Microgel anionic

The arm-first synthesis of star microgels by initiating polymerization or copolymerization of a divinyl monomer such as diviny lbenzene or a bis-maleimide with a polystyryl alkoxyamine was pioneered by Solomon and coworkers.692 693 The general approach had previously been used in anionic polymerization. The method has now been exploited in conjunction with NMP,692 6 ATRP69 700 and RAFT.449 701 702 The product contains dormant functionality in the core. This can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP ATRP704). 

Among the divinyl monomers, 1,4-DVB and EDM A are the most extensively studied monomers for microgel formation by anionic polymerization. Com-... 

Fig.46. Dependence of [r ] on theMn of polymers prepared by anionic polymerization of 1,4-DVB in THF. The symbols represent linear ( ) branched (V) and microgel ( ) structures. The dashed line represents the [iq]/Mn relationship of anionically prepared polystyrene. [Reproduced from Ref. 231 with permission, Hiithig Wepf Publ., Zug, Switzerland].

Anionic polymerization of 1,4-DVB by n-BuLi leading to the microgels was also reported by Eschwey et al. [236,237]. In their experiments, n-BuLi was used at very high concentrations of 17 and 200 mol % of the monomer. Contrary to the results of Hiller and Funke [231], they observed a transition from microgel to macrogel with decreasing n-BuLi concentration. Similar results were also reported by Lutz and Rempp [238]. They used potassium naphthalene as the initiator of the 1,4-DVB polymerization and THF as the solvent. Soluble polymers could only be obtained above 33 mol % initiator, whereas below this value macrogels were obtained as by-products. 

Fig. 49. Calculated dependence of the polymer structure on the initial 1,4-DVB and n-BuLi concentrations in the anionic 1,4-DVB polymerization. The numbers I to IV represent the region for the formation of linear, branched, microgel and macrogel structures, respectively. The solid and dashed curves represent the transition regions between these structures. [Reproduced from Ref. 239 with permission, Hiithig Wepf Publ., Zug, Switzerland]. 

Fig. 51. Schematic illustration of the mechanism of microgel formation in the anionic dispersion polymerization of 1,4-DVB initiated by living PBS chains in heptane. [Reprinted with permission from Ref. 247, Copyright 1995, American Chemical Society].

Fig. 54. Dependence of Mw of the microgels on the polymer yield in the anionic polymerization of EDMA in toluene by n-BuLi [254] (see Figure 53 caption for the reaction conditions). 

Fig. 55. Gel-permeation chromatogram(GPC) of a microgel sample of Mw = 10X106 g/mol obtained in the anionic polymerization of EDMA in toluene. Microgel concentration = 1 g/L solvent = butyl acetate elution temperature = 70 °C is the weight-average molar mass of linear polystyrene used for comparison. [Reproduced from Ref. 256 with permission, Huthig Wepf Publ., Zug, Switzerland].

Pille et al. used living PBS chains to initiate the anionic polymerization of EGDM and 1,4-butanediol dimethacrylate. They obtained highly crosslinked microgels together with slightly branched oligomers of PBS of a low molar mass [260]. 

Almost linear polymers with pendant vinyl groups are formed as intermediates in the anionic polymerization of 1,4-DVB due to the different reactivities of monomers and pendant vinyl groups. 1,4-DVB microgels are formed towards the end of monomer conversion. In the anionic polymerization of EGDM or 1,3-DVB, reactive microgels are formed already at the beginning of the polymerization. 

Only a few publications have appeared in which for the synthesis of reactive microgels other monomers were used than 1,4-DVB or EDMA. Hiller and Funke studied the anionic polymerization of 1,4-diisopropenylbenzene (1,4-DIPB) by n-BuLi in 1,2-dimethoxyethane and by sodium naphthalene in THF [231]. 

Values for RU differed by up to 100% with 1,4-DVB-microgels [286]. The reliability of methods for determining the RU of 1,4-DVB-microgels was checked [287] with poly(4-vinylstyrene) which was prepared by anionic polymerization of 1,4-DVB (Table 3). From these results, it can be concluded that only quantitative IR-spectroscopy is a reliable method for determining the RU of 1,4-DVB-... 

