PET is formed via the condensation of ethylene glycol (ethane-1,2-diol, aka EG) and terephthalic acid (1,4-benezenedicarbyoxylic acid, aka BDC). In the process,

PET is formed via the condensation of ethylene glycol (ethane-1,2-diol, aka EG) and terephthalic acid (1,4-benezenedicarbyoxylic acid, aka BDC). In the process, a proton (H, a hydrogen) is lost from the carboxylic acid (COOH -> COO) and a hydroxyl is lost from the alcohol (R-OH -> R). The H and OH combine to form a water (hence "condensation") and leave, and an ester bond (RCOO-R') is formed. Amide bonds (RCONR'), the backbone proteins and materials like Kevlar, spider silk, and Nylon, are similar, but less flexible and harder to break. This condensation reaction (and others) is reversible. If you heat PET in boiling water, you can begin to depolymerize the polymer into its constituent monomers by having a water molecule consumed in the reaction (RCOOR' + H2O -> RCOOH + HO-R'). However, PET is a rigid plastic that acts as a barrier material (that's why it's used for bottles and packaging), and has a high glass transition temperature (the temperature at which polymer chain mobility becomes broadly possible -- the polymer is in equilibrium, while it is kinetically trapped below the Tg). This inhibits the breakdown of PET, particularly at low temperature. Hence, the sentence from the abstract: > With a high ratio of aromatic terephthalate units—which reduce chain mobility—PET is a polyester that is extremely difficult to hydrolyse. Aromatic rings are just that -- rings. They don't flex a lot and can't undergo a lot of thermal motion, can't change conformation, etc. > Here we describe an improved PET hydrolase that ultimately achieves, over 10 hours, a minimum of 90 per cent PET depolymerization into monomers, with a productivity of 16.7 grams of terephthalate per litre per hour (200 grams per kilogram of PET suspension, with an enzyme concentration of 3 milligrams per gram of PET). > We also show that biologically recycled PET exhibiting the same properties as petrochemical PET can be produced from enzymatically depolymerized PET waste, before being processed into bottles, thereby contributing towards the concept of a circular PET economy. So, they depolymerize the PET, an subsequently repolymerize the terephthalic acids to produce new PET. I haven't clicked through to find if they recycle the EG as well, though.