Molecular Mass of Polymers and Biodegradable Polymers

Molecular mass of polymer :
Number average molecular mass : \overline{M_{n}} = \frac{\sum N_{i}M_{i}}{\sum N_{i}}
Ni → number of molecules of m1 mass
Mi → molecular mass
Weight average molecular mass :
\overline{M_{w}} = \frac{\sum N_{i}M_{i}^{2}}{\sum N_{i}M_{i}}
Poly Dispersity Index :
PDI = \frac{\overline{M_{w}}}{\overline{M_{n}}}
For natural polymers PDI = 1
Synthetic polymers PDI > 1
Biodegradable polymer :
Aliphatic polyesters.
eg : Poly β-hydroxybutyrate-co-β-hydroxyvalerate) (PHBV)
Obtained by copolymerisation of 3 - hydroxy butanoic acid. It is used in packing Orthopedic devices and controlled release of drugs.
LDPE (Low density polyethylene)
Ethylene \xrightarrow[350 - 570 \ K]{1000 - 2000 \ atm} LDPE
Free radical addition
High branched chemically inert poor conductor of electricity.
HDPE (High density polyethylene)
Ethylene \xrightarrow[1 \ atm]{330 - 350 \ K} HDPE
Zeigler Natta catalyst → (C2H5)3Al + TiCl4
High tensile strength relatively tough and hard, making containers, bottles, pipes, house ware etc

Part1: View the Topic in this Video from 15:22 to 20:40

Part2: View the Topic in this Video from 52:11 to 58:15

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1. Number-Average Molecular Mass \tt \left(\overline{M}_{n}\right)
If N1 molecules have molecular mass M1 each, N2 molecules have molecular mass M2 each, N3 molecules have molecular mass M3 each and so on, then
\overline{M}_{n}=\frac{\Sigma N_{i}M_{i}}{\Sigma N_{i}}

2. Mass-Average Molecular Mass \tt \left(\overline{M}_{w}\right)
Supposing, as before that N1, N2, N3 etc., molecules have molecular mass M1, M2, M3 etc., respectivley, then,
\overline{M}_{w}=\frac{\Sigma N_{i}M_{i}^{2}}{\Sigma N_{i}M_{i}}

3. Polydispersity Index
It is the ratio of the mass average molecular mass to the number average molecular mass