Polybutene and polyisobutylene are liquid oligomer

Oligomer

In chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...

s widely used as plasticizers for high-molecular weight polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s, such as polyethylene

Polyethylene

Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...

. They are not to be confused with the high molecular weight polymer polybutene-1.
The liquid is clear and colourless, and is sold in many different grades. The grades are determined mainly by molecular weight, with the higher molecular weight grades having increasing viscosity

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

.