Macromonomers via 1,3,6-trioxocane polymerization

The ring-opening polymerization of 1,3,6-trioxocane (TO) in the presence of 2-hydroxyethyl methacrylate (HEMA) leading to macromonomers is described. According to the activated monomer mechanism the oligo(1,3,6-trioxocane)s are terminated with HEMA-groups. The obtained macromonomers are non-uniform in end functionality. With high-performance liquid chromatography (HPLC) at the critical point of adsorption and supercritical fluid chromatography (SFC) it is possible to characterize the molar mass and functionality distribution of the macromonomers. Using these chromatographic methods functionality uniform fractions are available.     

Polymerization of acrylates by atom transfer radical polymerization. Homopolymerization of glycidyl acrylate

Atom transfer radical polymerization (ATRP) was applied to the homopolymerization of glycidyl acrylate (GA). This functionalized monomer can be polymerized to high conversions and high molecular weights using halogenated initiators and CuBr/4,4′-bis(5-nonyl)-2,2′-bipyridine (dNbipy) as the catalyst. The polymerizations exhibit first order kinetics and molecular weights increase linearly with conversion. The M?_n of the products is controlled by the ratio Δ[M]₀/[I]₀. The polymers obtained showed narrow molecular weight distributions, M?_w/M?_n = 1.25.     

Bifunctional initiators for group transfer polymerization

Bifunctional initiators of group transfer polymerization (GTP) were prepared in which two silyl ketene acetal groups are linked by an oligomethylene bridge, either at their C?C double bond or at their alkoxy groups. Their bifunctionality was proven for polymers of methyl methacrylate (MMA) prepared with initiators of the latter type in tetrahydrofuran (THF) with tetrabutylammonium cyanide, (C₄H₉)₄NCN, as catalyst. Hydrolysis after complete conversion of monomer yielded chains exhibiting only half of the molecular weight of the original polymer. GTP of MMA with the new initiators in THF with (C₄H₉)₄NCN is “living”, as could be established by the increase of number-average molecular weights after repeated monomer addition, providing the opportunity of production of ABA block copolymers of MMA and acrylates.     

Studies of atom transfer radical polymerization (ATRP) of acrylates by MALDI TOF mass spectrometry

Atom transfer radical polymerization (ATRP) of methyl, butyl and tert‐butyl acrylates was studied at conditions when low molecular weight polymers (M_n ≅ 2·10³) are formed, i. e., at relatively high concentration of initiator (ethyl 2‐bromopropionate) and catalyst (CuBr/amine). MALDI TOF analysis of the polymer samples isolated at different stages of polymerization revealed that in the course of polymerization potentially active macromolecules terminated with bromine are gradually converted into inactive macromolecules devoid of terminal bromine. A possible transfer mechanism, involving amine is a component of the catalytic system, is proposed. It was shown that quantitative analysis of the MALDI TOF spectra allows one to estimate the ratio of apparent rate constants of propagation and degradative transfer, providing quantitative information to what extent the system conforms to the contolled polymerization scheme.     

Polychloroalkane initiators in copper‐catalyzed atom transfer radical polymerization of (meth)acrylates

A series of polychloroalkanes CCl_nR_4‐n (n = 2, 3, or 4) was tested as initiators for atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) and methyl acrylate (MA) using CuCl/2,2′‐bipyridine as the catalyst. 2,2‐Dichloropropane and 2,2‐dichloroethanol initiate the ATRP of MMA very slowly. 1,1,1‐Trichloroalkanes, RCCl₃, are good initiators. For all the R groups tested, the number‐average molecular weight M_n increases with conversion and polydispersities are low (1.1 < M_w/M_n < 1.3). The initiator efficiency factor increases with electrophilicity of the initiating radical (0.7 < f < 1). CCl₄ is a multifunctional initiator and the final M_n values are lower than targeted. This is explained by the generation of new polymer chains occurring once the third active site is created per chain. ATRP of MA initiated by CCl₃CH₂CF₂Cl or CCl₃C₈H₁₇ results in polymers with M_n values predetermined by the Δ [M]/[Initiator]₀ ratio (f close to 1) and narrow molecular weight distributions (M_w/M_n < 1.3 at high conversion). The polymerization is much slower than that of MMA, but can be considerably accelerated by use of Cu(0) metal while maintaining an excellent control over molecular weights and polydispersities.     

