Atactic Polymerizations

In the simplest case of a one-path polymerization, the ultimate monomer unit of the growing chain has no influence on the kind of addition that will subsequently occur. Consequently, isotactic and syndiotactic diads are produced with the same probability. 

Since a growing chain end may only add on a syndiotactic or an isotactic diad, the sum of the individual probabilities must equal unity. The individual probabilities are defined as the ratios of the reaction rate for the addition under consideration to the sum of the reaction rates for all possible additions  

The is and si triads are measured together as heterotactic triads. Consequently, a genuinely atactic polymerization leads to a random array of 50% isotactic and 50% syndiotactic diads, and 25% isotactic, 25% syndiotactic, and 50% heterotactic triads (see also Chapter 3.2). 

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On the basis of the X-ray structural data as well as the mode of polymerization, Yasuda et al. [3a] proposed a coordination anionic mechanism involving an eight membered transition state for the organolanthanide-initiated polymerization of MM A (Fig. 6). The steric control of the polymerization reaction may be ascribed to the intermolecular repulsion between C(7) and C(9) (or the polymer chain), since completely atactic polymerization occurred when the monomer was methyl or ethyl acrylate. 

Fig. 35. Specific radioactivity of the non-atactic polymeric fraction (and corresponding values of —CjHs mol. per mol. of polymerized CjH ) plotted vs, the amount of polymers obtained at different polymerization times. (Tests performed in the conditions reported in Fig. 34.)...

Ashikari, Kanemitsu, Yanagisawa, Nakagawa, Okomoto, Ko-bayashi and Nishioko (59) have studied the copolymerization of propylene and styrene. They found decreasing styrene content and conversion of the copolymer by increasing aluminum to titanium ratios with triisobutyl aluminum and titanium trichloride catalysts. The trialkylaluminum titanium tetrachloride catalyst had relatively low steric control on the polymerization while trialkylaluminum-titanium trichloride had higher steric control. The ionicity which is required for atactic polymerization is more cationic for styrene than for propylene which is more cationic than that for ethylene. Some of the catalyst systems for these three monomers are shown on the ionicity chart in Fig. 9. 

It is known that in propylene polymerization, both with conventional and supported Ziegler-Natta catalysts, at least two types of active centers can be distinguished. Such species can be associated with the so-called isotactic and atactic polymeric fractions, which have different configurations and may be separated by simple extraction with boiling heptane. Based on the 13C NMR analysis of the microstructure of the atactic and isotactic fractions, Inoue 1451 has recently proposed a two site model. At one site the stereospecific polymerization proceeds according to the Bernouillian model, and at the other it proceeds according to the enantiomorphic site model. However, it is understood that a two site model is an oversimplification. As a matter of fact, the crude polypropylene can usually be separated into several fractions having different tacticity 51>. 

When the configurations at the centers are more or less random, the polymer is not stereoregular and is said to be atactic. Polymerizations which yield tactic polymers are called stereospecific. Some of the more important stereospecilic polymerizations of vinyl polymers are described briefly in Chapter 9. 

Figure 3-10 Tactic and atactic polymerization, C NMR spectra at 75 MHz, solvent CDCI3, "SSL". Carbonyl regions of tactic (top) and atactic polylactide (botton).

It follows from this that Bernoulli and Markov mechanisms differ in whether the transition probabilities of the crossover, or hetero, steps are the same as or different from the homo steps (see Table 15-6). In addition, both types of mechanism can be subclassified as to whether the transition probabilities for the homo linkages are symmetric or asymmetric. In copolymerization, a symmetric Bernoulli mechanism with constitutionally different monomers is called azeotropic copolymerization with configurationally different monomers, it is called random flight polymerization and in stereocontrolled polymerization with nonchiral monomers, it is also called ideal atactic polymerization. ... 

Synthesis of the complexes usually requires the separation (normally by fractional crystallization) of the racemic isomer pair from the meso isomer, since the latter leads only to atactic polymerization of propylene. The syntheses explored by Jordan, described above in the section Preparation, tend to raise the overall yield of racemic metallocene. 

Depending on reaction temperatures, first-generation catalysts produce increasing amounts of atactic polypropene (8 to 20%) next to the isotactic main product. By modification with electron donors (Lewis bases see also Table 14) of the desired complexation tendency, the atactic polymerization sites can be largely deactivated, thus raising the isotaxy index to 94 to 98% [245]. It is obvious that atactic polymerization... 

