Study On The Preparation Of PDMS Stabilizers And Its Application To Dispersion Polymerization In Supercritical CO₂

Polylactic acid(PLA)is the most extensively researched and utilized biodegradable and renewable aliphatic polyester.As a leading biomaterial,PLA has been used for numerous applications in medicine.But traditional methods to prepare these polyesters had many disadvantages such as the use of organic solvents,high reaction temperatures and the uncontrolled characters of the products.CO2 is inexpensive,nonflammable and nontoxic,making it an attractive solvent for large-scale synthesis.The preparation of polylactic acid by dispersion polymerization in ScCO2 has become a focus of research,while the preparation of dispersion polymerization stabilizer is the most important.In this paper,the research work in this field is reviewed.Then the synthesis of stabilizers and their applications in the preparation of PLLA by dispersion polymerization in ScCO2 are mainly introduced.The main results and conclusions are as follows:Four kinds of triblock-stabilizers and three kinds of diblock-stabilizers were prepared by using poly(dimethylsiloxane)(PDMS)as the CO2-philic chain,and the biodegradable poly(L-lactide)(PLLA)and polycaprolactone(PCL)as the lipop hilic chain.The structure of the stabilizers was characterized by FTIR and 1HNMR.The optimum preparation conditions were determined as follows:the reaction temperature was 110? and the molar ratio of catalyst to initiator was 1:1.With the increase of temperature and pressure,the solubility of stabilizers increased gradually,while the solubility of stabilizers decreased with the growth of lipophilic segment lengths.Whether the stabilizer was end-capped or not had little influence on its solubility.The PLLA powders were prepared successfully by using butyl alcohol as initiator and stannous acetate as catalyst with the synthesized stabilizers in ScCO2.The influence of the stabilizer amount,the reaction temperature and the reaction pressure on the characteristics of PLLA were studied by orthogonal test.The optimum preparation conditions were determined as follows:the reaction pressure was 20 MPa,the reaction temperature was 90?,the stirring speed was 300 rpm and the stabilizer amount was 5 wt%(stabilizer/monomer).The dispersion rule of the stabilizers and their influences on the PLLA were studied.The results were summarized as follows:The common stabilizers were involved in the initiation of the reaction system and the anchoring mode was chemisorption.The dispersion effect of the stabilizers was better with the shorter lipophilic segment lengths.When the optimum block mass ratio of stabilizer was 1:4:1 and the dosage was 5 wt%,the ideal average particle size and size distribution of PLLA were obtained.The anchoring mode of the end-capped stabilizer belonged to physical adsorption,and the anchoring ability increased with the increase of lipophilic segment lengths,while the solubility of stabilizer becames worse.When the optimum block mass ratio of the end-capped stabilizer was 1:2:1 and the dosage was 5 wt%,the dispersion effect of stabilizer was optimum,and the dispersion effect of end-capped stabilizer was better than the common stabilizer.The experimental results showed that the similarity between the lipophilic chains and the product was more helpful to anchoring and adsorbing polymers,and the dispersion effect became better.In other words,the anchoring capability of PLLA chain in stabilizers was better than that of PCL chain.By comparing the dispersion effect of stabilizers,the proportional relationship between the block mass ratio and the anchor-soluble balance(ASB)was determined.The triblock stabilizer contained two anchored PLLA segments,which had stronger anchoring property than diblock stabilizer.The block mass ratios of the acetyl teminated PLLA-b-PDMS-b-PLLA were 1:1:1,1:2:1 and 1:3:1,which were expressed as S111 Ac,S121As and S131Ac,respectively.S111 Ac had the longest lipophilic chain and good anchoring capability,but the solubility was worse in ScCO2.Although the solubility of S131 Ac was improved,the anchoring property decreased and the ASB of stabilizer tended to be CO2-philic property.However,S121 Ac had the best dispersion effect in three kinds of end-capped stabilizers.When the amount of S121Ac was 10 wt%,the average particle size of PLLA was the smallest and the size distribution was the narrowest.Poly(?-caprolactone)-block-polydimethylsiloxane-block-poly(?-caprolactone)triblock copolymer(PCL-PDMS-PCL)was synthesized via the ring-opening polymerization of?-caprolactone with dihydroxypropyl-terminated PDMS(HTPDMS)as the initiator and Bronsted acid as catalyst.The reaction time was shortened from 24 h to 4 h.