Kinetics of group transfer polymerization of tert-butyl methacrylate in tetrahydrofuran

The reaction kinetics for the group transfer polymerization (GTP) of tert-butyl methacrylate (TBMA) using a silyl ketene acetal initiator and a nucleophilic catalyst are investigated. The reaction is shown to be of first order in both monomer and catalyst concentrations. The “livingness” of this system appears to be influenced by the reaction temperature. At temperatures above ?20°C, deactivation is observed, with its severity increasing with increasing temperature. This deactivation is attributed to a depletion of catalyst by side reactions. It was demonstrated that reactivation is made possible by the addition of more catalyst. This result is in contrast to the anionic polymerization of TBMA, where no deactivation was observed even at ambient temperature. At temperatures below ?20°C no deactivation is observed; however, at these temperatures, the reactions manifest induction periods with lengths increasing with decreasing temperature. The rate constants are lower than those for the GTP of methyl methacrylate (MMA) by a factor of 1,5 to 2. The following Arrhenius parameters were obtained for the propagation rate constants: activation energy, E_a = (19,₁ ± 3) kJ/mol, preexponential factor, log₁₀ A = (7,0₅ ± 0,3). These values are comparable with those obtained for MMA. The molecular weight distributions are similar to those obtained in the GTP of MMA, i.e. the ratio of weight-to number-average molecular weights is rather high for low monomer conversions and narrows to M?_w/M?_n ≥ 1,3 for full conversion. This is attributed to the rates of the catalyst exchange equilibrium.     

New initiators for group transfer polymerization. Triphenylphosphonium-containing ketene silyl acetals

New initiators for group transfer polymerization (GTP), triphenylphosphonium-containing ketene silyl acetals, were synthesized by addition of triphenylphosphine and chlorotrimethylsilane to methyl acrylate or methyl methacrylate, respectively, giving 3-methyoxy-3-trimethylsiloxy-2-propenyltriphenylphosphonium chloride ( 4a ) and its 2-methyl substituted derivative ( 4b ), together with their corresponding oligomers ( 8 ). These phosphonium salts initiate GTP of ethyl acrylate using zinc halides as catalysts. The resulting polymers, containing a terminal triphenylphosphonium group, show a small polydispersity. In the presence of zinc halides, triphenylphosphine and chlorotrimethylsilane can be used directly to initiate GTP of acrylates and methacrylates. In this case triphenylphosphonium terminated polymers with a larger polydispersity are obtained.     

Cross-linking and ring opening during polymerization of heterocyclic methacrylates and acrylates

Heterocyclic methacrylates have been shown to have low polymerization shrinkage. In this paper, the extent of ring opening has been assessed by the degree of consequent cross-linking. In polymers of tetrahydrofurfuryl methacrylate and acrylate, tetrahydropyranyl and tetrahydropyran-2-ylmethyl methacrylate, fewer than 1% monomer units were involved in cross-linking and hence ring-opening reactions. 2-epoxypropyl methacrylate, when polymerized, was very extensively cross-linked indicating prolific ring opening. However, the polymerization shrinkage of this material was the highest of the heterocyclic methacrylates studied and was that predicted from the known molar volume change of methacrylate esters. Ring opening does not, therefore, appear to be a significant factor in the polymerization shrinkage of heterocyclic methacrylates.     

