Poly isotactic syndiotactic

Figure 7.10 shows the 60-MHz spectra of poly (methyl methacrylate) prepared with different catalysts so that predominately isotactic, syndiotactic, and atactic products are formed. The three spectra in Fig. 7.10 are identified in terms of this predominant character. It is apparent that the spectra are quite different, especially in the range of 5 values between about 1 and 2 ppm. Since the atactic polymer has the least regular structure, we concentrate on the other two to make the assignment of the spectral features to the various protons. 

Polymers that incorporate steric centers into their backbones can display various types of tacticity. The three principal types of tacticity are isotactic, syndiotactic, and atactic, as illustrated in Fig. 1.8 for polypropylene. Other polymers that display tacticity include polystyrene and poly a-olefins,... 

FIGURE 2.5 Skeletal formulas of isotactic, syndiotactic, and atactic poly(vinyl chloride) (PVC). 

The reader should note that stereoisomerism does not exist if the substituents X and Y in the monomer 4-14 are identical. Thus there are no configurational isomers of polyethylene, polyisobutene, or polyfvinylidene chloride). It should also be clear that 1,2-poly-butadiene (reaction 4-3) and the 1,2- and 3,4-isomers of polyisoprene can exist as isotactic, syndiotactic. and atactic configurational isomers. The number of possible structures of polymers of conjugated dienes can be seen to be quite large when the possibility of head-to-head and head-to-tail isomerism is also taken into account. 

The influence of preformed stereoregular polymethylmethacrylate on the polymerization mechanism is particularly interesting. Grignard compounds at — 50 C in toluene give syndiotactic poly(methylmethacrylate) when preformed isotactic poly(methyl-methacrylate) is present, and vice versa [30,31]. In this replica polymerization, the primary structure formed is the 1 1 (isotactic/syndiotactic) complex. Further association between this complex and syndiotactic macromolecules results in the 1 2 (isotactic/syndiotatic) complex [32]. In the absence of preformed polymer, isotactic poly(methylmethacrylate) was obtained under the same conditions. 

The polymer of 13C-labeled 2-phenyl-1-methylenecyclopropane by the same catalyst contains the isotope-labeled carbon exclusively at the exo-methylene group. The polymer consists of the 1-phenyl-2-exo-methylenepropan-1,3-diyl structure, exclusively. The 13C 1H NMR signals of the ipso carbon of the phenyl group are split into four due to the four triads, rr, rm, mr, and mm, based on comparison of the spectrum with that of poly(styrene-co-CO) with isotactic, syndiotactic, and atactic arrangement of the monomer units [160] (Scheme 16). Thus, the polymer produced from 2-phenyl-1-... 

Figure 2.16 Isotactic, syndiotactic, and heterotactic triad configurations for poly(methyl methacrylate).

There are several types of biodegradable synthetic polymer with vulnerable chemical moieties susceptible to enzymatic attack. The most typical ones are aliphatic polyesters, such as poly(glycolic acid), poly(lactic acid), and poly(e-caprolactone). Highly isotactic-, syndiotactic- and atactic-homopolyesters of... 

Table 2. Mechanical properties of isotactic, syndiotactic poly(MMA) and the stereocomplex...

Whereas poly(a-olefins) have only two microstructures of maximum order (isotactic, syndiotactic). cyclopolymers ° have /bur microstructures due to the rings present in their main chain which can be either cis or trans in configuration (Scheme 17). While the key issues concerning selectivity in the polymerization of a-olefins are regioselectivity (head-to-tail monomer incorporation) and enantioface selectivity (tacticity). cyclopolymerization of a.co-diolefins has added concerns. First, since the monomer has two olefins, either cyclization or cross-linking of the... 

This can be seen from the data in Table 10.1, where isotactic, syndiotactic, and atactic poly(methyl methacrylate) (PMMA) have different a values. If the size of a polymer chain can be affected by its configuration, the microstructure must be well characterized before an accurate assessment of experimental data can be made. This can be achieved using nuclear magnetic resonance (NMR) and infrared techniques. 

Figure 8. NMR spectrum in CDCI3 of poly(1,2-epoxypropane) prepared by the polymerization with chloroaluminum tetraphenyl-porphyrin (1c). m, r, I, S and H represent meso, racemo (diad) and isotactic, syndiotactic, and heterotactic (triad) sequences, respectively.

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... 

Other symbols occasionally used in the literature include (br) for branched materials and (iso), (syndio), and (a) for isotactic, syndiotactic, and atactic structures respectively. No symbol appears to exist for mechanical blends, although these materials are obviously important. Where necessary the symbol -m- will denote a mechanical blend, for example, poly(styrene-m-butadiene) for a mechanical blend of polystyrene with polybutadiene. 

Polymerization of trimethylsilyl methacrylate to alternatively form the isotactic, syndiotactic and atactic polymers has been described previously (22). The molecular weight and tacticity of these polymers could be determined after acidic workup of the polymerization reaction followed by isolation of the polymer and treatment with diazomethane to form poly(methyl methacrylate). The molecular weight and tacticity of these polymers are given in Table II. The values obtained represent dramatic differences in tacticity as a function of preparation method and thus provide a reasonable basis for the testing the effect of polymer microstructure on its subsequent behavior. 

In unsymmetrically disubstituted chains, the substituents of one type can be on the same side of the all-trans chain, on opposite sides, or on either side in a random arrangement, yielding isotactic, syndiotactic, and atactic forms. Poly(methylphenylsiloxane) was one of the chains chosen to illustrate this stereochemical variability. The relatively large Si-0 bond length and Si-O-Si bond angle place apposed side groups at distances of separation (ca. 3.8 A) at which there is a favorable energy of interaction. 

Conformational factors. The most important conformational factor is the tacticity of vinyl-type polymers. A polymer such as poly(methyl methacrylate) can have quite different values of Tg, depending on whether it is isotactic, syndiotactic, or atactic. See Table 3 for a collection of literature data (1) on the effects of tacticity on Tg. A theoretical analysis of the effects of tacticity variations on Tg has been provided (52). 

In addition, the 3,4- and 1,2-polyisoprenes can both exist in three forms isotactic, syndiotactic, and atactic. Thus there are eight possible structures if we disregard head-to-head possibilities. The part of the structure elements in the polymer depends on the catalysts. In general, the polymerization activity is lower compared to poly butadiene. Of the various structures of polyisoprene, only cis- and raw -1,4-polyisoprene and atactic 3,4-polyisoprene are important (Table 8) [217-219]. 

The crystal structures of polyesters based on o-hydroxy and 3-hydroxyacids have been extensively studied since the realization that a family of high melting polymers was achievable based on these structures (29,30). By suitable substitution of the monomers, optically active polymers can be prepared and the deliberate adjustment of the relative optical antipode content and distribution leads to steric copolymers which can present all the characteristics of isotactic, syndiotactic and stereoblock structures now familiar in the poly-a-olefin series. 

Three steric arrangements are shown as Fischer projections for poly(propylene) with the corresponding descriptors isotactic , syndiotactic , and atactic . 

The H—NMR spectra are rather complex. The deuteration in tertiary position (on H ) simplifies the situation and at 100 MHz one can observe for poly(methyl thiirane) [38] and poly(methyl oxirane) [39] two partially overlapping quartets corresponding to protons Ha and Hb of the methylene group. At 300 MHz it was shown for poly(methyl thiirane) [37] and for poly(methyl oxirane) [13] that isotactic (/), syndiotactic (s) and heterotactic... 

---