Oriented monomers

The two-step process of epitaxial polymerization has been applied to symmetrically substituted diacetylenes First, the monomers have been crystallized epitaxially on alkali halides substrates from solution and the vapor phase. The oriented monomer crystals are then polymerized under the substrate s influence by gamma-irradiation. The diacetylenes in this study are 2,4-hexadiyn-l,6-diol (HD) and the bis-phenylurethane of 5,7-dodecadiyn-l,12-diol (TCDU). The polydiacetylene crystal structures and morphologies have been examined with the electron microscope. Reactivity and polymorphism are found to be controlled by the substrate. 

A concept along similar lines was recently developed to account for the auto-accelerated character of the polymerization of carboxylic monomers and of acrylonitrile. Accelerated propagation is assumed to occur in oriented monomer-polymer association complexes. This conclusion is reached on the basis of kinetic evidence and the investigation of molecular associations present in these systems. 

Auto-acceleration is determined by a "catalytic" action of the polymer formed in the early stages of the reaction. The monomer selectively "solvates" the polymer to form a pre-oriented monomer-polymer complex in which propagation occurs at a much higher rate. At this point it seems difficult to determine to what extent the conclusions reached above can be generalized to other systems. Experiments along these lines are in progress. 

Abstract. Auto-accelerated polymerization is known to occur in viscous reaction media ("gel-effect") and also when the polymer precipitates as it forms. It is generally assumed that the cause of auto-acceleration is the arising of non-steady-state kinetics created by a diffusion controlled termination step. Recent work has shown that the polymerization of acrylic acid in bulk and in solution proceeds under steady or auto-accelered conditions irrespective of the precipitation of the polymer. On the other hand, a close correlation is established between auto-acceleration and the type of H-bonded molecular association involving acrylic acid in the system. On the basis of numerous data it is concluded that auto-acceleration is determined by the formation of an oriented monomer-polymer association complex which favors an ultra-fast propagation process. Similar conclusions are derived for the polymerization of methacrylic acid and acrylonitrile based on studies of polymerization kinetics in bulk and in solution and on evidence of molecular associations. In the case of acrylonitrile a dipole-dipole complex involving the nitrile groups is assumed to be responsible for the observed auto-acceleration. 

From this equation, it is clear that concentration of the solvent, S, influences a number of sites on the template which are occupied by the monomer, M. As the result of monomer units association with the template, the orientation of the substrate takes place and some special type of structure can be created. The structures, in which the monomer is aligned in a regular manner on the polymer template, were described by Chapiro in the case of polymerization of acrylic acid and acrylonitrile and details are described below. The ordered structure increases concentration of monomer at the reaction site, affects distances between pre-oriented monomer molecules, and changes a steric hindrance. This change in structure leads to the change in the kinetics of the polymerization reaction and it is responsible for stereo-control of the propagation step. 

Roha et ah 367) claim that the addition of a strong electrophile, such as AlClg, to a Ziegler catalyst produces a more active, homogeneous Ti+4 catalyst for ethylene polymerization. They proposed that electrophilicity is involved in the formation of the alkyl metal-transition metal complex and in complexity and orienting monomer. 

In coordinated polymerizations with alkyl metal and Ziegler-type catalysts, vacant p- or d-orbitals of a metal component coordinate with the jr-electrons of olefins, diolefins and non-polar monomers. When the polymer chain end is fixed in position and partially stabilized by its metal-containing gegen-ion, repetitive insertion of the polarized and oriented monomer between the chain end and gegen-ion yields stereoregular polymers. Of the various factors which affect polymer stereoregularity, the most important appears to be the gegen-ion structure and its ability to coordinate and orient the monomer. 

The unoccupied d orbitals of transition metals are suitable for monomer coordination. A certain structure of the complexes of these metals can result in an extremely useful link between space-oriented monomer coordination and polymerization. 

The entropy of polymerization is negative, i.e., randomly oriented monomer molecules are transformed to a highly ordered chain molecule. In order to have a... 

While some of the above discussion is in the general reedm of prognostication, some mote specific comments are appropriate. It is appeuent that em increase in both the number and types of solid-state polymerization is a desirable research objective. Here, the major initial burden lies with the chemist to design (, new reactive molecular structure and appropriately oriented monomer crystal structures. Successful development of new lattice-controlled processes resulting in the availability of well-defined fully ordered mticromolecules will readily attract the more physically and theoreticedly oriented research communities. The PDA and poly (sulfur nitride) cases bear strong witness to this point. 

In order to clarify whether the presence of meta-oriented units would change the rate of oxadiazole methylation and thus would change the ultimate oxadiazole/hydrazide ratio in the fiber, identical polymerizations of all-para- and 50/50 para/meta-oriented monomer mixtures were sampled at various intervals, the polymer dope samples were coagulated and hydrolyzed in dioxane/ water and the polymer compositions determined. As data in Table III show, essentially identical oxadiazole/N-methyl hydrazide ratios were obtained for the all-para and the mixed para/meta compositions. 

The chain propagation via-the anionic coordination mechanism, as shown above, involves insertion of an oriented monomer molecule. In more detail, since the catalyst fragment G (containing transition metal) is coordinated with both the... 

Anisotropic mechanical properties are exhibited by ordered, highly crosslinked polymers obtained by bulk polymerization of oriented monomers [117, 119]. These networks show a high degree of ordering and behave anisotropically in a number of physical properties, sueh as the refractive index, the thermal expansion coeffieient, and the modulus of elasticity. Figure 31 shows the tensile moduli in the two major directions for uniaxially oriented sample, and also for an isotropic sample. As can be observed, the modulus in the direetion of moleeular orientation is much higher than in the perpendicular direction. 

As discussed in A Word About... Aramids, the Latest in Polyamides (p. 427), Nomex is similar in structure to Kevlar (Figure 14.3), except that Nomex is prepared from meta- rather than para-oriented monomers. Draw... 

Syntheses in reverse micelles are carried out to order and orient monomers before polymerization to control further the polymer backbone, architecture, and functional properties and thereby enhance optical, mechanical, and processing properties compared with the polymers synthesized in bulk [11-16]. These membrane-templated interfacial enzyme-based polymerizations are explored to couple synthesis and processing in one step, with the formation of polymers as spheres. Experimental approaches for the enzyme-catalyzed syntheses of polymers in reverse micelles are described in the following. 

Similar results are obtained on the polycondensation of long chain ct-amino acid esters in monolayers. As shown in fig. 6 for compound (21) the polymer (b) exhibits a lower surface area per molecule compared to the monomer (a). The polypeptide formation in the oriented monomer film is shown in fig. 7. 

The presence of head-to-head" and tail-to-tail units in addition to the head-to-tail ones in propylene homopolymers (11,32), and of a number of even methylene sequences in ethylene/propylene copolymers (11,31,33-35) denotes lack of regiospecificity. As a consequence the syndiotactic polymerization of propylene is conveniently described as a binary copolymerization (11, 31, 36) between head-to-tail" and "tail-to-he ad oriented monomer molecules ... 

When in 1967 it was stated by Amerik and Krentsel (35) that vinyloleate can be polymerized in bulk in its smectic state to produce a stereoregular polymer, it was an exciting suggestion along the lines of Kargin s concept of polymerization through "oriented monomer sequences."... 

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