The present work reports on the application and the evaluation of a multitude of crosslinking approaches including high-energy irradiation, redox-initiating systems and conventional carbodiimide-coupling chemistry for frozen and/or freeze-dried porous gelatin scaffolds. The latter is particularly relevant for a plethora of biomedical applications such as tissue engineering supports, wound dressings, adhesive and absorbent pads for surgery, etc. Moreover, the results obtained for gelatin can be considered a proof-of-concept to be extrapolated to other polymer systems containing double bonds and/or amines and carboxylic acids to also realize scaffold crosslinking in dry or frozen state. The results showed that high-energy irradiation at −5 °C enabled sufficient segmental mobility to induce chemical crosslinking after performing a cryogenic treatment of methacrylamide-modified gelatin scaffolds. Alternatively, although several redox-initiating systems were unable to chemically crosslink functionalized gelatin, the combination of ammonium persulphate and TEMED resulted in the formation of scaffolds with a reasonable gel fraction. Interestingly, carbodiimide-coupling was found suitable to crosslink freeze-dried gelatin matrices.

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Crosslinking strategies for porous gelatin scaffolds

The term ‘Organometallic-Mediated Radical Polymerization (OMRP)’ encompasses all controlled radical polymerizations that make use of dormant species containing a direct bond between the metal atom and the growing radical chain. There are two versions of such techniques, one resting on the reversible activation of the dormant species by dissociation of the metal–carbon bond (OMRP-RT for ‘reversible termination’) and the other one based on the principle of degenerate transfer, with the growing polymer chain being released by an associative exchange with a second radical (OMRP-DT for ‘degenerative transfer’). The technique has allowed the control of a wide range of monomers from very reactive ones such as AN to vinyl acetate. Transition metals may be involved in a number of other one-electron processes and indeed all of them interplay with OMRP depending on conditions (type and amount of initiator, nature of metal and ligands, coordination number, type of monomer). A general analysis of this interplay is provided in this chapter. In addition to the interplay between dissociative and associative radical exchange processes in the OMRP dormant species, the chain-carrying radical may also interact with the OMRP trapping complex by H • transfer leading to catalytic chain transfer. Finally, transfer of atoms (particularly halogens) may positively or negatively interfere with OMRP. A strategy for the elimination, recovery, and recycling of the metal complex is also discussed.

Organometallic Mediated Radical Polymerization

The kinetics and mechanism of thermal polymerization of acrylonitrile initiated by Mn(III) pyrophosphate — poly(ethylene glycol) (PEG, molecular weight 6000) redox system in aqueous sulfuric acid medium was studied in the temperature range 30–60°C. The overall rates of polymerization and the disappearance of Mn3+ were determined. The polymerization was initiated by the organic free radical produced from the Mn3+-PEG reaction and the termination was by the metal ions. The rate of polymerization of acrylonitrile was found to be directly proportional to the square of the monomer concentration and first power of PEG concentration, and inversely proportional to the concentration of Mn3+. The rate of manganic ion disappearance was found to be directly proportional to manganic ion concentration and PEG concentration, and independent of the monomer concentration. Based on these observations, a plausible reaction scheme was suggested and suitable kinetic expressions were evaluated.

https://link.springer.com/content/pdf/10.1007/BF00652418.pdf

Block copolymerization initiated by Mn(III)-poly(ethylene glycol) redox system - general features and kinetics

The photodegradation of thin films of poly(para-tert-butylstyrene) with 265 nm radiation in the presence of oxygen, and as a function of irradiation time, has been studied using fluorescence, Fourier transform-infrared spectroscopy, and ultraviolet-visible spectroscopy. The influence of phthalate and terephthalate plasticizers on photo-oxidative degradation was also investigated. Blending with phthalate plasticizers was found to cause a higher efficiency of degradation than that obtained for doping with terephthalate plasticizers. The intensity of absorption was also found to increase with time of irradiation and in the intensity of a new absorption band at longer wavelength, thus indicating a possibility of photodegradation of polymer films. The analysis of the Fourier transform-infrared spectra of the irradiated and non-irradiated samples showed an increase in the intensities of the carbonyl and hydroxyl regions of the Fourier transform-infrared spectra providing evidence for the photodegradation as well as the photo-oxidation of polymeric chains. The increase in the analyzed ranges is attributed to the formation of alcohols, aliphatic ketones and to the increase in the number of polyene structures that resulted from hydrogen abstraction during photodegradation reactions.

Study of the influence of UV-irradiation on the photodegradation of plasticized poly(para-tert-butylstyrene) films

Kinetic study of aqueous polymerization of ethyl acrylate (EA) was carried out at 30 °C in dilute nitric acid medium by employing ammonium ceric nitrate (ACN)–methyl cellosolve (MC) and ACN–ethyl cellosolve (EC) as redox initiator systems. The ceric ion consumption was found to be first order with respect to ceric ion concentration with both initiator systems. The formation of complexes between Ce(IV) and reducing agents (RA) was observed. The order with respect to Ce(IV), reducing agents and monomer was evaluated for aqueous polymerization of EA by Ce(IV)–MC and Ce(IV)–EC redox initiator systems. The overall activation energy, Eoverall, for aqueous polymerization of EA was evaluated in the temperature region of 27–40 °C with both initiator systems. A kinetic mechanism for aqueous polymerization of EA initiated by redox initiator systems is presented.