Fig. 56. Dependence of Mwof the microgels on the polymer yield in the anionic polymerization of EDMA in toluene by n-BuLi [254] (see Figure 53 caption for the reaction conditions). Reduced viscosity vs concentration of microgels a) Composition (mol %) N,N -methyl-enebisacrylamide (55%), methacrylamide (33%), methacrylic acid (2%), methacrylamido acetaldehyd-dimethylacetal (10%),measured at 20 °C in water, b) Composition (mol %) 1,4-DVB (35%), propenic acid amide-2-methyl-N-(4-methyl-2-butyl-l,3-dioxolane prepared by emulsion copolymerization and measured in dimethylformamide.

In a related application, polyelectrolyte microgels based on crosslinked cationic poly(allyl amine) and anionic polyfmethacrylic acid-co-epoxypropyl methacrylate) were studied by potentiometry, conductometry and turbidimetry [349]. In their neutralized (salt) form, the microgels fully complexed with linear polyelectrolytes (poly(acrylic acid), poly(acrylic acid-co-acrylamide), and polystyrene sulfonate)) as if the gels were themselves linear. However, if an acid/base reaction occurs between the linear polymers and the gels, it appears that only the surfaces of the gels form complexes. Previous work has addressed the fundamental characteristics of these complexes [350, 351] and has shown preferential complexation of cationic polyelectrolytes with crosslinked car-boxymethyl cellulose versus linear CMC [350], The departure from the 1 1 stoichiometry with the non-neutralized microgels may be due to the collapsed nature of these networks which prevents penetration of water soluble polyelectrolyte. 

Stars with high arm numbers are commonly prepared by the arm-first method. This procedure involves the synthesis of living precursor arms which are then used to initiate the polymerization of a small amount of a difunctional monomer, i.e., for linking. The difunctional monomer produces a crosslinked microgel (nodule), the core for the arms. The number of arms is a complex function of reaction variables. The arm-first method has been widely used in anionic [3-6,32-34], cationic [35-40], and group transfer polymerizations [41] to prepare star polymers having varying arm numbers and compositions. 

In comparison to the polybutadiene stars under similar reaction conditions, the polyisoprene stars showed slightly lower degrees of branching. The added steric hindrance from the methyl group on the polyisoprene anion perhaps makes entry into the DVB "microgel" nodule difficult. 

The unique property of conjugated polymers to uptake/release the anions upon transformation from oxidized to reduced state can be used for the targeted deposition of NPs. To demonstrate the principal viability of such approach, VCL/AAEM/PEDOT microgels have been used as templates for the incorporation of AuNPs [148], Figure20 shows the procedure of AuNP deposition into microgels filled with PEDOT nanorods. 

Fig. 20 Selective AuNPs deposition on PEDOT nanorods in microgel structures (a) PEDOT nanorods are not fully oxidized and attract a small amount of counterions (green circles). The small amount of positively charged groups (red circles) are due to the incorporation of initiator residues into polymer chains of microgel. (b) After addition of H+, [AuCU]- PEDOT nanorods become oxidized in acidic pH and [AuCLt]" anions (yellow circles) are drawn into the microgel to compensate for a charge on the nanorod surface (white circles), (c) After addition of NaBH4 and a reduction process, AuNPs are predominantly formed on the PEDOT nanorod surface. Taken from [148], Copyright Wiley-VCH. Reproduced with permission...

Soppimath KS, Kulkarni AR, and Aminabhavi TM. Chemically modified polyacrylamide-g-guar gum-hased cross-linked anionic microgels as pH-sensitive dmg delivery systems Preparation and characterization. Journal of Controlled Release 2001 75 331-345. 

Soppimath, K.S. Kulkarni, A.R. Aminabhavi, T.M. Chemically modified polyacrylamide-g-guar gum-based crosslinked anionic microgels as pH-sensitive drug delivery systems preparation and characterization. J. Control. 

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