The Molar mass distribution in a polymer describes the relationship between a polymer fraction and the molar mass of that polymer fraction. In linear polymers the individual polymer chains rarely have the exact same degree of polymerization and there is always a distribution around an average value. Polymer is a generic term used to describe a very long molecule consisting of structural units and repeating units connected by covalent chemical bonds. ... Molar mass is the mass of one mole of a chemical element or chemical compound. ... The degree of polymerization, or DP, is the number of repeat units in an average polymer chain at time t in a polymerization reaction. ... In mathematics, there are numerous methods for calculating the average or central tendency of a list of n numbers. ...

Different average values can be defined depending on the statistical method that is applied. The weighted mean can be taken with the weight fraction, the mole fraction or the volume fraction: In statistics, given a set of data, X = { x1, x2, ..., xn} and corresponding weights, W = { w1, w2, ..., wn} the weighted mean is calculated as Note that if all the weights are equal, the weighted mean is the same as the arithmetic mean. ...

• Weight average molar mass or M_w
• Number average molar mass or M_n
• Viscosity average molar mass or M_ν
• Z average molar mass or M_z

The weight average molecular weight is a way of describing the molecular weight of a polymer. ... The number average molecular weight is a way of determining the molecular weight of a polymer. ...

These different definitions have true physical meaning because different techniques in physical polymer chemistry often measure just one of them. For instance osmometry measures number average molar mass and small angle laser light scattering measures weight average molar mass.M_v and M_z are obtained from respectively viscosimetry and sedimentation analysis. The quantity a in the expression for the viscosity average molar mass varies from 0.5 to 0.8 and depends on the interaction between solvent and polymer in a dilute solution. In a typical distribution curve the average values are related to each other as follows M_n < M_v < M_w < M_z. Polydispersity of a sample is defined as M_w divided by M_n and gives an indication just how narrow a distribution is. The polydispersity index, or PDI, is the ratio of the weight average molecular weight to the number average molecular weight. ...

The most common technique for measuring molecular weight used in modern times is a variant of high pressure liquid chromatography (HPLC) known by the interchangeable terms of size excluision chromatography (SEC) and gel permeation chromatography (GPC). These techniques involve forcing a polymer solution through a matrix of crosslinked polymer particles at a pressure of several thousand psi. The interaction between the crossliked polymer stationary phase and the polymer in the mobile phase results in higher retention times for low molecular weight species. The use of low polydispersity standards allows the user to correlate retention time with molecular weight.

The most common detectors used for size exclusion chromatography include online methods similar to the bench methods used above. By far the most common is the differential refraxtive index detector which measures the change in refractive index of the solvent. This detector is mass sensitive and only slightly molecular weight insensitive so it is ideal for a single detector GPC system as it allows the generation of mass v's molecular weight curves. Slightly less common are the molecular weight sensitive detectors of differential viscometry and low angle laser light scattering. These detectors directly measure the molecular weight of the polymer and are often used in conjunction with differental refractive index detectors

The molar mass distribution of a polymer sample depends on factors such as chemical kinetics and work-up procedure. Ideal step-growth polymerization gives a polymer with polydispersity of 2. ideal living polymerization results in a polydispersity of 1. By dissolving a polymer a insoluble high molar mass fraction may be filtered off resulting in an large reduction in M_w and a small increase in M_n thus reducing polydispersity. In physical chemistry, chemical kinetics or reaction kinetics study reaction rates in a chemical reaction. ... Step-growth polymerization is a polymerization process that involves a chemical reaction between multifunctional monomer molecules. ... In polymer chemistry, Living polymerization is a special form of addition polymerization where the ability of a growing polymer chain to terminate has been removed . ...

Molecular weight distribution is often found in the literature but this phrase is technically incorrect.