Triads isotactic

An isotactic triad [XV] is generated by two successive meso additions XXX... 

A mechanism in which the stereochemistry of the growing chain does exert an influence on the addition might exist, but at least two repeat units in the chain are required to define any such stereochemistry. Therefore this possibility is equivalent to the penultimate mechanism in copolymers. In this case the addition would be described in terms of conditional probabilities, just as Eq. (7.49) does for copolymers. Thus the probability of an isotactic triad controlled by the stereochemistry of the growing chain would be represented by the reaction... 

Millan and coworkers (99-101) also studied the effect of tacticity on the nucleophilic substitution reactions of PVC. Sodium thiophenate and phenol were used for these reactions. The central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads was found to be most reactive. It was concluded that initiation of degradation may occur by normal structures, and polyene build-up may be favored by syndiotic sequence. This... 

As discussed in Section III.G, the possibility of simultaneous initiation of degradation from normal units in the PVC chain is receiving increased attention. The work of Millan and coworkers who showed that the normal secondary chlorines in isotactic triads are also labile and can act as points of initiation for thermal degradation was also discussed. This favors simultaneous participation of normal chlorines along with defect sites in the thermal degradation of PVC. The content of nondefect labile structures in PVC is much higher than that of the defect structures [125]. 

All these kinetic features may be readily taken into account within the 3 following assumptions a) because of lower steric hindrance, isotactic triads exhibit a better accessibility than the syndiotactic ones kQ(AmAmA)>kQ(ArArA) b) autoretarded kinetics arises from increasing steric hindrance around the reaction sites as the substitution proceeds further, and from electrostatic repulsion between the anionic reagent j RCH2 ) and the modified negatively charged chain (AAB and AAB-AB-) c) this electrostatic effect is partly canceled in isotactic triads by anchimeric assistance of the substituted B unit to the SN2 step WVmAJ-MAmV -). 

TPP)MnOAc (18) is an excellent initiator of the polymerization of the ringopening polymerization of PO. An example is illustrated in Fig. 52, where 400 equiv of PO was consumed completely in about 20 h at 30 °C. The NMR spectrum of the polymer shows a simple resonance pattern due to the CH3 group (6 17.4 ppm), indicating that the polymer consists exclusively of head-to-tail linkages. The NMR spectrum was also informative concerning the stereoregularity of the polyether, where the obtained polyether was almost atactic (i/s= 0.54 0.46,1/H/S=0.28/0.50/0.22). This is in contrast to the case with the aluminum porphyrin as initiator, which produces a polymer rich in isotactic triad sequences under similar conditions. 

An unambiguous stereochemical assignment of the alpha methyl absorptions is difficult to make. On the basis of the similarity of the observed spectra to the closely similar poly (alphamethylstyrene) ( , 10, Jl ), it is tempting to assign the upfield and downfield absorptions to syndiotactic and isotactic triads, respectively. This assignment would be supported by the 300 and 600 MHz 1h spectra of tetramer [14] that suggest predominant (>90%) formation of a racemic dyad. 

In agreement with the data on poly-a-olefins and poly-vinyl-ethers having the side chain asymmetric carbon atoms in the y-position with respect to the main chain, stereoregularity does not exert a remarkable influence on the rotatory power of poly-acrylates and poly-methacrylates. In fact, according to the quantitative data reported by H. Sobue, K. Matsuzaki, S. Nakano (135), and to the qualitative indications given by Liu, Szuty and Ullmann (64), concerning respectively poly-menthyl-meth-acrylate and poly-(l-methyl-benzyl)-methaciylate, the specific optical activity of the polymers does not vary by more than 30% when varying, within a wide interval, the isotactic triads content of the polymers (Table 18). 

In all of these systems, there is a tendency to form more isotactic triads than heterotactic triads (coming from an s to an i addition) with increasing temperature. With the exception of vinyl formate, all differences (AHzf/i — Aff ) are positive however. A formation of heterotactic triads (from an i to an s addition) is thus favored over the formation of syndiotactic triads with increasing temperature. Because the syndio-... 

A similar result has been recently found for the free radical polymerization of methyl methacrylate in 14 solvents (32). All differences (Aff. — AHf/8) were found to be positive, but only three of the 14 differences (AH /g — AH. ). Again, isotactic triad formation is favored over heterotactic triad formation and heterotactic triad formation over syndiotactic with increasing temperature as long as the individual modes of addition are considered and not the net result. Except for meth-acrylic acid in alcohols (cf. Lando et al. (28)) no model is known which shows why a certain solvent acts differently from another one with respect to stereocontrol in free radical polymerization. 

Within the limits of error, the ratio fc / /fci/i is always greater than 1—i.e., the formation of syndiotactic triads is preferred over the formation of isotactic triads (column I). The system methacrylic acid/ alcohol with (fc / /fcj/j) = 7.3 is outstanding in this respect. 

The molecular weight distribution is very broad, and the high-molecular-weight part only contains rara-(isotactic) triads. Further studies with other initiators are described in the same paper [7]. All of the results show a dependence of the microstructure on the initiator being used and allow an insight into the mechanism of the polymerization process. The nature of the active species in these special anionic polymerizations is responsible for the actual course of the polymerization. 

