Weights molecular

The molecular weight of a polymer is of prime importance in the polymer s synthesis and application. Chemists usually use the term molecular weight to describe the size of a molecule. The more accurate term is molar mass, usually in units of g mol-1. The term molecular weight is the ratio of the average mass per formula unit of a substance to - th of the mass of an atom of 12C and is dimensionless (IUPAC, 1991, in press). This text will use molecular weight throughout irrespective of the units, because molecular weight is the more familiar term for most chemists. 

The characterization of a polymer by Mn alone, without regard to the polydispersity, can be extremely misleading, since most polymer properties such as strength and melt viscosity are determined primarily by the size of the molecules that make up the bulk of the sample by weight. Polymer properties are much more dependent on the larger-sized molecules in a 

The molecular weight of a substance is a number that gives the weight of one molecule of that substance compared with the weight of one atom of oxygen taken as exactly 16 (or one atom of carbon takes as exactly 12). 

The molecular weights of substituted polymers are typically low compared with parent polymers due to the bulky nature of the ring substituents, which can hinder polymerization however, solubility in common organic solvent is increased. Dao et al. [193] have studied the effect of alkyl substituents on the molecular weight of polyaniline. The molecular weight of chemically prepared poly(2-methyl aniline) and poly(2-ethyl aniline) was reportedly 7000 and 5000g/mol, respectively, which is 15-times lower than polyaniline (80 000) prepared under identical conditions. A similar effect of alkyl substituents was observed on molecular weights of polythiophenes [207, 208]. 

The average molecular weight of a polymer can be viewed and measured in three different ways [14,212] as indicated in Table 16 below. 

Type of average molecular weight Symbol Method of determination 

As these methods of determining the average molecular weight are relatively complicated, for povidone it is expressed in terms of the K-value, in accordance with the European and U.S. Pharmacopoeias (see also Section 2.2.3.2). 

The number-average of the molecular weight, Mn is determined by methods that measure the number of molecules. This value is very seldom determined or published for povidone. Table 17 shows some older values. 

The viscosity-average of the molecular weight, Mv has attracted greater interest recently, as it can be calculated direct from the relative viscosity, the intrinsic viscosity or the K-value (see Section 2.3.2.2). Table 18 shows typical viscosity-average values for the different povidone types. 

There are various definitions of the molecular weight of a polymer. By multiplying 

If instead of the number of molecules Uj with a molecular weight the weight of the fraction Wj is used, then the weight average molecular weight (M,y) is obtained. 

Vv hen a relative) small amount of ven,- long chains is present in the p-ol mer, M, may diner quite considerably from M . A small amount of long chains has a great effect upon h-L but hardly influences M ., .  

There are several ways of defining average molecular weight. The number-average molecular weight M is obtained by dividing the chains into a series of size ranges and 

Hypothetical polymer molecule size distributions on the basis of 

FigMre 14.4 Distribution of molecular weights for a typical polymer. 

Degree of polymerization— dependence on nnmber-average and repeat unit molecular weights 

An alternate way of expressing average chain size of a polymer is as the degree of polymerization, DP, which represents the average number of repeat units in a chain. DP is related to the number-average molecular weight M by the equation 

A polymer is a large chain molecule of high molecular weight which is composed of a single molecule (monomer) that is repeated many times in the chain. In contrast, a macromolecule is a large molecule composed of many small molecules bound together with chemical bonds, e.g., a protein or DNA. An oligomer is a small polymer of only several monomer units. 

Plastics are prepared by the melting, molding, extruding, or the compression of polymers. The word polymer implies a molecule consisting of a long chain of units of smaller molecules or monomers. Thus, the polymer is also called a macromolecule. Such large molecules exist in nature and common examples of these are cellulose, rubber, cotton, silk, wool, starch, and keratin. 

The annual world production of polymers has increased from 11.5 Mt in 1940 to about 27 Mt in 1960, after which time production almost doubled every decade to more than 150 Mt in 1990. Fiber production at about 36 Mt is almost equally divided into natural and synthetic. The production of elastomers (flexible plastics) represents about one-tenth of the total polymers, with production of synthetic elastomers being about twice that of natural rubber. 

Normally the number of monomers in a polymer molecule varies considerably, but the interesting range for the fabricator of plastics is generally between 10 and 10 units. 