© 2001 Society of Chemical Industry

Aqueous polymerization of ethyl acrylate initiated by ceric ion‐reducing agent systems in nitric acid medium

The polymerization of acrylonitrile initiated by the redox system 2,2′-thiodiethanol/Ce4+ in dilute sulfuric acid was investigated in the temperature range of 15–25°C. Oxidation of the substrate in the absence of the monomer has also been studied. The reaction involves the formation of an intermediate complex between the metal ion and the protonated species of the reductant, whose decomposition gives rise to the initiating free radicals. Multual interaction of the growing macroradicals accounts for the termination of polymerization. A suitable kinetic scheme has been proposed and rate and equilibrium constants evaluated.

Polymerization of acrylonitrile initiated by the redox system 2,2′‐thiodiethanol/Ce4+ in aqueous sulfuric acid

The kinetics of the polymerization of acrylonitrile, methyl methacrylate, and acrylamide initiated by the redox system cyanoacetic acid/manganese triacetate in dimethylformamide solution were investigated in the temperature range of 25 – 40°C. It was found that an initial complexation between the reactants followed by an electron transfer yields radicals. In contrast to acrylamide, acrylonitrile and methyl methacrylate effectively suppress further oxidation of the primary radicals. Exclusively the mutual termination accounts for the kinetics of these polymerizations. Data on the average degrees of the polymerizations support the suggested mechanisms. Rate and equilibrium constants were evaluated and their significance is discussed.

Redox polymerization, 4. Kinetics of the polymerization of acrylic acid derivatives initiated by the system cyanoacetic acid/Mn(OCOCH3)3 in dimethylformamide†

Polymerization of acrylonitrile and methyl methacrylate by the redox systems propanedinitrile-Mn(III) in aqueous sulfuric acid and propanedinitrile-Mn(OAc)3 in DMF and glacial acetic acid was investigated in the temperature range 20–40°C. The kinetics are consistent with the formation of an intermediate complex whose irreversible decomposition yields the initiating radical. With both monomers mutual termination predominates. The main difference between aqueous H2SO4 and acetic acid compared with DMF lies in the ease of oxidation of the primary radicals by the oxidant. Rate and equilibrium parameters were estimated and their significance discussed.

Redox polymerization‐kinetics of the reaction inititated by the system CH2(CH)2MN(III) in aqueous H2SO4 and the system CH2(CN)2MN(OAC)3 in DMF and acetic acid

Polymerisation of acrylonitrile and methyl methacrylate initiated by the system cyanoacetic acid-Mn(III)(acac)3 is investigated in DMF and in DMSO at 25–35°C. Kinetics of reduction of the complex by cyanoacetic acid shows the formation of a mixed ligand complex whose decomposition yields the primary radicals. Efficient initiation of polymerisation by these radicals followed by exclusive mutual termination accounts for the kinetics of polymerisation. Rate and equilibrium parameters are evaluated and their significance discussed.

Redox polymerisation, 5. Kinetics of the polymerisation of acrylonitrile and methyl methacrylate initiated by the system cyanoacetic acid‐Mn(III)(acac)3 in DMF and DMSO

The kinetics of polymerization of acrylamide, acrylonitrile and methyl methacrylate by the systems cyanoacetic acid/Mn(OCOCH 3 ) 3 as well as ethyl acetoacetate/Mn(OCOCH 3 ) 3 were investigated in glacial acetic acid medium in the temperature range of 25-45°C. Oxidation proceeds through intermediate complex formation between reactants. Competitive complexation of the substrates by Mn(II) leads to retadation of the rates by added manganous ions. Primary radicals from the substrate account for the initiation when acrylamide is used as the monomer, but with acrylonitrile and methyl methacrylate secondary radicals formed by the interaction of the 1:1 complex with monomers account for initiation. In presence of the monomers the retardation due to added manganous ions is absent. Termination in all cases is of the mutual type. Rate and equilibrium constants were evaluated.

Redox polymerization, 7. Kinetics of the polymerization of acrylic acid derivatives initiated by the systems cyanoacetic acid/Mn(OCOCH3)3 and ethyl acetoacetate/Mn(OCOCH3)3 in glacial acetic acid†

Mahadevan Haragopal

Papers

Polymerization of acrylonitrile initiated by the redox system 2,2′‐thiodiethanol/Ce4+ in aqueous sulfuric acid

Redox polymerization, 4. Kinetics of the polymerization of acrylic acid derivatives initiated by the system cyanoacetic acid/Mn(OCOCH3)3 in dimethylformamide†

Redox polymerization‐kinetics of the reaction inititated by the system CH2(CH)2MN(III) in aqueous H2SO4 and the system CH2(CN)2MN(OAC)3 in DMF and acetic acid

Redox polymerisation, 5. Kinetics of the polymerisation of acrylonitrile and methyl methacrylate initiated by the system cyanoacetic acid‐Mn(III)(acac)3 in DMF and DMSO

Redox polymerization, 7. Kinetics of the polymerization of acrylic acid derivatives initiated by the systems cyanoacetic acid/Mn(OCOCH3)3 and ethyl acetoacetate/Mn(OCOCH3)3 in glacial acetic acid†