CHAIN INITIATION

As the comments have mentioned, it probably begins by the protonation of butene impurities.

A secondary carbocation is formed, which undergoes methyl shift to form a primary carbocation.

Propagation

An alkane attacks the carbocation, and supplies a "hydride" to the carbocation to complete the carbocation.

Meanwhile, another carbocation is formed which also undergoes methyl shift to regenerate the carbocation at the beginning of this step.

Termination

 here to suggest a isobutyl chloride (1-chloro-2-methylpropane) termination product. I can't find other Lewis bases to complete the carbocation for the termination step