Tacticity, effect

The presence of hydroxyl end groups in PPG can be assigned from 13C NMR data (Figure 24), with resonances (g) observed between 65-68 ppm in the spectrum shown in Figure 24. There are two different resonances here (g) due tacticity effects in the polymer (meso/racemic (m/r) isomerisation of the terminal dyad). Estimations of the amount of these ends in the polymer, along with the average molecular weight of the PPG, can be made from the 13C NMR data. 

To obtain fuller picture on the cyclolinear polyorganosiloxanes tacticity effect on formation of crys-talline and mesomorphous states, by N.N. Makarova and her co-workers the PMPCS-4 and PPCS-4 were synthesized and their properties were studied. 

In previous chapters have seen that chemicals to be tactically effective must be brought into contact with the body or the body must be surrounded by a toxic atmosphere in which the concentration of the... 

The results pursued by both groups in the case of the FARCs, the intention was to cause a tactic effect, without the objective of producing an extreme damage also they did not want to give publicity to this special attack abroad. In the case of Aum, the objective was to cause a great impact in the society, and even promote a political change. 

Substitution of a larger group (such as methyl) or a larger atom (such as chlorine) for the hydrogen atom on the a-carbon results in a large tacticity effect on the Tg s of acrylic... 

Even in the absence of substitution on the a-carbon, extremely large side groups attached directly to the polymer chain, as in poly(N-vinyl carbazole), can cause a large tacticity effect. 

Fig. 39. Mean relaxation time (sec) as a value for the period of the space tactical effectivity of a viscoelastic substance (sorted according to the classes of substances and the period of the relaxation time, both with rising order n=6)...

KAT Katsumoto, Y. and Kubosaki, N., Tacticity effects on the phase diagram for poly(A/-isopropylaciylamide) m-wat x,Macromolecules, 41, 5955, 2008. 

When present, tacticity effects do not necessarily vary regularly with... 

An example of tacticity effects in poly(epoxides) is shown in the backbone spectrum of anionically polymerised poly(butylene oxide) in Figure 1.27 [35]. The CH resonance is narrowly split into eight peaks of essentially equal intensity, consistent with chemical shift sensitivity at the tetrad level. Evidently the CH carbon senses the tacticity of its nearest neighbours in both directions in the chain, but senses the tacticity of its next-nearest neighbour in only one direction. The CH2 carbon is split into four equal peaks, consistent with sensitivity to tacticity at the triad level. The fact that the triad and tetrad peaks are essentially equal in intensity indicates a perfectly random tacticity distribution, i.e., = 0.5. 

In the assignment of the roethine carbon resonances in the E-VOH copolymers we have to take in account both tacticity effects, as in PVOH, and sequence distribution effects. For the calculation of the chemical shifts in the sequence triads of solid E-VOH copolymer, the chemical shift additivity rules for aliphatic alcohols, as calculated by Ovenall [5] from solution NMR measurements, have been used ... 

Much less recognized is the possible influence of tacticity on copolymer properties when a-olefin monomer units are a minor component, and crystallinity is not based on a tactic a-olefin sequence but on a different comonomer such as ethylene. In this chapter, this tacticity effect is shown for ethylene-rich ethylene/propylene (EP) copolymers, where the crystallizable sequences are based on ethylene, that is, a comonomer that does not have tacticity requirements. In particular, this chapter describes in detail the microstructure of EP copolymers having industrially relevant compositions (ethylene content 80-55 mol%), with particular focus on the placement of propylene units along the ethylene-based macromolecular chains and their influence on copolymer properties. This subject is, of course, related to the industrial relevance of EP copolymers and ethylene/propylene/diene monomer terpolymers (EPDMs) (collectively referred to as EP(D)Ms), which presently represent the most widely produced saturated rubbers. ... 

The H-NMR spectrum of isotactic PBMA at 100 C does not show the quartet for the butyl methyl group (H-T), which instead seems to be the expected triplet. Further scrutiny reveals smaller peaks between the peaks of the triplet. The same long-range tacticity effect exists for isotactic PBMA, but the larger triplet corresponds to the dominant mm triad sequence. The much smaller peaks correspond to the mr sequence. 

One may, by careful choice of monomer, study the potential of different catalysts to behave in a stereoselective or regioselective manner. Thus, with a symmetrical monomer such as norbomene [58], norbomadiene [59] or their 5,6 [60] or 7-substituted derivatives [61,62] we have obtained polymers with a variety of cis main chain double bond contents and distributions. In a number of the 7-substituted examples, fine structure on certain NMR resonances is observed which is attributable to tacticity effects. Conversely, one may use the unsymmetrical monomers such as 1-substituted derivatives [63] and delineate the propensity of the different catalysts to regioselectivity, which manifests itself as head-tail bias in the polymer. 

In Figure 3.19(b) is shown the fully proton decoupled spectrum obtained in pyridine at 38 "C of a (MMA/MAA) copolymer of approximately 33% acid composition. While the spectrum resolves into a-CHj, CHj, OCH3 >C<, and C - O regions, the structural similarity of the comonomers results in resonance overlap (at 20 MHz) between ester and acid carbons in all regions but the carbonyl. The complete structure in the carbonyl region arises from the sensitivity of the carbons to the microstructural features of the copolymer chain, i.e., sequence and tacticity effects. To be sure that these effects do not lead to overlap of ester and acid carbonyl resonance, chemical shifts of both homopolymers and homosteric copolymers were examined with the result that the carbonyl resonance region does separate into distinct acid and ester regions in pyridine. 

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