One of the changes that you observe is the reduction in double bond peak with increasing degree of polymerization when vinyl group based monomers are used.

Most vibrational modes are localized and care little about whether you are in a monomer or a polymer.  The exceptions include the group frequencies corresponding to things that are removed or added during the polymerization process.  There are extensive articles on this issue for polypeptides; the new modes are the amide modes.  If you can get down to 0 - 10 cm^{-1} (I am here thinking of Raman, not IR) there might in the solid be substantial changes in the modes corresponding in the monomer to whole-molecule libration.

Thank you for the reply. @ Garima Agrawal...what do you think is the reason behind the reduction of double bond intensity?

Thank you for the reply. @ Garima Agrawal...what do you think is the reason behind the reduction of double bond intensity?

Infrared spectroscopy is a useful tool to control the majority of chemical reactions. In the case of the polymerization is possible to observe  some structural differences between the monomer and the polymer using IR . For example in the case of vinyl polymers, sone of  remarkable changes are the reduction in intensity of the double bond as a result of the occurrence of polymerization (agreeing with @Garima Agrawal). In general, removal or addition of certain groups  can significantly modify the intensity and the frequency of vibration in the IR.

Polymerization means almost always some change in functional groups. You have double bonds in ethylene monomer but you don't  have it in polyethylene. Same about condensation polymers: -NH2 and carboxylic groups in the monomers for nylon synthesis vs amide bonds in polyamides, etc. All these changes can be followed easily by IR.