Aromatic diisocyanates (such as TDI) are generally more reactive with diols than aliphatic diisocyanates (such as hexamethylene diisocyanate). But you are on the right track when you switch from using TDI into another diisocyanate in order to form a sticky polyurethane of low average molecular mass (thermoplastic prepolymer). Of course, aromatic diols are usually avoided in producing polyurethanes but this is another story.

Mix equimolar amounts of aliphatic diisocyanate with aliphatic diol and follow the standard procedures that are given in laboratory manuals while shortening the period of time for the reaction. {Please take care since you are using toxic materials. After reaction, the polymer is relatively safe.}.

Take a sample of the product and apply it as an adhesive between wood or glass parts ...etc. & see the effect after one day. Wishing you success in your research.

But I want to obtain a non-brittle and flexible prepolymer formula using aromatic isocyanates or modified isocyanates. I use TDI and PMDI with polyether triol polyol in my current formula. I want to eliminate TDI from this form, due to the toxic effect of TDI. What can you suggest me now?

All diisocyanates are toxic but once they react with a polyol or a diol, the toxicity is reduced sharply. No problem in trying aromatic diisocyanates although the benzene ring is considered as "stiffening" group when it is present in the polymeric main chain & that is why I suggested aliphatic diisocyanates.

I am attaching this 30 pages PDF file that was prepared by a leading company in polyurethanes:

http://www.pmahome.org/files/5414/6263/3433/03_WhatPolyurethaneWhere.pdf

Please notice pages 18 & those after since they will help you. Best wishes.

All diisocyanates are toxic, but we have a non toxic for example HDI or PDI which no has aromatic on the main chain and about the stickiness prepolymer you can decrease the molar ratio of diisocyanate, decrease the temperature or reaction time.

Best wishes.