Linkages head-to-tail

By noting the amount of chlorine that could be removed in this way they were able to determine whether the polymer was formed by head-to-tail linkage (Figure 12.10(a)) or head-head and tail-tail linkage (Figure 12.10(b)). 

Poly(PO) formed with 17 as initiator under irradiation showed virtually the same NMR pattern as that for the polymer formed in the dark at 70 C, where the resonance due to the methyl group was very simple, indicating that the polymer consists of regular head-to-tail linkages. The diad and triad tactic-ities of the polymer, as determined by NMR [82], indicated the atactic struc-... 

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. 

The crystalline fraction was found to be formed via a cleavage of the C1g-0(CH2—O) bond in the monomer molecule and proved many years ago to be an isotactic polymer (with regular head-to-tail linkages) (Figure 9.1) [42]. The structure of the amorphous fraction, on the other hand, varies depending on the kind of catalyst. Some amorphous polypropylene oxide)s prepared with catalysts such as diethylzinc-methanol [43] or aluminium isopropoxide-zinc chloride [44] consist of regular head-to-tail linked units, but they are atactic (the mole fraction of isotactic diads is less than 0.6) [43]. Some other amorphous polypropylene oxide)s obtained with catalysts derived from reactions in the diethylzinc-water [44,45], and triethylaluminium-water [46] systems, and with aluminium isopropoxide [44], have been found to be irregular, i.e. to contain head-to-head and tail-to-tail enchained monomer units. 

A procedure has recently been developed for the regioselective polymerization of an 3-(4-octylphenyl)thiophene with FeCl3 [253]. The slow addition of a slurry of FeCl3 in CHC13 to a solution of the monomer resulted in the formation of a polymer with 94% head-to-tail linkages. The slow addition of FeCl3 keeps the ratio of Fe3 + to Fe2 + low during the... 

Head-to-Head and Head-to-Tail Linkages. Errors in the linkages between successive monomeric units of the poljrmers are possible (and always statistically present). The effect of head-to-head (HH) and head-to-tail (HT) bonds in the XPS core levels spectra of substituted polymers have been computed and found at the limit of the sensitivity of the technique (e.g. ). The control of these linkages during the synthesis is difficult and the number of polymers that can be prepared with 100% of HH or HT linkages is small (43). 

For free-radical polymerization, classical results have been obtained concerning the tacticity of hydroxytelechelic poly(methyl methacrylate)109) and copolymers, 46) initiated by H202/UV. Most of the units are in a syndiotactic (64 %) or heterotactic (30 %) configuration. For poly(vinyl acetate) obtained in the presence of H202 at 120 °C 98), the polymer contains less syndiotactic (22%) and somewhat more heterotactic (38%) units with 80% of head-to-tail linkage mode. For the copolymerization of alkyl methacrylate by the H202/UV system113) quite different results, explained by the nature of the medium, especially by the solubility effect (see Table 1.1), have been obtained. 

In principle, the monomer can be enchained by head-to-tail linkages or head-to-head, tail-to-tail enchainments (4-4). Poly(vinyl fluoride) actually has about 15% of its monomers in the head-to-head, tail-to-tail mode. This is exceptional, however. Head-to-tail enchainment appears to be the predominant or exclusive constitution of most vinyl polymers because of the influence of resonance and steric effects. 

The presence of head-to-tail linkages in such tetramers is responsible for the relatively high value of [M]g, while the low value for [M] (approximately zero) observed in the solution of the directly formed tetramer shows that its molecule does not contain the terminal head-to-tail linkages. This leaves the formula... 

The polymer is commonly obtained from vinyl chloride with a peroxide initiator such as peroxydicarbonates, fert-butylperpivalate, benzoyl or lauroyl peroxide, acetyl cyclohexylsulfonyl peroxide, or azobis(2,4-dimethylvaleronitrile). The polymerization can be done by suspension, emulsion or solution techniques. Low polymerization temperatures are used when high MW material is required. Suspension polymerization employs water suspension agents, such as poly(vinyl alcohol) or methylcellulose. The resulting polymer Is a partially syndiotactic material but with low crystallinity. The macromolecules typically have head to tail linkages (H-T) and a small proportion (less than 1.5%) of branching. Ziegler-Natta catalysts are not used to produce PVC. 

In the cyclotrimerization of propynoic acid, trimellitic (l,2,4-fb,l l3((X)2H),) and trimesic acid (l,3,5-CgH3(CO2H)3) are obtained as the product of head-to-head or head-to-tail linkage, respectively. For the sake of comparison, reactions were conducted both in neat water and in neat THF. In all cases, the yield of the trimer was higher in the aqueous system, evidencing the high tolerance of the rhodium catalyst towards water. 

The alkaloids in this group are characterized by a single diphenyl ether head-to-tail linkage. 

Non-Isoprenoid Monoterpenoids.—There has been activity in the field of monoterpenoids formally related to chrysanthemic acid (43) and belonging to the odd artemesyl (44), santolenyl (45), and lavandulyl (46) groups where the customary head-to-tail linkage of isoprene units is not followed. Yomogi alcohol (47), the allylically rearranged artemisia alcohol (51), has been isolated from Artemisia feddei. The santolinyl class now includes two alcohols. 

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