Free-radical polymerization of acrylates and vinyl acetates initiated by transition metal/hydrosilane two-component initiation systems

The polymerization of vinyl acetate (VA), methyl methacrylate (MMA), and methyl acrylate (MA) in ethyl acetate and tetrahydrofuran solution was initiated by the two-component initiating systems Fe(III) 2-ethylhexanoate or Co(II) acetylacetonate and pentamethyldisiloxane as co-initiator. Whereas Fe(III) 2-ethylhexanoate initiated the polymerization of all three monomers also in the absence of the siloxane, with an activity increasing from VA to MA, Co(II) acetylacetonate was completely inactive under these conditions. Addition of the siloxane co-initiator raised the monomer conversions with both transition metal initiators, particularly pronounced for MMA and MA. Number-average degrees of polymerization for poly(vinyl acetate) and poly(methyl acrylate) amounted to some hundred or less, but increased to several thousand for poly(methyl methacrylate). Electron spin resonance (ESR) spin trap experiments with N-tert-butyl-α-phenylnitrone (PBN) revealed that the transition metal compounds and the siloxane undergo a reaction in which free radicals are produced by oxidative cleavage of the co-initiator to generate hydrogen atoms which initiate the polymerization. With Fe(III) 2-ethylhexanoate PBN adducts of carbon-centered radicals are observed in the absence and in the presence of the monomers. By way of contrast, with Co(II) acetylacetonate only the hydrogen atom adduct to PBN is observed in the absence of the monomers. Co(acac)₂ is likely to form a new complex with the co-initiator and the spin-trap molecules. The co-ordinated PBN competes with the two parent ligands of the complex for the liberated hydrogen atoms, however, due to favorable reaction rate the PBN-H adduct is formed preferably. The PBN adducts of carbon-centered radicals replace the PBN-H adduct when the monomers are added to the initiator solution, i.e. the H atoms are now preferably trapped by the monomer molecules.     

Acrylate‐Terminated Macromonomers by Michael Addition

A series of diacrylate macromonomers bearing alkoxysilyl units was prepared by convenient Michael addition of aminopropyl methyl diethoxysilane to 1,2‐ethylene glycol diacrylate (EGDA), p‐phenylene diacrylate (PDA) and 1,4‐cyclohexanediol diacrylate (CHDA). The resulting macromonomers have been characterized in detail by NMR spectroscopy, vapor pressure osmometry (VPO) measurements and fast‐atom bombardment mass spectroscopy (FAB‐MS). Average molecular weights M̄_n ranged between 530 and 1 300 g·mol^–1 (VPO). FAB‐MS and size exclusion chromatography (SEC) showed the formation of a homologous macromonomer series. Viscosities of the liquid monomers are relatively low, ranging from 0.082 to 8.30 Pa·s. This renders these compounds interesting as reactive diluents in dental composite formulations. Upon polymerization of the macromonomers, low volumetric shrinkage occurred, which was in the range of ΔV = 2.4 and 3.9 vol.‐% at high conversion. Crosslinking was monitored by photo‐differential scanning calorimetry (photo‐DSC). Furthermore, composites were prepared by mixing 2,2‐bis‐[p‐(2‐hydroxy‐3‐methacryloxypropoxy)‐phenyl]propane (Bis‐GMA) with the new macromonomers, initiator and glass filler. The composites showed compressive strengths up to 244 MPa, flexural strengths from 22 to 42 MPa and Young's moduli between 870 and 3 070 MPa. The composite materials exhibited low volume shrinkage of about 2 vol.‐% in comparison to over 3 vol.‐% shrinkage of commercially available composites.     

Synthesis of functional polymers by atom transfer radical polymerization

The preparation of monochelic polystyrene by atom transfer radical polymerization (ATRP) was investigated. The polymer analogous pathway by substitution of the bromide by an alcoholate resulted in significant elimination of the bromide and the degree of functionalization was low. Better results were achieved by the use of functional initiators. Carboxylic acid- and anhydride-bearing initiators were prepared by bromination of the commercially available 4-ethylbenzoic acid and the 4-methylphthalic anhydride, respectively. With these two products monochelic polystyrenes were synthesized. Further modification of the 4-(1-bromoethyl)benzoic acid led to initiators with a hydroxy or an oxazoline moiety. Again, the respective functional polystyrenes were obtained.     

