the most interesting and will be discussed in the following sections. 

The atactic form of polyfmethyl methacrylate) is amorphous and exhibits only one transition temperature (Tg). The stereoregular isotactic and syndiotactic forms are partially crystalline and undergo both a glass transition and melting. 

Finally, it proved possible to obtain an acceptable fit for polyfmethyl methacrylate) using a carbomethoxy shift parameter of —9.50 ppm. This shift was not very well defined by the data, although it must certainly be large. (When it was permitted to float freely in the minimisation, it drifted upward to —14 ppm, but with only a very minor resulting improvement in the least-squares fit.) If the conformations that we calculate to have highest probability are instead set to be dominant, then the syndiotactic chain becomes all-trans, as observed in the solid state by Speracek et al. [20], and a -I-8.14 ppm shift increment is predicted on going from the syndiotactic to the isotactic polymer. The experimental figure is between -l- 6.0 and -l- 8.45 ppm. 

The functionalized initiator, 26, has also been used to initiate polymerization of methyl methacrylate in THE at -78 °C to form the corresponding a-dimethylamino-functionalized polyfmethyl methacrylate), 34, as shown in Scheme 16 [50]. The resulting a-dimethylamino-poly(methyl methacrylate) with 82% syndiotactic diads exhibited a narrow molecular weight distribution as determined by SEC analysis (M /M =1.06) and the amine functionality was 1.0 as determined by end-group titration. A telechelic a,0)-bis(dimethylami-no)poly(methyl methacrylate) (35) was prepared by reacting living amine-functionalized poly(methyl methacrylate)lithium enolate (33) with a one-half... 

The frequency differences and assignments for polyfmethyl methacrylate) for the spectra of syndiotactic and... 

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