Diameter Polydispersity Of Microspheres

Control of diameter polydispersity is a key factor in all dispersion and emulsion polymerizations. Usually it depends on such parameters as monomer and initiator concentrations, and interfacial properties of surface-active agent (e.g., its critical concentration of micellization). For a given composition of the liquid phase, interfacial properties of surfactant usually strongly depend on its chemical structure. There are reports on using poly(dodecyl acrylate)-g-poly(e-capro-lactone)s with various ratios of molecular weights of poly(e-caprolactone) grafts and total molecular weights of copolymer molecules in syntheses of poly(lac-tide) microspheres [44,49].

Figure 1 illustrates dependence of the optical density at 320 nm of samples poly(dodecyl acrylate)-g-poly(e-caprolactone) [P(DA-CL)] with various ratios of Mn(CL)/Mn(DA-CL) dissolved in 1,4-dioxane-heptane (1:4 v/v) on P(DA-CL) concentration. The rapid increase of optical density indicates that copolymer concentration exceeded the critical concentration of micellization. Values of critical concentrations of micellization (ccm) for these copolymers are listed in Table 2.

Data in Table 2 indicate that ccm for copolymers with essentially the same total content of poly(e-caprolactone) and with similar molecular weights (from 22,000_to 32,000) strongly depends on the Mn(CL)/Mn(DA-CL) ratio. Initially, when Mn(CL)/Mn(DA-CL) decreases ccm decreases as well to about 5 g/L for copolymers with Mn(CL)/Mn(DA-CL) close to 0.2. However, when Mn(CL)/

FIG. 1 Dependence of the optical density at 320 nm (OD320) on the concentration of poly(dodecyl acrylate)-g-poly(e-caprolactone); samples with ratio Mn(CL)/Mn(DA-CL) equal 0.033, 0.107, 0.178, 0.307, and 0.413. Conditions: 1,4-dioxane/heptane (1:4 v/v) solvent, 1 cm cell. (From Ref. 44.)
TABLE 2 Critical Concentrations of Micellization (ccm) of P(DA-CL) in 1,4-dioxane-heptane (1:4 v/v) for Copolymers with Various Ratios of Mn(CL)/Mn(DA-CL)a

Mn(CL)/Mn(DA-CL)

Weight fraction of poly(e-caprolactone) in P(DA-CL)

ccm (g/L)

0.033

0.23

21

0.107

0.20

16

0.178

0.24

5

0.307

0.19

> 60 if any

0.413

0.24

> 60 if any

"Weight fraction of poly(e-caprolactone) in these copolymers was in the narrow range from 0.19 to 0.24. Source: Data from Ref. 44.

"Weight fraction of poly(e-caprolactone) in these copolymers was in the narrow range from 0.19 to 0.24. Source: Data from Ref. 44.

Mn(DA-CL) becomes higher than about 0.3 the copolymers do not form micelles even at concentrations of 60 g/L. Apparently, copolymers with many short poly-(e-caprolactone) chains are more soluble in a 1,4-dioxane/heptane mixture than those with one chain and thus form micelles inefficiently. On the other hand, copolymers with only one long poly(e-caprolactone) chain per macromolecule do not form micelles. Probably in these molecules the long poly(e-caprolactone) chains exceed a certain critical length, fold onto themselves, and become embedded in poly(dodecyl acrylate) long segments from the same copolymer macro-molecule. Such behavior is characteristic, for example, for proteins in water solutions. Proteins in water are usually folded so as to hide their hydrophobic fragments inside and expose their hydrophilic segments to exterior. These hy-drophilic envelopes limit their tendencies for aggregation. Schematically, the two above-discussed situations are illustrated in Fig. 2. In the figure, A corresponds to poly(DA-CL) having many short and thus soluble poly(e-caprolac-tone) side chains and B corresponds to copolymer molecule with one long insoluble poly(e-caprolactone) chain.

Poly(dodecyl acrylate)-g-poly(e-caprolactone) copolymers with various Mn(CL)/ Mn(DA-CL) ratios were used as stabilizers in dispersion polymerizations of ra-cemic mixture of d,d- and l,l-lactide and in polymerization of the optically pure l,l-lactide enantiomer [43,44,49]. Dependence of the uniformity parameter [1/ (Dw/Dn - 1)] on Mn(CL)/Mn(DA-CL) ratio is shown in Fig. 3.

Figure 3 indicates that the lowest polydispersity of diameters of poly(lactide) [poly(d,l-lactide) and poly(l,l-lactide)] microspheres could be obtained for Mn(CL)/Mn(DA-CL) ratio close to 0.23. Indeed, in polymerization of racemic mixture of d,d-lactide and l,l-lactide carried on in the presence of P(DA-CL) with Mn(CL)/Mn(DA-CL) = 0.25, microspheres with very narrow diameter distributions were obtained (Dw/Dn = 1.03). The number average diameter of these particles was 2.71 pm. The SEM image of these uniform microspheres is shown in Fig. 4.

FIG. 2 Schematic illustration of poly(dodecyl acrylate)-g-poly(e-caprolactone) copolymer chains in 1,4-dioxane/heptane mixture: A, a copolymer chain with many short poly-(e-caprolactone) grafts; B, a copolymer with one long and insoluble poly(e-caprolactone) side chain.

FIG. 3 Dispersion polymerizations of lactides (l,l-lactide and racemic mixture of d,d-lactide and l,l-lactide). Dependence of the diameter monodispersity parameter (l/(Dv/Dn - l)) on the ratio of molecular weight of poly(e-caprolactone) grafts and molecular weight of poly(dodecyl acrylate)-g-poly(e-caprolactone) copolymers. (From Ref. 58.)

FIG. 3 Dispersion polymerizations of lactides (l,l-lactide and racemic mixture of d,d-lactide and l,l-lactide). Dependence of the diameter monodispersity parameter (l/(Dv/Dn - l)) on the ratio of molecular weight of poly(e-caprolactone) grafts and molecular weight of poly(dodecyl acrylate)-g-poly(e-caprolactone) copolymers. (From Ref. 58.)

It was also important to find out to what extent diameter distribution of microspheres would be affected by concentration of P(DA-CL) surfactant. In a series of polymerizations of l,l-lactide initiated with stannous octoate and carried on in the 1,4-dioxane/heptane (1:4 v/v) medium, the concentration of P(DA-CL) [Mn(CL)/Mn(DA-CL) = 0.178] has been varied. Dependence of diameter polydispersity parameter (Dw/Dn) and yield of polymer in the form of microspheres on concentration of P(DA-CL) is shown in Fig. 5. According to Fig. 5, Dw/Dn was essentially independent of concentration of P(DA-CL) and varied in the range 1.06-1.08. However, it is important to stress that for P(DA-CL) concentrations lower than about 0.6 g/L the fraction of poly(l,l-lactide) in the form of microspheres decreased significantly. The remaining portion of polymer was in the form of a shapeless precipitate.

Was this article helpful?

0 0

Post a comment