Since glass bottles and alu kegs are quite expensive. We have seen different approach to cut costs at breweries, one of those is to use plastic in stead of bottles or alu kegs.
One major problem with plastic is that is expandible and also allow permeation. So the CO2 run out much faster than glass or metal containers. But you also can not reuse it. You can do it, but it can reduce your quality of your beer. And that's not an good idea.
Still there are brewerys that put beers on plastic kegs. I asked some very known into beers in Belgium, and the respond was that the brewery put beer into plastic kegs mostly for export.
We can seen 4-litre PET Kegs on sale for personal use by known breweries [5].
Some kegs are quite similar to one alu one [2].
But some fillers are special designed to plastic kegs [1].
We see also that home brewers use plastic bottles. [3]
From japan the Mitsubishi Plastics Inc are creating one-litre plastic bottle that are covered with a thin carbon film on the inside of the bottle. That makes them 10 times better at preventing the loss of CO2 [4]. Thats is a kind of good approach.
In China they are doing drastic to cut prices, not so worried about the quality. They use plastic bag [6]. A kind of doggy bag?
One nasty issue with plastics is hormone inhibitors. For only that reason I'm very sceptical about using plastic with so important and life savings product.
[1] http://eshop.czechminibreweries.com/product/pkf-50/
[2] http://dolium.eu/product/dolium-30l/
[3] http://thebrewersmarket.com/product/500ml-plastic-beer-bottle-with-caps/
[4] https://blogs.wsj.com/japanrealtime/2015/08/20/beer-in-plastic-bottles-mitsubishi-plastics-works-to-make-it-happen/
[5] https://bestinpackaging.com/2011/11/13/brau-beviale-2011-party-beer-kegs/
[6] https://internchina.com/the-best-about-china-street-barbecue/