Molecular weight distribution definition

Model Networks. Constmction of model networks allows development of quantitative stmcture property relationships and provide the abiUty to test the accuracy of the theories of mbber elasticity (251—254). By definition, model networks have controlled molecular weight between cross-links, controlled cross-link functionahty, and controlled molecular weight distribution of cross-linked chains. Sihcones cross-linked by either condensation or addition reactions are ideally suited for these studies because all of the above parameters can be controlled. A typical condensation-cure model network consists of an a, CO-polydimethylsiloxanediol, tetraethoxysilane (or alkyltrimethoxysilane), and a tin-cure catalyst (255). A typical addition-cure model is composed of a, ffl-vinylpolydimethylsiloxane, tetrakis(dimethylsiloxy)silane, and a platinum-cure catalyst (256—258). 

These data for typical properties of EP polymers are either as measured or as advertised by respective manufacturers. This table is not intended to be definitive either in terms of the total grade slate or the specific data reported for each producer. Note that the molecular weight distribution data are based on a qualitative comparison of GPC curves. Mooney viscosities are repotted for final product form (i.e.. in the case of oil-extended rubbers, the viscosity is that of the EP plus oil. 

The fact that this polycondensation process takes place at room temperature, with careful control of both molecular weight and molecular weight distribution of the final polymers or copolymers produced are definitive advantages over the corresponding ROP reaction of (NPCl2)3 in molten state. 

From these definitions one may corroborate the intention of HTS in chemistry and materials science. The total speed-up factor of this part of the R D (Research and Development) process, as stated earlier, is between 5 and 50, but contrary to most of the pharma applications true (semi-) quantitative answers will result. As a result, this approach is essentially applicable in any segment of R D. On the other hand, this approach requires methods of experimentation that have almost the same if not the same accuracy as in the traditional one-experiment-at-the time approach. This is key as (i) in process optimisation accuracy is key and (ii) in research, also in academic research, accuracy is important as some polymer properties do not span a wide range of values (e.g., the elastic modulus of amorphous polymers) or may depend critically on molecular weight distribution or molecular order. 

Now the function displays the number fraction of molecules with a certain molecular mass. Its integral is 1 by definition. Nevertheless, we still call it the number molecular weight distribution because the factor /N (A/) dM is nothing but a constant. 

Definition of a Complex Polymer. A simple polymer is one vrtiich has at most one broad molecular property distribution (e.g., a broad molecular weight distribution). A complex polymer is one which has two or more broad molecular property distributions (e.g., a broad molecular weight distribution and a broad copolymer composition distribution) ( ). Properties such as molecular weight and composition, Aiich can be in so much variety in a polymer that they must be described as a distribution, are here termed "distributed properties". It is the presence of simultaneous breadth (i.e., variety) in more than one distributed property which is the defining characteristic of a "complex" polymer and the source of analysis difficulties. 

The problem of molecular weight distribution (MWD) and functionality type distribution (FTD) belongs by definition to an extensive problem of molecular heterogeneity of polymers. In the synthesis of a polymer with the requested properties, e.g. a telechelic polymer, one is always faced with different types of polydispersity the macromolecules can be of different length, they can have a different number of functional groups, i.e. be mono-, bifunctional, etc., they can be branched (star-, comb- or tree-like) and, finally, they can be cyclic. 

The occurrence of definitive bimodal molecular weight distributions in polymers prepared by normal cationic techniques was first discovered by Hi-... 

The opposite of the large diameter pipeline with little axial or radial mixing is the perfect backmixed reactor with instantaneous mixing and uniformity. For polystyrene reactors with several hours of residence time, complete mixing in 1-2 min is usually adequate to satisfy a practical definition of perfectly mixed. The probability of exit of any fluid element from this type of reactor is independent of when it entered. The residence time distribution is exponential and the molecular weight distribution in the case of no termination is Mw/Mn = 2.0, which will spread out to 2.3 when chain transfer controls. If product requirements necessitate a narrower residence time distribution, one can utilize several of these reactors in series. This becomes necessary to control the grafting distribution in rubber modified polystyrene. 

It has not so far been possible to perform a definitive set of experiments to show whether this holds for all possible molecular weight distributions, but it seems certain from existing experimental data that this result will be adequate for the type of polydispersity normally encountered. 

Gel permeation chromatography indicates a polymodal polydispersity with a broad molecular weight distribution. Currently it is not possible to specify the definite values of the average molecular weights due to a lack of comparable standards. 

This review covers mainly the scientific literature that has appeared in relevant journals until early 2001 concerning the metal-catalyzed (or atom-transfer) living radical polymerization. The word living employed here simply refers to polymerizations that provide control over the molecular weights, the molecular weight distributions, and the chain end reactivity as do other living polymerizations. Its definition and use criteria, along with the word controlled , are still under discussion and have recently been discussed elsewhere.39 Thus, the discussion on the difference between these words is not to be treated here. 

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