# Utilization of Polymeric Materials toward Sustainable Biodiesel Industry: A Recent Review
## 2 Polymeric Biodiesel
### Production Techniques  
PHAs are polymeric ester-linked molecules consisting of considerable quantities of carbon and energy. Each molecule comprises 600-35,000 units of fatty acid monomers [36]. There are more than 150 recorded monomers, each monomer contains chain (R) group which are often saturated alkyl group or sometimes unsaturated or branched alkyl group. Based on the number of carbon atoms, PHAs are categorized into three groups, short (3-5 carbon atoms), medium (6-14 carbon atoms), and long (above 15 carbon atoms) chain length PHAs biopolymers. The difference in PHAs characteristics comes from the diversity of monomers. If the synthesis is made up of one type of monomer, it is called homopolymer while the synthesis of different types of monomers is called copolymer [19]. Table 1 shows examples of PHAs with the sources of carbon and synthesis microbes. Under unbalanced nutrition conditions, prokaryotic microorganisms are responsible about synthesizing PHAs as stored intracellular matter [19]. There are more than 300 micro-organic species, most of them are bacterial species, capable to produce PHAs. They were classified into two groups. The first bacterial group requires excessive carbon source and limited nutritional elements of nitrogen, sulfur, magnesium, and phosphorus. _A. eutrophus_, _Protonnas extorquens_, and _P. olevovans_ species are examples of this bacterial group. In contrast, the other group such as _Alcaligenes latus_, _Azotobacter vielandii_ and _E. coli_ requires excessive sources of both carbon and the other elemental nutrients [53]. Figure 2: Production of polymeric biodiesel HAME and HBME from mcl PHA and P3 HB. P3 HB and mcl PHAs are the polymeric raw materials to synthesize the polymeric biodiesel HBME and HAME. They are sourced from rich carbonic source, such as de-watered activated sludge, and isolated using particular bacterial strains. The produced polymeric materials undergo esterification through acid-catalyzed hydrolysis, in which alcohol methanol or ethanol are added in optimized ratios to complete the conversion into alkyl esters. This chemical reaction is usually catalyzed by acid catalyst, and sulfuric acid (H\({}_{2}\)SO\({}_{4}\)) is used most frequently. However, hydrochloric acid (HCl) and phosphoric acid (H\({}_{3}\)PO\({}_{4}\)) were used to catalyze this chemical reaction. In addition to the acid catalysts, base catalysis was reported to synthesize HAME and HBME from mcl PHAs and P3 HB polymeric materials. After the formation of HAME and HBME, saturated solution of salted water is added and stirred intensively. Then, the mixed solution is let to settle down into two layers. The more transparent layer is biodiesel HAME and HBME that should be dewatered and filtered to remove undesirable debris [19; 35]. Since the raw materials PHAs for the polymeric biodiesel is non-toxic an utilized in medical, agricultural, and industrial applications, the wastes of this acid-catalyzed hydrolysis do not form threats [54]. However, a toxicity test is highly recommended for the polymeric biodiesel and byproduct wastes in future investigations. Figure 3 illustrates the production line of polymeric biodiesel (HAME and HBME). There are four parameters affecting acid catalyzed esterification to synthesize polymeric biodiesel, i.e., the type and ratio of alcohol, type and ratio of the catalyst, reaction temperature, and reaction time. Four alcohols can be used in this chemical reaction, methanol, ethanol, propanol, and butanol. However, methanol and ethanol are the most frequently used due to their preferable physicochemical properties and low costs. It was reported that alcohol methanol is more efficient for converting mcl PHAs and P3 HB into alkyl esters. The reason behind this efficiency is the short chain of the alkyl group within methanol which has higher solubility and activity. For alcohol ratio, higher amount of alcohol results in higher conversion percentage [19; 55].