# Bio-Sourced and Biodegradable Membranes
## 2 Green Membrane Materials
### Synthetic Biodegradable Polymers
#### 2.2.2 Petrochemical Polymers  
Poly-\(\epsilon\)-caprolactone  
Poly-\(\epsilon\)-caprolactone is an aliphatic polyester, produced by ring-opening polymerization of \(\epsilon\)-caprolactone (see Figure 6A) [77]. This polymer has a low Tg = -55-60 \({}^{\circ}\)C and low melting point of Tm = 60-65 \({}^{\circ}\)C and good resistance to oil and water. Examples of  
Figure 4: Synthesis routes PLA. using poly-\(\varepsilon\)-caprolactone for membranes include dye removal [78], wound dressing [79], antifouling [80], treatment blood infections [81], and guided tissue regeneration [82] applications. Furthermore, poly-\(\varepsilon\)-caprolactone is highly degradable polymer. Thus, Pseudozyma japonica-Y7-09 enzyme, which is present in different microorganisms, results in Poly-\(\varepsilon\)-caprolactone degradation by 93.3% for 15 days for polymer film and 43.2% for 30 days when the polymer was made as foam plastic [83]. Poly-propylene Fumarate  
Polypropylene fumarate (PPF) is an aliphatic polyester, containing a double bond in the main chain. This polymer can be produced by multistep synthesis (see Figure 6B) [84]. Because of the presence of double bonds, this polymer can be cross-linked by various methods which, results in the broad mechanical properties of the resultant structures [85]. Although, the mechanical properties are still low and limit the use of PPF for the membrane and other applications. However, some new approaches have been applied by the introduction of nanofillers into PPF to improve its properties [86; 87; 88]. The in vitro biodegradation of PPF scaffolds revealed nearly 17% mass loss at 6 weeks [89]. Figure 5: Circuit diagram of polybutylene succinate synthesis. Figure 6: Synthesis routes of some petrochemical derived biodegradable plastics. (**A**) Poly-\(\varepsilon\)-caprolactone; (**B**) Poly-propylene Fumarate; and (**C**) Poly-ethylene Glycol. Polyethylene glycol (PEG) is a synthetic polyether that can be produced by various ways: from ethylene oxide, ethylene glycol, or ethylene glycol oligomers (see Figure 6C). PEG production has been reported in 1859 by both [PERSON] and [PERSON], independently. PEG membranes are predominately used for drug release systems [90; 91; 92; 93; 94]. In addition, PEG is well known to possess sensitivity to oxidative degradation [95]; thus, this material is substantial to oxidative biological degradation by different microorganisms [96; 97; 98]. Poly-vinyl Alcohol  
Polyvinyl alcohol (PVA), unlike most of other vinyl polymers, cannot be produced by polymerization of its monomer units because vinyl alcohol is thermodynamically unstable. PVA is predominately produced by hydrolysis of polyvinyl acetate (see Figure 7). Hydrolysis reaction is usually proceeded in the presence of ethanol but can be easily done without it. PVA has been reported to use for membrane preparation for wound dressing applications [99; 100]. In addition, PVA is biodegradable in the presence of different microorganisms under two-step metabolic processes, including oxidation and hydrolysis [101]. Polyurethanes (PU) are one of the most produced polymers in the world. Around 25 million tons of this class of polymer was produced in 2019, which is about of 6% of all manufactured polymers [102]. The interest to polyurethanes is determined by the broad chemical variety of monomer units, resulting in a wide range of physical properties-from rigid and strong plastics to flexible elastomers [103]. Linear PU are basically produced during polycondensation reaction between diisocyanates and diols (see Figure 8A, step 2A) with the ability to produce polyurethane urea when a diamine chain extender is used on the second synthesis step (see Figure 8A, step 2B). Polyurethanes were known for a long time as non-degradable polymers. For the last decades, biodegradable PU were developed by using monomer units with hydrolysate bonds (e.g., ester, amide) groups [104; 105]. Examples of oligomeric diols [106; 107] used for biodegradable PU synthesis are given on Figure 8B.