Head-tail polymerization

Figure 10-10 Polymerization of 1,6-diynes using a molybdenum alkylidene catalyst [Rp is (CFjljCHjC] [67]. The 1,6-diyne monomer is drawn in two different exaggerated conformations to illustrate that head-tail polymerization leads to six-membered rings, and tail-tail polymerization leads to five-membered rings. See Fig. 10-8 for a more mechanistic diagram of acetylene metathesis.

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. 

The macromolecules of cells are built of units—amino acids in proteins, nucleotides in nucleic acids, and carbohydrates in polysaccharides—that have structural polarity. That is, these molecules are not symmetrical, and so they can be thought of as having a head and a tail. Polymerization of these units to form macromolecules occurs by head-to-tail linear connections. Because of this, the polymer also has a head and a tail, and hence, the macromolecule has a sense or direction to its structure (Figure 1.9). 

Terpenoid (Chapter 6 Focus On. Section 27.5) A lipid that is formally derived by head-to-tail polymerization of iso-prene units. 

The results presented above indicate that the previously unknown head-to-tail polymerization is the major reaction product of the iminium methide species. To investigate the generality of this reaction, we next studied a neutral ene-imine species shown in Scheme 7.9.48 As illustrated in this scheme, the generation of this reactive species requires quinone reduction followed by elimination of acetic acid. The ene-imine is structurally related to the methyleneindolenine reactive species that is a metabolic oxidation product of 3-methylindole (Scheme 7.9).57 59... 

Head-to-tail polymerization of propylene produces a polymer in which every other atom is a stereocenter. 

Before the mechanism of vinyl polymerization was understood, the question of the structure of vinyl polymers was of considerable interest. Staudinger had written these polymers as having a head-to-tail arrangement of recurring units, but he had not really furnished evidence of the structure. As Carothers once said, Staudinger had assigned the structure by pronouncement. He was as usual correct, and chemical evidence was developed to establish such structures. For example, when monovinyl methyl ketone polymerized, it could produce by head-to-head, tail-to-tail reaction a 1,4-diketone. By head-to-tail polymerization it would give a 1,5-diketone. These two types have different reactions. The study of the polymer proper showed that the polymer was a 1,5-diketone. In the case of polyvinyl chloride, a head-to-head, tail-to-tail polymerization would lead to a 1,2-dihalide compound, and a head-to-tail polymerization would lead to a 1,3-dihalide. 

HEAD-TO-TAIL POLYMERIZATION ACTIN ASSEMBLY KINETICS MICROTUBULE ASSEMBLY KINETICS Heat and matter transfer,... 

A line of research that has aroused much interest in recent years is the study of head-to-head, tail-to-tail polymers (96-98). Their direct synthesis has little likelihood of being successffil as head-to-tail sequences usually predominate in vinyl polymerization. One possibility for their preparation is through the chemical modification of suitable preformed polymers. In the case of the head-to-head, tail-to-tail polypropylene, different stereoisomeric forms have been isolated, depending on the method of preparation. In the general scheme, the precursor is an unsaturated polymer obtained by polymerization of the disubsti-tuted butadiene (2,3-dimethylbutadiene or 2,4-hexadiene) then, by chemical or catalytic reduction, this polymer is converted into the desired polypropylene, whose stmcture can then be examined by NMR spectra. Head-to-head, tail-to-... 

Wegner, A. 1976. Head to tail polymerization of actin. J Mol Biol. 108 139—50. 

As shown by H NMR, the activated catalyst mixture reacts with norbornene (or a series of methyl-substituted norbornenes) to be partially converted to a new carbene species [53]. From the ratio of product carbene and residual initiator carbene concentrations, it was estimated that the rate constant for propagation is at least 3 times that for initiation. The three species present in the equilibrium situation of Eq. (39) may all possess their own intrinsic activities, resulting in a more complex polymerization behavior. In addition, a substantial amount of secondary metathesis occurs, as shown by changes in both the cis content of the polymer and head/tail ratio of the substituted carbenes when the catalyst was left in solution (120 min at 20 °C) after consumption of the monomer. 

IQ. Unusual Head-to-Tail Polymerizations 1. Cationic Intra-Intermolecular Polymerizations... 

The reagent will always react with the end carbon atom because then the conjugation of the n electrons of the phenyl or carbonyl group with the free electron, the sextet or the free pair on the a-carbon atom, remains intact. The predominant head to tail polymerization is therefore produced. 

The linear polymerization of DVB was accomplished with Pd(PPh3)2(BF4)2 in acetonitrile-chloroform solvent at temperatures ranging from 40 to 70° C. The polymer contained structural units derived from both head-tail and head-head coupling. The polymerization proceeded by a step-growth mechanism. The molecular weight increased exponentially as a function of time. This reaction was also used for a one-pot synthesis... 

In radical polymerization, the more substituted radical always adds to the less substituted end of the monomer, a process called head-to-tail polymerization. 

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