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POLYMER CAPSULE
SUPHANNEE PONGKITWITOON
The Polymer Capsule
The Capsule Formation
- Ring Opening Metathesis (ROM) 1
- Cross Metathesis 7
Amphiphilic Graft Copolymer prepared by Ring Opening Metathesis Polymerization 9
Applications
- The Reactive Capsule Formation 10
References 11
The Capsule Formation
The polymeric capsules and hollow particles can be prepared from either monomeric starting materials or from oligomers and preformed polymers. Mostly, the process involves a disperse oil phase in an aqueous continuous phase. The precipitation of polymeric materials at the oil-water interface causes each oil droplet to be enclosed within polymer shell. The interfacial polycondensation is used to prepare poly(urea), poly(amide), or poly(ester) capsules by reaction between an oil-soluble monomer and water soluble monomers. Vinyl polymers such as polystyrene, acrylates and methacrylates have been used to prepare hollow or capsule polymer particles. The dispersed oil phase usually serves as the polymerization medium, which is as a good solvent for the monomers but acts as a non-solvent for produced polymers. Therefore during polymerization, the system is comprised of three mutually immiscible phases. The studies of Torza and Manson show that the low viscosity oils are able to diffuse rapidly and assume the lowest interfacial energy morphology within the time frame of the experiment. Sunberg et. al. proposed the theoretical model based on the Gibb’s free energy change of the process of morphology development. The Gibbs free energy change per unit area for the process leads to a core shell morphology with oil encapsulated within the polymer phase. Thus the core shell morphology with the core oil engulfed by polymer is the thermodynamically stable morphology. Berg et. al. studied by using the system of poly(methyl methacrylate), which prepared from free radical polymerization of dispersing n-decane or hexadecane, methyl methacrylate and an oil soluble initiator in water containing a surfactant or stabilizer. McDonald et. al. and Sundberg et. al. have encapsulated highly non polar core oil such as decane and octane, while Kasai et. al. and Okubo et. al. have studied on slightly more polar materials such as benzene, toluene, and xylene. All those groups synthesize hollow polymer particles, so the nature of techniques of encapsulation should allow both hydrophilic and hydrophobic core materials.
Core shell particles with polymer gulfing an oil core only form if the total tension of the oil-polymer and polymer-water interfacial tensions is less than the oil-water interfacial tension. Additionally, the polymerization conditions must permit the thermodynamic morphology by the interfacial conditions to form within time frame of the experiment. Consequently, the encapsulation of a more hydrophilic-material demands the ability to synthesize sufficiently amphiphilic polymers, which will satisfy the interfacial requirement as well as the time frame for thermodynamic morphology. For instance, the ATRP synthesis of cross linked amphiphilic terpolymers is reached to the requirements both of thermodynamics and kinetics.
Ring Opening Metathesis (ROM)
The term metathesis is referred to an interchange reaction of alkylidene group between alkenes. The total number of double bonds remains unchanged. For many years, the ring opening metathesis polymerization (ROMP) and simultaneously investigated Metathesis of cyclic olefin, which in the literature was originally considered olefin disproportionation were regarded as two distinct reactions. Calderon (1972) recognized that they were two sides of the same coin and then introduced the term olefin metathesis for this reaction type. Since the metathesis depends on the particular substrate and transformation, Metathesis is formally grouped into several sub-types such as ring closing metathesis (RCM), ring opening metathesis (ROM) and ROMP as shown in Figure 1 and 2. The first developments of ROMP are contributed to Ruthenium based catalysts. Also there are other catalysts such as acyclic diene metathesis (ADMET).
Mechanism of Ring Opening Metathesis
Calderon in 1968 identified double bonds as the reactive centers in the metathesis of acyclic olefins. The metathesis reaction of d8-2-butene with 2-butene yielded only d4-2-butene, so we will be able to exclude the cleavage of any single bond. In 1971, Dall Asta and Motroni could draw the same conclusion for ROMP by their experiment using isotope-labeled cycloolefins. Early mechanism proposed for olefin metathesis is a pairwise exchange of alkylidene groups with various quasi-cyclobutane, metal tetracarbene or metallacyclopentene.
Chauvin and He’risson in early 1970 used the experiments of cross-metathesis, a nonpairwise reaction via metal carbine intermediates so called metallacyclobutane mechanism. This mechanism is supported by the fact that ROMP yields high molecular weight polymers already at low yields. For a simple pairwise mechanism that is a step growth polymerization, in which high polymers should yield only at high conversion.
The metallacyclobutane mechanism is further supported by many other discoveries such as the characterization of intermediates including metallacyclobutane and olefin metalcarbene complex.
There are four basic steps proposed. The first steps, there is coordination of the olefin to the metal center of a carbine complex. Secondly, cycloaddition forms the metallacyclobutane intermediate. The third step, cycloreversion is occurred and finally there is decoordination of the olefin. All of these reactions are reversible and shown in Figure 3.
However, Rooney et al. recently reported the contrast details of mechanism, in which the persistent metal anion radicals presents in metathesis reactions by using Grubbs’ catalyst, (PCy3)2Cl2RuCHPh as Figure 4.