The stereochemistry of GTP of MMA polymerization was measured for Lewis acid as well as for bifluoride catalysis. Lewis acid catalysts gave a ratio of syndiotactic heterotactic triads of 2 1 while bifluoride catalysis gave ratios near 1 1 [9, 41]. The amount of isotactic triads was about 5%. The effect of temperature on triad and diad composition provided data to calculate the difference in activation enthalpy (AAH ) and activation entropy (AAS ) for... 

However, when (5,5)-3,3 -(2,3-butanediol)-2,2 -bipyridine or (/ )-3,3 -(l, 2-propanediol) -2,2 -bipyridine was used as the ligand [125], copolymers were obtained that had a higher content of isotactic triads. An effective control towards the isospecificity of copolymerisation (<98 % isotacticity in the copolymer) is fulfilled for the copolymerisation of ring-substituted styrene such as p-t-butylstyrene and carbon monoxide with catalysts containing cationic methylpalladium species [117] ... 

Oxetane, a four-membered cyclic ether, is highly susceptible to cationic polymerisation [83]. However, this monomer also undergoes coordination polymerisation in the presence of catalysts such as zinc dimethoxide [84], triethylaluminium water acetylacetone [85-87], aluminium isopropoxide zinc chloride and di-ethylzinc water [87,88], as well as tetraphenylporphinatoaluminium chloride methylaluminium di(2,6-di-/-butyl-4-methylphcnoxidc) [89]. Studies of the microstructure of the polymer derived from the polymerisation of 2-methylox-etane with the triethylaluminium-water-acetylacetone (2 1 2) catalyst showed that the polyether obtained consisted of regioregular monomer unit sequences, fairly rich in isotactic triads [87] ... 

Very recently, Miller, Brey, and Butler (40) have carried out a very similar study of the stereochemical configuration of polymetha-crylic anhydride and have reported results which are at. considerable variance with ours. In particular, they have reported a very much more marked increase in the proportion of isotactic triads with increasing temperature of polymerization, a reaching values as high as 0.7—0.8. The source of this discrepancy remains to be discovered. 

The spectra of the methyl methacrylate-a-methylstyrene copolymers are similar to those of the methyl methacrylate-styrene copolymers and are not shown. The backbone methylene regions (8.1—8.6t) of the two comonomer units cannot be distinguished from each other. On the other hand, the methyl methacrylate a-methyl resonances are clearly multiple except at the two lowest values of f . In the spectrum of the 46 54 methyl methacrylate-a-methylstyrene copolymer, the shape of the methyl methacrylate a-methyl resonance is markedly altered from that observed at higher f , the peak at lower field (8.83r), which corresponds to isotactic triads in the homopolymers and is normally very small in free radical polymers, appears to be the most prominent. The a-methyl protons of the a-methylstyrene units appear at highest field, 9.6r—9.8t. Peak positions for the a-methylstyrene copolymers are... 

Two quantities—Pi, the concentration of the isotactic triad and ai, the probability that an isotactic triad be followed by another isotactic triad—are sufficient to describe the sequence length of the isotactic units. The concentration of the various triad states can be expressed in terms of the various transition probabilities of the four-parameter model discussed previously (1). It can be shown that in the expressions for the probabilities of the various triads, the four transition probability parameters occur in such a combination that the concentration of the various triad states like Pi, etc. depend on only two independent parameters (Appendix I). 

Commercial poly(vinyl ethers) range from viscous oils to soft tacky resins and elastomeric solids (specific viscosity r)sp = 0.1-6). Due to the polymerization conditions (initiator, low-polarity medium), they are amorphous and atactic with a majority of isotactic triads [96,97], Glass-transition temperature is -34° C for PVM, -42° C for PVE, and - 19° C for... 

In a triad, the focus is on the central methine between two neighboring monomer residues. An isotactic triad (mm) is produced by two successive meso placements ... 

The poly(methyl methacrylate)s prepared in this experiment, as well as polymers formed in model reactions with silanes that contain bulky substituents (such as phenyldimethyl and diphenylmethyl groups), have predominantly syndiotactic structures identical to polymers prepared by the conventional group-transfer process. This result supports a two-step dissociative mechanism for a group-transfer process, because steric hindrance from bulky silyl groups should increase the proportion of isotactic triads in the hypothetical associative concerted propagation step. 

At present, no evidence exists for long runs of either meso or racemic placements in the Wurtz dechlorination reaction used in the synthesis of polysilylenes. Deviation from a random stereochemical polymerization has been suggested by West et al. (3) in the case of poly(phenylmethylsilylene) (PPMS). An analysis of Si NMR triads indicates that addition of monomer units with the same stereochemistry is preferred, which results in an increase in the intensity of syndiotactic and isotactic triad structures compared with a random stereochemical polymerization. 

Poly(methacrylic acid) and poly(acrylic acid) (PAA) were prepared by AIBN initiated polymerization of the freshly distilled monomer in deoxygenated methyl ethyl ketone at 60°C. The incorporation of 9,10-dimethylanthracene (9,10-DMA) end-groups in the polymer was achieved by the addition of the chain transfer agent (1% by weight) to the polymerization mixture. Unreacted 9,10-DMA was separated from the polymer by gel permeation chromatography using a column packed with Sephadex LH-20 and methanol as the eluent. Analysis of the PMA sample by NMR indicates that the polymer produced under these conditions consists of 57% syndiotactic, 33% heterotactic and 10% isotactic triads (15). Solution concentrations were 0.02 M in repeating units of the polymer. 

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