Thus, it was found that an aspecific catalyst can be easily converted into an isospecific one either by replacing the cocatalyst or by using ethyIbenzoate. It is well recognized that the propagation rate constant of Isotactic polymerization is several times faster than that of atactic polymerization. The present results, therefore, indicate that only some aspecific species can be converted into isospecific ones the rest are deactivated. Such a deactivation may also occur in commercial catalysts, suggesting that catalyst efficiency could be much improved if this problem is overcome. 

When propene is polymerized under free radical conditions the polypropylene that results IS atactic Catalysts of the Ziegler-Natta type however permit the preparation of either isotactic or syndiotactic polypropylene We see here an example of how proper choice of experimental conditions can affect the stereochemical course of a chemical reaction to the extent that entirely new materials with unique properties result... 

The statistical nature of polymers and polymerization reactions has been illustrated at many points throughout this volume. It continues to be important in the discussion of stereoregularity. Thus it is generally more accurate to describe a polymer as, say, predominately isotactic rather than perfectly isotactic. More quantitatively, we need to be able to describe a polymer in terms of the percentages of isotactic, syndiotactic, and atactic sequences. 

Gas-phase polymerization of propylene was pioneered by BASF, who developed the Novolen process which uses stirred-bed reactors (Fig. 8) (125). Unreacted monomer is condensed and recycled to the polymerizer, providing additional removal of the heat of reaction. As in the early Hquid-phase systems, post-reactor treatment of the polymer is required to remove catalyst residues (126). The high content of atactic polymer in the final product limits its usefiilness in many markets. 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

Microstructure. Interest in PVP microstmcture 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(methacryhc acid) (or poly(MAA)), VP will apparentiy polymerize with syndiotactic microstmcture, although proof is lacking (41—45). The reverse, polymerization of MAA in the presence of PVP, affords, as expected, atactic poly(MAA) (46,47). 

Modified Bitumen Membranes. These membranes were developed in Europe during the late 1950s and have been used in the United States since the late 1970s. There are two basic types of modified asphalts and two types of reinforcement used in the membranes. The two polymeric modifiers used are atactic polypropylene (APP) and styrene—butadiene—styrene (SBS). APP is a thermoplastic polymer, whereas SBS is an elastomer (see Elastomers, thermoplastic elastomers). These modified asphalts have very different physical properties that affect the reinforcements used. 

Polystyrene. Polystyrene [9003-53-6] is a thermoplastic prepared by the polymerization of styrene, primarily the suspension or bulk processes. Polystyrene is a linear polymer that is atactic, amorphous, inert to acids and alkahes, but attacked by aromatic solvents and chlorinated hydrocarbons such as dry cleaning fluids. It is clear but yellows and crazes on outdoor exposure when attacked by uv light. It is britde and does not accept plasticizers, though mbber can be compounded with it to raise the impact strength, ie, high impact polystyrene (HIPS). Its principal use in building products is as a foamed plastic (see Eoamed plastics). The foams are used for interior trim, door and window frames, cabinetry, and, in the low density expanded form, for insulation (see Styrene plastics). 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin polymers). 

Polypropylene. There is an added dimension to the catalytic polymerization of propylene, since in addition to the requirement that the catalyst be sufficiently active to allow minute amounts of catalyst to yield large quantities of polymer, it must also give predominantly polypropylene with high tacticity that is, a highly ordered molecular stmcture with high crystallinity. The three stmctures for polypropylene are the isotactic, syndiotactic, and atactic forms (90) (see Olefin polya rs, polypropylene). 

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

Epichlorohydrin Elastomers without AGE. ECH homopolymer, polyepichlorohydrin [24969-06-0] (1), and ECH—EO copolymer, poly(epichlorohydrin- (9-ethylene oxide) [24969-10-6] (2), are linear and amorphous. Because it is unsymmetrical, ECH monomer can polymerize in the head-to-head, tail-to-tail, or head-to-tail fashion. The commercial polymer is 97—99% head-to-tail, and has been shown to be stereorandom and atactic (15—17). Only low degrees of crystallinity are present in commercial ECH homopolymers the amorphous product is preferred. 

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