Edit: some answerers seem to be slightly misunderstanding the nature of the premise. Remember that the concept here is this; In the Precambrian of Earth, bacteria evolve to extract the abundant chloride in seawater and use it to emit chlorine gas as a defense mechanism. Their predators adapt, and the predators' predators adapt, until all organisms (Only microbes at this stage) emit chlorine in one way or another. With the evolution of multicellular land plants, which occurs after the point-of-divergence with Earth's timeline, all the chlorine-emitting organisms covering the land keep on pumping it into the atmosphere until it is 1% chlorine. They have the same biochemistry as Earth bacteria; albeit an evolved version, but because of all the chlorine in the atmosphere, organic molecules have become heavily chlorinated; hence the PVC instead of cellulose.
In his book World-building, Steven L. Gillett proposes a world called Clorox, where in the planet's life's early history, microbes evolved to use the chloride in the water around them to produce chlorine gas as a defense. An evolutionary arms race ensued, and eventually all of the food chain produces the gas and the atmosphere contains 1% chlorine because of their emissions. The life forms are carbon-based and oxygen-breathing, but are highly tolerant to chlorine. Their cells have thick walls/membranes, and their bones, shells and other hard parts are made of plastics.
Apparently, the plants on this world produce PVC and use it like cellulose, making them have plastic bark, stems and leaves. There are a bunch of other unusual things about the planet, such as its greenish-yellow sky, mild acid freshwater bodies (And mild bleach seas), smogginess, and lack of an ozone layer - the chlorine does the job well enough, however.
I'd like to use a similar idea, but have it in an alternate Earth timeline, where Proterozoic bacteria evolves to emanate chlorine gas instead. One thing I'd like to check though is; Are Gillett's plastic plants viable? Would PVC instead of cellulose work biologically, and are there any repercussions on other aspects of the life it would have? You can leave out the last question if you like, but it'd be all the more appreciated if you didn't. Any other comments on such an atmosphere's effects are welcome.
Please inform me of any errors; and seriously - please don't downvote without leaving criticism. It can get annoying sometimes.