Single-site beta-diiminate zinc catalysts for the ring-opening polymerization of beta-butyrolactone and beta-valerolactone to poly(3-hydroxyalkanoates).
Polymerization of beta-butyrolactone (BBL) and beta-valerolactone (BVL) using the zinc alkoxide initiator (BDI-1)ZnO(i)()Pr [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] proceeds very rapidly under mild conditions to produce poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxyvalerate) (PHV), respectively. For the monomer-to-initiator ratio 200:1, PHB number-average molecular weights (M(n)) are proportional to conversion throughout the reaction and polydispersity indices (PDIs) are narrow, consistent with a living polymerization. Higher monomer-to-initiator ratios can be used to achieve high molecular weight PHB (M(n) > 100 000). End-group analysis verifies that the polymerization of BBL follows a coordination-insertion mechanism, where complexes of the form (BDI-1)ZnOCH(Me)CH(2)CO(2)R are the active species. Variable temperature NMR experiments show that (BDI-1)ZnO(i)()Pr is monomeric in benzene-d(6) solution. In contrast, (BDI-2)ZnO(i)()Pr [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene] is a poor initiator at room temperature because it prefers to form a bis-mu-isopropoxide dimer in solution. According to kinetic studies, propagation by (BDI-1)ZnO(i)()Pr is first order in both monomer as well as (BDI-1)ZnO(i)()Pr concentration. These results lead us to propose a monometallic active species. Several results suggest that elimination side reactions are slowly catalyzed by zinc alkoxides, leading to degradation of the polymer.
Rieth
Lee R
LR
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, USA.
Moore
David R
DR
Lobkovsky
Emil B
EB
Coates
Geoffrey W
GW
D015107:4-Butyrolactone / Q000031:analogs & derivatives* / Q000737:chemistry*; D002384:Catalysis; D018360:Crystallography, X-Ray; D006885:Hydroxybutyrates / Q000138:chemical synthesis*; D007700:Kinetics; D007783:Lactones / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D009942:Organometallic Compounds / Q000737:chemistry*; D011091:Polyesters / Q000138:chemical synthesis*; D012996:Solutions; D013237:Stereoisomerism; D014631:Valerates / Q000138:chemical synthesis*; D015032:Zinc / Q000737:chemistry*; D000094122:Polyhydroxybutyrates
D015107:4-Butyrolactone; D006885:Hydroxybutyrates; D007783:Lactones; D009942:Organometallic Compounds; D011091:Polyesters; D012996:Solutions; D014631:Valerates; D015032:Zinc; C471863:(2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene)isopropylzinc; C471864:beta-valerolactone; C003182:poly-beta-hydroxybutyrate; D000094122:Polyhydroxybutyrates; C013426:beta-butyrolactone; C073314:poly(3-hydroxyvalerate)