Ruthenium based metathesis catalysts
There are several varieties of initiators ranging from simple metal salts to highly sophisticated alkylidene complexes used for olefin metathesis. Early homogeneous metathesis catalysts are formed in situ from a transition metal halide and a man group metal alkyl co-catalyst. Generally, multi-component catalysts mostly consist of carbonyl, nitrosyl halide or oxyhalide complexes of molybdenum, tungsten, or rhenium n combination with lithium, aluminium, or tin organyl compounds. A typical catalyst is the highly active Calderon catalyst, WCl6 / EtAlCl2 / EtOH.
RuCl3 hydrate, known as a metathesis initiator, is remarkably tolerant to oxygen, water, and functional groups. With that tolerance enables polymerization taking place in alcohol or water. Additionally the solvents might act as a co-catalyst and also may shorten the induction period. However, the use of RuCl3 is limited only for polymerization of highly strained cycloolefins such as NBE or cyclobutene and their derivatives. RuCl3/HCl in butanol was used for the first metathesis polymerization performed on an industrial scale. In general, high trans-polymers are yielded with RuCl3, but the addition of small amounts of chelating di-olefins, such as NBD (norbornadiene) or endo-dicyclopentadiene, will lead to high cis-polymers. This is attributed to the higher sterical crowding at the Ru center in te presence of these chelated bi-dentate ligands and lower steric demand in cis-metallacyclobutane. The improved initiator is Ru(tos)2(H2O)6, which is higher activity and better initiation than RuCl3. Moreover there are developments of Ru for the metathesis polymerization with various mono-meric and di-meric ruthenium arene complexes, which are shown in Figure 5, 6, 7, 11 [Frenzel et. al., 2002].
Cross Metathesis
However, there are several ideas different from the ring opening metathesis (scheme I), which called Cross Metathesis. In early 1970, Chauvin and He’risson used the experiments of cross-metathesis, which is a nonpairwise reaction via metal carbine intermediates, so called metallacyclobutane mechanism. This mechanism is supported by the fact that ROMP yields high molecular weight polymers already at low yields. For a simple pairwise mechanism that is a step growth polymerization, in which high polymers should yield only at high conversion.
There are many reports shown the limitations of ruthenium (I) based metathesis as one by C. Pietraszuk et. al. Since ruthenium has limited to vinylsilanes containing three alkoxy or siloxy substituents at silicon [Pietraszuk, et. al., 2003]. However, cross metathesis (scheme II) of chloro-substituted metathesis products is very useful for transformation of chloride at silicon replaced by variety of groups. So the use of Calderon (I) catalyst can be overcome by Grubbs (II) catalyst of cross metathesis.
The cross metathesis is of di- and tri-chloro- substituted vinylsilanes with alkyl-, aryl-, and silyl-substituted olefins at room temperature or mild temperatures at slightly elevated temperatures.
The example for cross metathesis is the formation of the bis (silyl) ethene that forms a silylcarbene as an intermediate during catalytic cycle. However, decomposition of either silylcarbene intermediate or ruthenacyclobutane likely causes the failure of observation for cross metathesis.
The Amphiphilic Graft Copolymer Prepared by Ring Opening Metathesis Polymerization
The principle of self assembly is a spontaneous aggregation and organization of subunits into a stable well-defined structure via non-covalent interactions driven by the lowest energy of the final products.
In Breitenkamp and Emrick’s work, 2003, the use of graft copolymers in assembly and capsule formation of PEGylated polyolefins at the oil-water interface help understand the mechanism of capsule formation. The interfacial activity and backbone reactivity of copolymers help them enable to form capsule as Figure 1. The ring opening cross metathesis generates cross linking without disrupting the initial assembly.
The hollow capsule can be filled with reagent and polymers with the appropriate solubility. This property makes capsule formation helpful and applicable for many fields.
Mechanism
The example of the amphiphilic capsule formation is following figure, in which PEG substituted cyclooctene macromonomers were prepared in two steps from 5-hydroxycyclooctene (1) as shown in scheme 1a. Ring opening succinic anhydride by (1) in the presence of 4-dimethylamino pyridine, DMAPgave the corresponding carboxylic acid (2). Then the following esterification with PEG monomethly ethers under dicyclohexanylcarbodiimide (DCC) coupling condition will take place. Macromonomers (4) and (5) were prepared in this fashion with PEG. The anionic polymerization of ethylene oxide providing an ester linkage is occurred.
The Applications
There are several works shown the developments and applications of encapsulation such as drug release, drug delivery, tissue fixation, DNA fragmentation, cell distribution, and so on. For this paper the example of application will present as a reactive capsule formation.
The Reactive Capsule Formation
Rosengren et. al., 1999, studied the system of LDPE implanted in the abdominal wall of rats to understand the reactive capsule formation. The relative encapsulation forms at soft tissue and the formation is related to cell necosis. Since foreign body capsule with the same principal features re formed around any foreign bodies introduced into soft tissues, the reactive capsule formation at the soft surface appeared thicker than that of the coarse surface at three times. The implant materials make tissue augmented.
References
-ester-substituted
enol ethers: application to the shortest synthesis of KDO, Tetrahedron,
Volume 59, Issue 35, 25 August 2003, Pages 6751-6758
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