Onepot Polycondensation of Rpet and FADD Advances Circular Economy
A one-pot melt polycondensation process can convert post-consumer recycled polyethylene terephthalate into higher-value block copolyesters by pairing recycled PET with a fatty acid dimer diol. The route uses controlled alcoholysis and torque-based endpoint monitoring to produce materials with defined soft and hard segments.
The work addresses a central question in circular materials research: how to use post-consumer recycled polyethylene terephthalate (rPET) efficiently in high-performance applications. The proposed route combines high-molecular-weight rPET with fatty acid dimer diol (FADD) in a one-pot melt polycondensation process.
FADD serves as a hydrophobic remote diol with an average molecular weight of about 550 g/mol. Its functionality is held at exactly 2.0, a specification intended to prevent excessive branching or chain capping during polycondensation and to preserve the target soft-hard segment ratio.
To degrade and reassemble rPET, the process uses alcoholysis to break the PET backbone. Two synthesis series were developed around different diols, allowing the hard segment to be varied while keeping the soft segment structure comparable.
| Process series | Key reactant | Hard segment | Core function |
|---|---|---|---|
| Series one | Ethylene glycol (EG) | PET | Promotes in situ degradation and random distribution |
| Series two | 1,4-Butanediol (BDO) | PBT | Transesterification replaces EG to form a PBT backbone |
In the first series, excess EG acts as a catalyst carrier and supports in situ depolymerization, supplying hydroxyl end groups that accelerate transesterification. The resulting material has a PET hard segment with a random distribution.
The second series introduces BDO. Because BDO and EG have different boiling points — roughly 230 °C and 197 °C, respectively — selective distillation can remove EG and convert the PET backbone into PBT through transesterification. This yields copolymers with similar soft segments but different hard-segment properties.
All reactions were carried out in a custom 1-liter melt polymerization reactor. Torque monitoring was used to improve batch consistency because agitator torque is directly related to melt viscosity.
The reaction was stopped when torque reached 35 N cm at a stirring rate of 25 rpm, standardizing molecular-weight evolution across runs. The final product was quenched in deionized water and dried under vacuum to support accurate thermal and mechanical analysis.
