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Thiol-Yne Photopolymerizations: Novel Mechanism, Kinetics, and Step-Growth Formation of Highly Cross-Linked Networks.

Fairbanks BD, Scott TF, Kloxin CJ, Anseth KS, Bowman CN - Macromolecules (2008)

Bottom Line: Chain-growth polymerization of alkyne and vinyl functionalities was only observed for reactions in which the alkyne was originally in excess.A tetrafunctional thiol was photopolymerized with a difunctional alkyne, forming an inherently higher cross-link density than an analogous thiol-ene resin, displaying a higher glass transition temperature (48.9 vs -22.3 degrees C) and rubbery modulus (80 vs 13 MPa).Additionally, the versatile nature of this chemistry facilitates postpolymerization modification of residual reactive groups to produce materials with unique physical and chemical properties.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biological Engineering and the Howard Hughes Medical Institute, University of Colorado, Boulder, Colorado 80309-0424.

ABSTRACT
Radical-mediated thiol-yne step-growth photopolymerizations are utilized to form highly cross-linked polymer networks. This reaction mechanism is shown to be analogous to the thiol-ene photopolymerization; however, each alkyne functional group is capable of consecutive reaction with two thiol functional groups. The thiol-yne reaction involves the sequential propagation of a thiyl radical with either an alkyne or a vinyl functional group followed by chain transfer of the radical to another thiol. The rate of thiyl radical addition to the alkyne was determined to be approximately one-third of that to the vinyl. Chain-growth polymerization of alkyne and vinyl functionalities was only observed for reactions in which the alkyne was originally in excess. Analysis of initial polymerization rates demonstrated a near first-order dependence on thiol concentration, indicating that chain transfer is the rate-determining step. Further analysis revealed that the polymerization rate scaled with the initiation rate to an exponent of 0.65, deviating from classical square root dependence predicted for termination occurring exclusively by bimolecular reactions. A tetrafunctional thiol was photopolymerized with a difunctional alkyne, forming an inherently higher cross-link density than an analogous thiol-ene resin, displaying a higher glass transition temperature (48.9 vs -22.3 degrees C) and rubbery modulus (80 vs 13 MPa). Additionally, the versatile nature of this chemistry facilitates postpolymerization modification of residual reactive groups to produce materials with unique physical and chemical properties.

No MeSH data available.


Related in: MedlinePlus

(A) Elastic modulus and (B) tan δ versus temperature for stoichiometrically balanced polymerization of PETMP/DDY reacted at 25 (○) and 80 °C (◻) and PETMP/BDDVE reacted at 25 °C (Δ). Samples were formulated with 3 wt % Irgacure 184 and irradiated at 365 nm, 10 mW/cm2, for 60 min.
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fig7: (A) Elastic modulus and (B) tan δ versus temperature for stoichiometrically balanced polymerization of PETMP/DDY reacted at 25 (○) and 80 °C (◻) and PETMP/BDDVE reacted at 25 °C (Δ). Samples were formulated with 3 wt % Irgacure 184 and irradiated at 365 nm, 10 mW/cm2, for 60 min.

Mentions: Dynamic mechanical analysis (DMA) was performed on PETMP/DDY and, for comparison, a tetrathiol/divinyl ether formulation using a divinyl ether monomer of similar molecular weight to DDY (i.e., PETMP/BDDVE), as shown in 7 and Table 1. BDDVE was used as a difunctional analogue for DDY as 1,9-decadiene is immiscible with PETMP; however, the mechanical properties of fully cured PETMP/BDDVE are representative of those generally found in flexible thiol−ene polymer networks.12,13 Each sample exhibits a similar transition from a glassy regime, where the elastic modulus is greater than 1 GPa, to a rubbery regime, where the modulus is proportional to the cross-link density, marked by a characteristic glass transition temperature, Tg. The elastic modulus of PETMP/DDY cured at room temperature displays a glass transition beginning at 25 °C; as seen in 2, this sample contains residual thiol and vinyl sulfide after the cessation of polymerization. As the polymerization of PETMP/DDY proceeds, there is a concomitant increase in its Tg; when the Tg reaches the polymerization temperature, the mobility of unreacted functional groups is significantly impaired, i.e., vitrification, which limits the final conversion. To overcome vitrification and reach higher functional group conversions, photopolymerizations of a PETMP/DDY sample were performed at an elevated temperature of 80 °C, and polymer properties are summarized in Table 1.


