PLA is a plastic made of corn (it is polymerized lactic acid). It's actually a pretty versatile thermoplastic as it's semi-crystalline and so although it can be formed at relatively low temperatures, if you heat treat it (which increases its crystallinity), it can withstand ~boiling temperatures like other common thermoplastics. It's also technically biodegradable, but usually won't actually break down unless thin and in proper industrial compost processes. (Which can be kind of good for many applications... you don't want your plastic breaking down while in use, although you definitely don't want to be littering PLA.)

It's pretty cheap, commonly used in one-time plastic items, and is the most common 3D printing plastic.

But also, you can make polypropylene and polyethylene pretty easily from methane (which can be from biological sources or synthesized from electrolyzed hydrogen and CO2) via methanol-to-olefins processes, common in China.

But you're missing the point. We can synthesize literally anything you can make from oil using hydrogen, direct air captured CO2, and the Fischer-Tropsch process. It's a complete replacement for crude oil. It's just more expensive than getting it from the ground.

With enough energy you can make plastic out of CO2 and water. So it's an economic question: What happens when oil is no longer useful as fuel? Will it be cheaper to just synthesize the hydrocarbons you need, or will it be cheaper to keep some of the oil infrastructure running?

I have no idea why you're being down-voted. You literally can make any hydrocarbons we get from crude using synthesis gas (from CO2 and water split into hydrogen) using the Fischer-Tropsch process: https://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_proces...

Are folks just not aware of this, so they think people who say you can "make plastic out of CO2 and water" are talking hand-wavy nonsense?

The process is used commercially today in Qatar, Malaysia, and South Africa plus has had pilot plants in the US. But many of these US pilot plant efforts stopped when the price of oil cratered. So again, we absolutely do have alternatives to crude oil, it's just a function of economics.

We can also play with some alternatives such as artificial spider silk. This is an opportunity not just to ape current methods, but actually come with something completely new.