Tacticity, influence

Polymers of MAA often exhibit syndiotacticity, the extent being influenced by the template effect (Figure 2.7B) [22], solvent and temperature. To what extent the tacticity influences the recognition properties of the materials is not clear. 

AB Stereoblock PMMA Synthesized From MMA and TrMA. In order to determine how tacticity influences sterocomplex formation, DMA was replaced by MMA in the syntheses of PMMA A-B stereoblock polymers. Homopolymer analysis showed that syndiotactic content was 10% lower for PMMA synthesized from MMA than that prepared from DMA. The AB stereoblock pol)7mers of PMMA obtained from MMA and TrMA were synthesized in the same manner as those obtained from TrMA (Scheme 3), the exception being that MMA was substituted for DMA. 

The Tg increases with pressure. The tacticity influences Tg (syndiotactic higher than isotactic). The higher the cross-linking density in a polymer network, the higher is Tg. Plasticizers are added to a polymer to reduce Tg. Content of solvents (absorbed water) can act as a plasticizer. 

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). 

Equation (9-234) illustrates the enormous influence that go-slow tactics can have on the profitability of companies and processes which have low contribution efficiencies, since a slowdown has little effect on Aee- B L sometimes the case that in different countries productivity per worker varies considerably in similar industries. When poor productivity is not the result of technical or capital inadequacy, it should be possible to increase profitability without a proportionate increase in Aee-... 

The shortest distance between A and B is not always a straight line. Keep in mind as you consider intangible factors that you must influence behavior in order to improve companywide safety performance. Achieving this may require a combination of skills and tactics, some of which may not be directly related to explicit PSM requirements. 

There is also clear evidence that penultimate group effects are important in determining the stereochemistry of addition in many homopolymerizations and copolymerizations. This is made evident from the fact that most homopolymers have tacticity (i.e. P(/ i)f0.5, Section 4.2). Indeed, for some homopolyinerizations there is evidence that the configuration of the pcnpcnultimatc unit may also influence the stereochemistry of addition/9 If penpen- and penultimate units... 

Puzin et al.h] reported that the tacticity of PMMA prepared in bulk is influenced (slight increase in syndiotacticity) by very small amounts of titanoecnc diehloride (1 O 3 M). Selective complexation of the propagating radical was postulated. 

Tacticity of VIA A is influenced by solvent,90 9 the presence of amines (Table 8.3)90 and complcxation. PYIAA appears more isotactic when formed in a nonhydrogen-bonding solvent,90 95 Polymerization of MAA in CHCh in the presence of 14 or 15 also yields a more isotactic polymer90 Polymerization of zinc complexes of MAA also yields more isotaclic polymers,9 >... 

One of the most used systems involves use of horseradish peroxidase, a 3-diketone (mosl commonly 2,4-pentandione), and hydrogen peroxide." " " Since these enzymes contain iron(II), initiation may involve decomposition of hydrogen peroxide by a redox reaction with formation of hydroxy radicals. However, the proposed initiation mechanism- involves a catalytic cycle with enzyme activation by hydrogen peroxide and oxidation of the [3-diketone to give a species which initiates polymerization. Some influence of the enzyme on tacticity and molecular... 

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. 

The three hierarchical levels are interconnected by information flowing from the strategic level via the tactical level to the operational level, and the other way around. From upper to the lower level, the information flow is related to the environment on the strategic level, which is the organizational values and norms. However, as Thompson (Thompson, 1967) identified, the tactical control level can allow the operational level to operate as a relatively closed system. The tactical level provides a buffer between the uncertain environment and stability of resources required for uninterrupted production on the operational level. In this way the influences from the external environment on the operational level will be reduced to a minimum. The information flow going from lower to upper level is related to the operational process or transformations. The top down flow provides the restrictions and conditions for the transformation, while the bottom up flow provides information about the status of inputs, outputs, and resources of the transformations. The horizontal information flows are between different control elements on one hierarchical control level. 

So where does our analysis leave the investigator Not necessarily up the creek without a paddle we can adapt Mme. de Stael s famous phrase to Tout comprendre, cest tout controller . The object of understanding is mastery, and once we are aware of the physicochemical influences at work in the cationoid systems, we can attempt to adjust the conditions in such a way that we achieve monoeidic systems on which we can make unambiguous measurements. Another aspect of chemical mastery is the ability to make wanted products, and if we understand the factors influencing the composition of the ionic population, it will be possible to make materials of different tacticity, DPD, etc., from the same monomer, and quite different copolymers from the same two monomers, by a well-informed choice of reaction conditions. 

The cyclization rate kc is higher for B B than for B B dyads and a drastic influence of tacticity is observed, the meso dyads being by far more reactive than the racemic ones,as expected (8). Some characteristic Kc values measured at 20°C in NaOH 0.1 N for different systems are collected in table 5. 

To conclude, kinetic measurements and structural analysis of the copolymers have allowed a quantitative and self-consistent description of the reaction of RCI Li species on PMMA taking into account PMMA chain reactivity through the simplified model of the nearest neighbouring group effects. Two main features are particularly relevant the definite influence of tacticity, and the independance of the reaction process on the total charge of the copolymer. In this sense, the R-C Li/PMMA systems are closed to the PMMA basic hydrolysis in presence of excess base. (29,31). 

In semi dilute (c>c, [B] = 0.3-0.5 mole.l- ) or dilute (c< c, [B] = 10-2 mole.l- -) solution, Kp is significantly greater for copolymers than for the model compounds whatever the solvent is. For semi-dilute solution in a given solvent, the complex influences of composition, unit distribution and tacticity do not result in definite trends on K, values, as illustrated in table 6 by some representative Kj data related to keto-2-picolyl structures at 25°C. 

If this is the case and if the rate of polymerisation increases with the propene concentration, the latter influences the tacticity. 

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