Thiol-Yne Photopolymerizations: Novel Mechanism, Kinetics, and Step-Growth Formation of Highly Cross-Linked Networks.

Fairbanks BD, Scott TF, Kloxin CJ, Anseth KS, Bowman CN - Macromolecules (2008)

(A) Elastic modulus and (B) tan δ versus temperature for stoichiometrically balanced polymerization of PETMP/DDY reacted at 25 (○) and 80 °C (◻) and PETMP/BDDVE reacted at 25 °C (Δ). Samples were formulated with 3 wt % Irgacure 184 and irradiated at 365 nm, 10 mW/cm2, for 60 min.
© Copyright Policy - open-access - ccc-price
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2651690&req=5

fig7: (A) Elastic modulus and (B) tan δ versus temperature for stoichiometrically balanced polymerization of PETMP/DDY reacted at 25 (○) and 80 °C (◻) and PETMP/BDDVE reacted at 25 °C (Δ). Samples were formulated with 3 wt % Irgacure 184 and irradiated at 365 nm, 10 mW/cm2, for 60 min.
Mentions: Dynamic mechanical analysis (DMA) was performed on PETMP/DDY and, for comparison, a tetrathiol/divinyl ether formulation using a divinyl ether monomer of similar molecular weight to DDY (i.e., PETMP/BDDVE), as shown in 7 and Table 1. BDDVE was used as a difunctional analogue for DDY as 1,9-decadiene is immiscible with PETMP; however, the mechanical properties of fully cured PETMP/BDDVE are representative of those generally found in flexible thiol−ene polymer networks.12,13 Each sample exhibits a similar transition from a glassy regime, where the elastic modulus is greater than 1 GPa, to a rubbery regime, where the modulus is proportional to the cross-link density, marked by a characteristic glass transition temperature, Tg. The elastic modulus of PETMP/DDY cured at room temperature displays a glass transition beginning at 25 °C; as seen in 2, this sample contains residual thiol and vinyl sulfide after the cessation of polymerization. As the polymerization of PETMP/DDY proceeds, there is a concomitant increase in its Tg; when the Tg reaches the polymerization temperature, the mobility of unreacted functional groups is significantly impaired, i.e., vitrification, which limits the final conversion. To overcome vitrification and reach higher functional group conversions, photopolymerizations of a PETMP/DDY sample were performed at an elevated temperature of 80 °C, and polymer properties are summarized in Table 1.

Bottom Line: Chain-growth polymerization of alkyne and vinyl functionalities was only observed for reactions in which the alkyne was originally in excess.A tetrafunctional thiol was photopolymerized with a difunctional alkyne, forming an inherently higher cross-link density than an analogous thiol-ene resin, displaying a higher glass transition temperature (48.9 vs -22.3 degrees C) and rubbery modulus (80 vs 13 MPa).Additionally, the versatile nature of this chemistry facilitates postpolymerization modification of residual reactive groups to produce materials with unique physical and chemical properties.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biological Engineering and the Howard Hughes Medical Institute, University of Colorado, Boulder, Colorado 80309-0424.

ABSTRACT
Radical-mediated thiol-yne step-growth photopolymerizations are utilized to form highly cross-linked polymer networks. This reaction mechanism is shown to be analogous to the thiol-ene photopolymerization; however, each alkyne functional group is capable of consecutive reaction with two thiol functional groups. The thiol-yne reaction involves the sequential propagation of a thiyl radical with either an alkyne or a vinyl functional group followed by chain transfer of the radical to another thiol. The rate of thiyl radical addition to the alkyne was determined to be approximately one-third of that to the vinyl. Chain-growth polymerization of alkyne and vinyl functionalities was only observed for reactions in which the alkyne was originally in excess. Analysis of initial polymerization rates demonstrated a near first-order dependence on thiol concentration, indicating that chain transfer is the rate-determining step. Further analysis revealed that the polymerization rate scaled with the initiation rate to an exponent of 0.65, deviating from classical square root dependence predicted for termination occurring exclusively by bimolecular reactions. A tetrafunctional thiol was photopolymerized with a difunctional alkyne, forming an inherently higher cross-link density than an analogous thiol-ene resin, displaying a higher glass transition temperature (48.9 vs -22.3 degrees C) and rubbery modulus (80 vs 13 MPa). Additionally, the versatile nature of this chemistry facilitates postpolymerization modification of residual reactive groups to produce materials with unique physical and chemical properties.

No MeSH data available.


Related in: MedlinePlus