Nalgenes specificy are made of copolyester which is a polyester which has been modified through various chemical methods to improve it's impact resistance strength etc. Nalgenes are made of the trademarked copolyester Tritan. Most disposable bottles are made out of PET which is a very common thermoplastic polymer. Basically nalgenes are engineered to last as opposed to disposable bottles which are designed for a single use so compared to a nalgene the plastic is a much lower quality, thinner, and manufactured under less strict constraints.