Since the precise number cannot be controlled, the molecular weight (MW) of a polymer is not a unique value, and the distribution can vary as a result of the method of preparation. There are two important average molecular weights of a polymer the number average MW, Mn, and the weight average MW, Mw. 

Biomacromolecules can be characterized by many numerical values. One of the most common and important is the molecidar weight (M) as represented in Table 3.12. It is an 

Method Biomacromolecule Property assessed Sample range 

Native gel Nucleic acids, proteins Size/charge ng 

Composition All Component content, specific chemical group variable 

As with all thermoplastics it is important to characterise the molecular weight distribution of PAEK. In practice simple measurements such as melt flow index (MFI) and capillary rheometry are used to give an indication of molecular weight. The lower shear rate of MFI is 

Early attempts to characterise the molecular weight of PEEK were hindered by its lack of solubility. Solution viscosity is typically measured in concentrated sulfuric acid. This technique is useful for small samples and samples that may lack thermal stability but it is not as sensitive as melt viscosity. Much effort was put into gel permeation chromatography (GPC) of sulfonated PEEK and the determination of Mark-Houwink parameters. Relationships were determined between melt viscosity and molecular weight [40]. Melt viscosity is related to molecular weight by  

The size of polymer chains can be characterized by measuring the molecular weight (M). Typically, the size of polymer ehains increases with increase in molecular weight. The molecular weight of a polymer is determined by the degree of polymerization (AO, i.e., the number of repeating units in the polymer chain, by the following equation  

A polymeric material may consist of polymer chains of various lengths or repeat units. Hence, the molecular weight is determined by the average or mean molecular weight which 

The properties of polymeric material are strongly linked to the molecular weight of the polymer as shown schematically in Fig. 1.2. A polymer such as polystyrene is stiff and brittle at room temperature with a degree of polymerization, n, of 1,000. Polystyrene with a degree of polymerization of 10 is sticky and soft at room temperature. 

A measure of the broadness of a polymer s molecular weight distribution is the polydis-persity index defined by 

Polymer molecular weight and molecule size. You are asked to compute the maximum possible separation between the ends of a high density polyethylene molecule with an average molecular weight of 100,000. 

Henkel et al. did not differentiate the natural products from natural product derivatives. The descriptors they used for comparison were molecular weight, number and type of hetero atoms, pharmacophore groups (e.g. CO, CONH, OH, CN, NH, etc), bridgehead atoms, rotatable C C bonds, rings per molecule, chiral centres per molecule and rotatable bonds per molecule. 

In 2001, Lee and Schneider17 compared the properties of trade drugs (taken from the Derwent World Drug Index, WDI, n=5151) and natural products (taken from the BioScreenNP database, n= 10 495). These investigators described the comparison of parameters applicable only to the rule of five (molecular weight, log P, number of H donors per molecule, number of N per molecule, number of O per molecule and percentage of rule of five alerts). 

The parameters used to compare these data sets18 were a combination of the two earlier investigations. 

We have also made similar descriptor comparisons of compounds from the Merck chemical collection, the top 200 selling drugs in 2006, and Merck s natural product collection. 

Although the three papers discussed above used data from different databases, the overall conclusions were essentially the same—as indeed were those from our analysis. The results from many of these descriptors are highlighted below. 

The detailed discussion of the structure-property relationship is a very complex issue and is not within the scope of this book. Further details can be found in many textbooks [e.g., 14]. However, many aspects of structure such as molecular weight, molecular weight distribution, crystallinity, etc., significantly influence the properties of PP and, hence, are briefly discussed here. 

More sophisticated techniques such as gel permeation chromatography are used for measuring the molecular weight (Section 3.2). 

The various methods are not described here since, as a rule, they are learned in the practical course in physics or physical chemistry. The cryoscopic process is much to be preferred to the ebullioscopic. The most frequently used solvents are benzene and glacial acetic acid, and the best apparatus is the closed one of Beckmann, with electromagnetic stirring. 

