1. Structural Analysis
For the hard-segment component of TPEE, highly crystalline polyesters-such as PBT or PET-may be selected; for the soft-segment component, polyethers (e.g., PTMG, PEG, PPG) or polyesters (e.g., PCL, PGA, PLLA) may be chosen. Based on the nature of the soft segment, TPEE is primarily classified into two types: polyether-type TPEE and polyester-type TPEE.
According to available literature, polyether-type TPEE-exemplified by brands such as Hytrel and Pelprene-P-is a block copolymer composed of highly crystalline polybutylene terephthalate (PBT) hard segments and polytetrahydrofuran ether glycol (PTMG) soft segments. Polyester-type TPEE-exemplified by brands such as Pelprene-S and Arnitel-U-also utilizes highly crystalline PBT for its hard segments, but employs polycaprolactone (PCL) for its soft segments.
The influence of differing segment structures on the block copolymerization reaction and the compositional uniformity of the resulting polymer stems primarily from the compatibility of the segments. The relative compatibility of various segment structures follows the sequence below:
When the polyether soft segments are identical: PBT > PET;
When the polyester hard segments are identical: PEG > PTMG > PTMG-PPG > PPG.
When comparing polyether-ester elastomers with identical intrinsic viscosities, those utilizing PBT as the hard segment and PTMG as the soft segment exhibit superior tensile strength and tear strength compared to those utilizing PEG as the soft segment. Furthermore, in terms of hydrolytic stability, polyether-esters with PTMG soft segments significantly outperform those with PEG soft segments.
Consequently, the majority of commercially produced polyether-ester elastomers utilize polytetrahydrofuran ether (PTMG) as the soft segment and polybutylene terephthalate (PBT) as the hard segment.
2. Synthesis Methods
Building upon the brief structural analysis presented above, we can now proceed to provide a more detailed exposition of the various synthesis methods employed for TPEE. Currently, the primary synthesis methods for TPEE include the hydroxyl-terminated polyether method (comprising transesterification and direct esterification routes), the acetoxy-terminated polyether method, and the chain exchange method, among others [2]. (1) Transesterification Method
The transesterification method involves using DMT (dimethyl terephthalate), EG (ethylene glycol) or BG (butanediol), and PTMG (polytetrahydrofuran ether) as raw materials, and preparing the product through polymerization processes such as transesterification reactions and melt polycondensation.
(2) Direct Esterification Method
The direct esterification method involves the direct melt polycondensation of terephthalic acid (PTA), BG, and PTMG to synthesize TPEE, thereby eliminating the transesterification step. Since PTA is inexpensive and readily available, this synthetic route has garnered increasing attention.
(3) Acetoxy-terminated Polyether Method
Acetoxy-terminated polyether serves as an intermediate in the ring-opening polymerization of tetrahydrofuran to produce hydroxyl-terminated polyether. Infrared analysis of fractions obtained at various temperatures has confirmed that acetoxy-terminated polyether can undergo transesterification reactions with DMT.
By conducting a transesterification reaction between DMT and EG, and subsequently subjecting the resulting product to melt polycondensation with acetoxy-terminated polyether, a multi-block copolymer of PET and polytetramethylene ether is synthesized. If the proportion of DMT in the reaction mixture is increased, block copolymers with a higher content of hard segments can be synthesized.
(4) Chain Exchange Method
The chain exchange method leverages the principle that, during the melt polycondensation process, the rate constant for chain exchange reactions is greater than that for chain growth reactions. This method employs pre-existing polymers as starting materials, facilitating a transesterification reaction between the polyester and the polyether-specifically, by substituting the low-molecular-weight diol units within the polyester chains with high-molecular-weight diol units.
