1. Introduction Polymers and prepolymers are usually charac- terised with a wide range of techniques. These tech- niques range from molecular weight determination to analytical methods. The molecular weight of a polymer or prepolymer quoted in a different form is dependent upon the techniques used for its determi- nation. Among them the most likely quoted are the number average molecular weight (M n ) and weight average molecular weight (M w ) while a third type of molecular weight referred occasionally is desig- nated as z-average molar mass (M z ). Usually M w is greater than the M n except for an ideal system where uniform molar mass (M w = M n ) is concerned. This difference in the values of M w and M z leads to the spread of molar masses i.e. dis- persivity (M w /M n > 1). It is an established fact that M w is sensitive to high molar masses while the M n is affected more with the lower molecular weight. There are quite a large number of methods [1] for determining the average molar mass of a polymer but relatively few for investigating the distribution of that molar mass. The knowledge of the poly dis- persivity of a polymer is important to understand the variation in its processing conditions [2–4] and subsequently help in tailoring it according to the requirements. In the present study liquid hydroxyfunctional poly- dimethylsiloxane prepolymers having different molar masses (quoted by the manufacturers) were characterised with different techniques and the respective results are compared with one another. The poly dispersivity of the polymer with respect to different techniques has been discussed. 829 * Corresponding author, e-mail: gbshah.gul @ gmail.com © BME-PT and GTE Characterisation of silicone prepolymers and disparity in results G. B. Shah * Applied Chemistry Laboratories (ACL), PINSTECH P. O. Nilore, Islamabad, Pakistan Received 11 September 2008; accepted in revised form 17 October 2008 Abstract. Liquid hydroxyfunctional polydimethylsiloxanes prepolymers (HOPDMS, Siloprens) with a range of relative molar masses, Mn (25 500–88 000 g/mol) quoted by the supplier have been characterised by a number of techniques such as OH group titration, bulk viscometry, Gel Permeation Chromatography (GPC), and swelling measurements. The resulting molar masses of these prepolymer obtained through these techniques have been compared with one another and that of the values quoted by the manufacturer. Unexpected results of insensitivity of the properties of these prepolymer to the initial molar masses quoted by the suppliers have been observed. It has been found out that these results are dependent upon the type of technique used. The reason for these unexpected results has been discussed in detail and attributed to the multi func- tionality of the prepolymers, a characteristic not previously suspected or reported. Keywords: material testing, prepolymers, viscosity average molecular weight, number average molecular weight, molecu- lar weight between crosslinks eXPRESS Polymer Letters Vol.2, No.11 (2008) 829–834 Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2008.96