INDEFINITE POLYMERIZATION

As illustrated in the diagram below, domain swapping can also result in indefinite polymerization to form linear supramolecular structures. These may correspond to present-day polymers of proteins such as microtubules, or they may represent abnormal structures, like the straight and paired-helical filaments in the neurofibrillary tangles observed in the brain tissue of those afflicted with Alzheimer s disease. 

Any polymerization reaction in which the product of each elongation step can itself also undergo further polymerization. When the same types of bonds and/or conformational states that are present in the reactant(s) are generated within product(s) during elongation, the process is referred to as isodesmic polymerization. Such is the case for the indefinite polymerization of actin, tubulin, hemoglobin S, and tobacco mosaic virus coat protein. See Nudeation Protein Polymerization Actin Assembly Kinetics Microtubule Assembly Kinetics Microtubule Assembly Kinetics... 

A reaction in which the types of bonds formed in the product(s) are the same types of bonds broken in the reactant(s). See Indefinite Polymerization... 

Any highly cooperative chemical or physical process that serves to generate structures that then grow by linear accretion or elongation. Nucleation is an important phase in indefinite polymerization processes (e.g., actin filament and microtubule assembly). Nucleation is typically of a high kinetic order,... 

Condensed phosphates are derived by dehydration of acid orthophosphates. The resulting polymeric stmctures are based on a backbone of P—O—P linkages where PO tetrahedra are joined by shared oxygen atoms. The range of materials within this classification is extremely broad, extending from the simple diphosphate, also known as pyrophosphate, to indefinitely long-chain polyphosphates and ultraphosphates (see Table 1). Both weU-defined crystalline and amorphous materials occur among the condensed phosphates. 

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

This example of step-growth polymerization has two monomers that can react with each other but not with themselves. It has only one dimer that can be self-reactive. This pattern continues indefinitely with two trimers, one self-reactive tetramer, and so on. Molecules with an odd number of mer units will... 

Free radical polymerization does not go on indefinitely, because sometimes two free radicals collide and react. This destroys two free radicais and terminates the chain. For example, another initiator fragment can react with the... 

With pure monomer and diluents a polymerization with a half-life of 8 hr has been recorded with these catalysts. There, stability would appear to be indefinite provided water or oxygen are not admitted to the system. In one experiment described in Table XII the monomer feed is switched off after 60 min and the dissolved monomer exhausted from the polymer slurry. If monomer is reintroduced some days later, polymerization begins again without an induction period and the rate was little changed from that previously observed. 

Figure 1.23 Protein crosslinking reactions done using homobifunctional reagents can result in large polymeric complexes of multiple sizes and indefinite structure.

Collins et al. reported in 1995 that catalysts based on hafnium are desirable for the production of elastomeric polypropylene in that they polymerize propylene to a high molecular weight polymer and are indefinitely stable under typical polymerization conditions [8], Based on the theory that hafnium as a catalytic center leads to a significant increase of molecular weight in propene polymerization compared with the zirconium-based catalyst, Rieger et al. searched for hafnocene systems to obtain polymers with new properties. 

The threshold concentration of monomer that must be exceeded for any observable polymer formation in a self-assembling system. In the context of Oosawa s condensation-equilibrium model for protein polymerization, the cooperativity of nucleation and the intrinsic thermodynamic instability of nuclei contribute to the sudden onset of polymer formation as the monomer concentration reaches and exceeds the critical concentration. Condensation-equilibrium processes that exhibit critical concentration behavior in vitro include F-actin formation from G-actin, microtubule self-assembly from tubulin, and fibril formation from amyloid P protein. Critical concentration behavior will also occur in indefinite isodesmic polymerization reactions that involve a stable template. One example is the elongation of microtubules from centrosomes, basal bodies, or axonemes. 

Although the self-assembly of polymeric structures can involve nucleation and elongation steps (See Actin Assembly Kinetics Microtubule Assembly Kinetics), one can simplify the assembly process through what is known as seeded assembly. At an initial monomer concentration [M], seeded assembly is induced by the addition of pre-assembled polymeric structures consequently the polymer number concentration must remain constant. The rate of monomer incorporation into indefinite length polymers can be written as follows ... 

Purified diene polymerizes within 24 hr If stored neat. Its lifetime can be indefinitely prolonged by storage as a 10% by weight solution In hexane under an argon atmosphere. 

This process can proceed indefinitely just as in ideal addition.polymerization, making an infinite set of free radicals AMj. . ... 

Di-/i-chloro(dichloro)bis[77 -T7 -2,7-dimethylocta-2,6-diene-l,8-diyl)diruthenium (compound A) is a purple crystalline compound, that is indefinitely stable at room temperature under a nitrogen atmosphere. For short periods it can also be handled in air. MS (El) m/z 616 (M + ). In the solid state the product shows symmetry Ci, ° but it exists as a mixture of two diastereomers in solution. It reacts with various Lewis bases L to form the monomers Ru(t7 77 -CioHi6)Cl2L and it shows catalytic activity in ROMP polymerization. ... 

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