A very elegant and simple method by which the molecular weight of organic substances can be determined in a melting point apparatus was recently described by K. East.1 

Camphor has a very high cryoscopic constant and its melting point is very greatly depressed by substances dissolved in it. The depression is about 8 times as great as that in benzene. Ebenzene = 5-1, Eoamphor = 40. This means that a molar solution in camphor melts 40° lower than the solvent, i.e. than camphor itself. Accordingly, even with relatively dilute camphor solutions the depressions obtained are so great that the sensitivity of an ordinary thermometer (which can be read to 0-25°) suffices completely for the determination.2 

Make the melting-point tubes as described on p. 40 from a clean test tube the internal diameter should be about 4-5 mm. Cut off pieces about 5 cm. long. Close the lower ends by sealing as uniformly as possible, also keeping the walls as thin as possible. By drawing the softened glass away sideways avoid reducing the bore more than a little. 

Eemove the funnel and seal the tube in a micro-flame but do not draw out too finely. Now heat the contents in a bath of concentrated sulphuric acid at 180° so that a homogeneous melt is produced. 

After subjected to purifying, the CGTase secreted by B. alkalophilus 1177 and the standard proteins were subjected to SDS-PAGE electrophoresis simultaneously. 

According to the SDS-PAGE electrophoresis performed on standard proteins with known molecular weight (Fig. 2.5), standard protein relative mobility was plotted against logarithm molecular weight of standard protein as shown in Fig. 2.6. 

regression equation (logMr = —l.l977Rf + 5.1504) was obtained (Rf is the migration rate). The molecular weight of the enzyme was calculated to be 69 kD. 

Km is a constant characteristic of the enzyme, only with the nature of the enzyme, and has nothing to do with the concentration of the enzyme. Its physical meaning is the substrate concentration when the enzyme reaction rate reaches half of the maximum reaction rate. Michaelis-Menten equation quantitatively describes the relationship between the substrate concentration and the maximum reaction rate. 

By using the reciprocal of the concentration of starch and the reciprocal of the reaction rate as the abscissa axis and the vertical axis, respectively, we obtained the double reciprocal curve y = O.OlOSx + 0.0087, R = 0.9996 (Fig. 2.7), where x is the reciprocal of the concentration of starch substrate, and y is the reciprocal of the reaction rate. According to the double reciprocal plot method of Linewear-Burk (i.e., draw a straight line and extrapolated to intersect with the abscissa axis, and intercept in the abscissa axis was — 1/Km, and the intercept of in vertical axis was 

A polymer is a large molecule, i.e., a macromolecule consisting of a large number of repeating small, simple chemical units called monomers covalently bonded to form a 

Polymer samples exhibit polydispersity and the molar mass is an average depending on the particular method of measurement used. One of the most important characteristerics of these long chain molecules is their degree of polymerisation (DP), i.e., the number of mers (repeating units) in a given macromolecule (Equation 6.1) [1, 2]. 

Certain average values of molecular weight (MW) are used to characterise a polymer and these average values are obtained by several different methods. They are  

The Mark-Houwink equation defines the viscosity average molecular mass as  

Tisp = Specific viscosity Tijei = relative viscosity Ti = the viscosity of the solution Tio = the viscosity of the solvent 

SEC has been used in the determination of a range of polymers including polyepichlorohydrin [99], polybis(carboxylato phenoxy phosphazine [100], polyarylene ethynylene [101], PMMA - polydimethyl siloxane grafts [102], surfactants [103], polycarbonate [104], PMMA [58], polybutadiene [105], polyelectrolytes [106], aliphatic oligoamide [107], PET [108], and polyvinyl alcohol [109]. 

Polyethylene Melting point (°C) Density (g/cm ) Tensile strength (MPa) Number/type of branches 

It can be seen that the properties of the polymer are strongly linked to molecular weight and molecular weight distribution, and it is necessary to match the molecular weight, material properties and flow characteristics in order to shape the material dming processing and give the desired final product. 

Plastics are made up of polymers and other materials that are added to increase the functionality. The actual polymer content within a plastic can vary widely from less than 20% to nearly 100%. Those plastics consisting of virtually 100% polymer are termed prime grades . The level and type of the other additives used depends on the application for which the plastic is intended. There is a vast range of materials available in the market, in the USA alone over 18,000 different grades of plastic material are available. 

