Bio-Renewable Polymers

Bio-derived poly(lactic acid) (PLA)

Lactic acid --> Polylactic acid

-Derived from corn starch or sugar cane

-Biodegradable (can be ester hydrolysed), biocompatible, bioresorbable

-Used in packaging, textiles, medical devices

Direct condensation synthesis of poly(lactic acid)

Lactic acid is used to give low molecular weight PLA

corn --enzymatic degradation--> sugars --fermentation--> lactic acid --step-growth polymerisation--> PLA

-Step-growth polymerisation is slow and unreliable

Ring-opening polymerisation synthesis of poly(lactic acid)

Lactide (cyclic lactic acid) is used to give high molecular weight PLA

corn --enzymatic degradation--> sugars --fermentation--> lactic acid --heat--> lactide --chain-growth polymerisation--> PLA

-Chain growth polymerisation is faster

-This method is preferred by industry

Applications of poly(lactic acid)

Used in sandwich packs as a window

Used as coffee cup lids

Issues with poly(lactic acid)

-Uses genetically modified crops

-Recycling of PLA was not always feasible

-PLA in landfill sites would not biodegrade but create methane over time (as slow to biodegrade)

Bio-based poly(butylene succinate) (Bio-PBS)

Succinic acid (dibutanoic acid) + 1,4-butanediol --> PBS

-Both monomers derived from renewable sources

-PBS fully biodegradable and compostable

-Strong, flexible, heat resistant

-Used in food packaging, textiles, medical applications

Production of bio-succinic acid

Produced via microbial fermentation

BIomass --extraction hydrolysis--> sugars --fermentation--> succinic acid

-Microbial fermentation using bacteria

-Converts sugars into succinic acid

-Purified via crystallisation or electrolysis

Production of bio-1,4butanediol

Produced from bio-succinic acid through catalytic hydrogenation (so derived from biomass)

Succinic acid --(H₂, metal catalyst)--> 1,4-butanediol

-Not environmentally friendly as uses metal catalyst (e.g. Ru, Pd, Ni)

Synthesis of bio-based poly(butlyene succinate) (Bio-PBS)

Formed through step-growth polymerisation via esterification between bio-BDO and bio-succinic acid

-Initial step temperature 160-190°C then second step temperature 220-240°C

-Requires Lewis acid catalysts (e.g. Sc(NTf₂)₃, Sc(CF₃SO)₂, Ti(OBu)₄)

-Solvents not required

Production of bio-ethylene for poly(ethylene)

-Sugars are processed to produced fruit juice

-Fruit juice is fermented and distilled to produce ethanol

-Ethanol is catalytically converted to ethylene

Ethanol --(H₂SO₄, 180°C)--> Ethylene

Low density polyethylene (LDPE)

High degree of branching on chain so less dense

-Better flexibility, impact toughness, resistance to environmental stress vs HDPE

-Crystallisation impeded by branching -> limits degree of crystallinity to ~40%

-Low density since more amorphous content (0.910-0.930 g cm⁻³)

-M_w of 80-40g mol⁻¹

-Used in drinks packaging as film to bundle them

High density polyethylene (HDPE)

Little branching on polymer chain so dense

-Increased crystallinity results in increased stiffness and higher density (0.935-0.960g cm⁻³)

-70+% crystalline

-M_w of around 35 kg mol⁻¹

-Large strength to density ratio

-Used in pipes, hard hats, LEGO

Urethanes

Formed by the reaction between an isocyanate (R-N=C=O) and an alcohol

-Alcohol attacks electrophilic C and electrons go to O

-O⁻ reforms double bond, N=C electron pair abstracts proton from O⁺-H

Polyurethanes

Produced by the step-growth polymerisation process between an isocyanate and alcohol

Derivation of polyurethane monomers

-Polyols can be derived from natural sources such as corn

-Isocyanates need to be derived from carboxylic acids, often using harsh conditions

-Isocyanates also highly reactive so limited stability -> usually prepared then used straight away

Applications of polyurethanes

Can be designed to provide strength (e.g. through addition of benzene rings) or to provide elasticity (through addition of alkane chains)

-Commonly used in textiles to make them durable, stretchy, waterproof

-E.g. Lycra, medical textiles, waistbands, fabrics

Biorenewable polymer

A polymer made wholly or partly from renewable biological resources, rather than from fossil-based feedstocks

Non-renewable production of ethylene for poly(ethylene)

Ethylene produced by thermal cracking of oil

-Energy intensive and produces significant amount of CO₂