We show in previous sections that the minimal amount of information to create pseudocomponents is a distillation curve and a specific gravity or density distribution. If only the bulk density is available, we can use the constant Watson X-Factor assumption to estimate the density distribution. If only a partial density distribution is available, we can use the beta function to extrapolate an incomplete 

Penr Stdte A pen K Nakv Aspen LeaslSq WhUson 

A related property is the acentric factor. The acentric factor accounts for the size and shape of various kinds of molecules. Simple molecules have an acentric factor close to 0, whereas large or complex hydrocarbon molecules may have values approaching 0.5 to 0.6 . The acentric factor is not measured, but defined as an exphdt function of the ratio of vapor pressure at the normal boiling point to the measured or estimated critical pressure. We show the definition of the acentric factor in Equation (1.19). 

Roees Edmbter Beigman Spencer.Daubeit Rowe 

Slanditg Maihews Penr Slate Matkv Aspen i Hu Eaten PcHlei Meissnef Redding Twu critical propeily ciZ. 

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American Petroleum Institute Research Project 42, "Properties of Hydrocarbons of High Molecular Weight," API, Division of Science and Technology, New York, 1966. 

CODE = six-letter abbreviated name MOL WT = molecular weight... 

The market values and molecular weights of the materials involved are given in Table 2.1. Oxygen is considered to be free at this stage, coming from the... 

TABLE 2.1 Molecular Weights and Values of Materials in Example 2.1... 

Before we can explore how reactor conditions can be chosen, we require some measure of reactor performance. For polymerization reactors, the most important measure of performance is the distribution of molecular weights in the polymer product. The distribution of molecular weights dictates the mechanical properties of the polymer. For other types of reactors, three important parameters are used to describe their performance ... 

Separation of low-molecular-weight materials. Low-molecular-weight materials are distilled at high pressure to increase their condensing temperature and to allow, if possible, the use of cooling water or air cooling in the column condenser. Very low... 

Separation of high-molecular-weight heat-sensitive materials. High-molecular-weight materials are often heat sensitive and as such are usually distilled under vacuum to reduce their boiling temperature. 

In summary, distillation is not well suited for separating either low-molecular-weight materials or high-molecular-weight heat-sensitive materials. However, distillation might still be the best method for these cases, since the basic advantages of distillation... 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. In absorption, a gas mixture is contacted with a liquid solvent which preferentially dissolves one or more components of the gas. Absorption processes often require an extraneous material to be introduced into the process to act as liquid solvent. If it is possible to use the materials already in the process, this should be done in preference to introducing an extraneous material for reasons already discussed. Liquid flow rate, temperature, and pressure are important variables to be set. 

Distillation is by far the most commonly used method for the separation of homogeneous fluid mixtures. The cost of distillation varies with operating pressure, which, in turn, is mainly determined by the molecular weight of the materials being separated. Its widespread use can be attributed to its ability to... 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. Liquid flow rate, temperature, and pressure are important variables to be set, but no attempts should be made to carry out any optimization at this stage. 

What is the minimum selectivity of decane which must be achieved for profitable operation The values of the materials involved together with their molecular weights are given in Table 4.1. 

Material Molecular weight Normal boiling point (K) Value ( kg- )... 

Soaps of heavy metals have been used but cationic surface-active agents have proved more suitable, notably organic amines of relatively high molecular weight. 

Beckmann thermometer A very sensitive mercury thermometer with a small temperature range which can be changed by transferring mercury between the capillary and a bulb reservoir. Used for accurate temperature measurements in the determination of molecular weights by freezing point depression or boiling point elevation. 

Clausius-Mosottf Jaw The molecular polarization (P) of a substance of molecular weight M, density d and dielectric constant O is ... 

Fischer-Tropsch reaction The catalytic reaction of hydrogen and carbon monoxide (synthesis gas ) to produce high-molecular weight hydrocarbons. 

They are colourless liquids with characteristic odours, and are prepared by the condensation of ketones with alkyl orthoformates in the presence of alcohols, or by the reaction of acetylenes with alcohols in presence of HgO and BF3. In some cases trichloroethanoic acid is used as the catalyst. They lose alcohol when heated and form vinyl ethers. Exchange of alcohol groups occurs when the ketals of the lower alcohols are boiled with alcohols of greater molecular weight. See acetals. 

It is a ratio. The molecular weight is equal to the sum of the atomic weights of the constituent nuclei. The molecular weight expressed in grams is known as the gram molecular weight. 

In RNA the sugar is ribose and uracil replaces thymine. Different varieties of RNA exist with widely different molecular weights. Messenger RNA conveys information in the form of the sequence of bases from DNA to RNA on the ribosomes, where proteins are... 

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