Poly(methyl methacrylate)-block-poly(ethyl acrylate) by group transfer polymerization: synthesis and structural characterization

Poly(methyl methacrylate)-block-poly(ethyl acrylate) (PMMA-block-PEA) was synthesized by sequential group transfer polymerization (GTP) in tetrahydrofuran at ?30°C using (1-methoxy-2-methyl-1-propenyloxy)trimethylsilane (MTS) as an initiator and tetrabutylammonium fluoride monohydrate (TBAF · H₂O) as a catalyst. First, the PMMA macroinitiator was prepared in situ in quantitative conversion and its lifetime was at least 15 min. In the second step, an equimolar amount of ethyl acrylate was polymerized with a conversion of 67–88% and PMMA-block-PEA with number-average molecular weight 5000 < M?_n < 11 500 and polydispersity 1,4 < M?_w/M?_n < 2,1 was obtained. The number of chains almost does not change during the polymerization and the initiating efficiency of MTS was in both steps ca. 0,65–0,80. The diblock structure of the copolymer was confirmed by the ¹³C NMR spectrometric direct proof of the bond-linking of both blocks and by comparing the DSC behaviour of the copolymer with that of the corresponding blend of the homopolymers. No contamination of the block copolymer with the homopolymers was detected by the analytical methods used for the structural characterizations.     

Monomolecular and bimolecular termination in the polymerization of di(meth)acrylates

The termination mechanism in the polymerization of a series of analogous di(meth)-acrylates differing only by the type of the heteroatom (S or O) in the ester group was investigated. The experimental method was based on differential scanning calorimetry measurements of photochemically initiated polymerization. The ratio of the bimolecular termination and propagation rate constants k/k_p and/or the monomolecular termination rate constant k were determined according to three termination models (monomolecular, bimolecular, and mixed). Statistical techniques were used for choosing between the models to find out by which mechanism (monomolecular, bimolecular or mixed) the termination occurs. The results obtained prove than monomolecular reaction is an important termination way in the polymerization of multifunctional monomers and must be taken into account when kinetic constants are determined, even at low degree of conversion. The sulfur-containing monomers are characterized by much lower k values than the analogous sulfur-free monomers.     

Group transfer polymerization of methacrylonitrile

The group transfer polymerization of methacrylonitrile in acetonitrile, initiated by 2-(trimethylsilyl)isobutyronitrile in presence of tetraethylammonium cyanide or tetrabutylammonium fluoride as coinitiator, is accompanied by an anionic polymerization initiated by the coinitiator alone so that polymers with bimodal molecular weight distributions are formed. In contrast, with tris(dimethylamino)sulfonium difluorotrimethylsilicate as coinitiator a pure GTP is observed leading to polymers with unimodal molecular weight distribution. In all systems, severe side reactions reduce conversions and molecular weights.     

Chain transfer polymerization of methyl methacrylate initiated by organolanthanide complexes

A novel chain transfer polymerization mediated by Cp*₂ Sm(III) species and organic acids is described. The chain transfer polymerization involves the reaction of organic acids such as thiols or ketones with an active bond between samarium(III) and the enolate at a living chain end of poly(methyl methacrylate) (PMMA). This chain transfer reaction resulted in termination of the living chain end and the regeneration of the active initiator which would consist of (C₅Me₅)₂Sm(III) and deprotonated organic acids. The chain transfer polymerization were confirmed by turnover numbers (TON). tert‐Butyl thiol exhibited the chain transfer reactivity effectively to control the molecular weight of PMMA without decreasing of the polymer yield and the stereoregularity. As a result of this chain transfer polymerization, thermal and optical properties of the PMMA obtained were improved by the control of chain end groups or by reducing a large amount of the samarium initiator.     

Anionic polymerization of fluorine-containing vinyl monomers, 6. Polymerizations of fluoroalkyl acrylates and methacrylates initiated by organoaluminium compounds

The anionic polymerizations of 2,2,2-trifluoroethyl acrylate ( 1a ), 2,2,2-trifluoroethyl methacrylate ( 1b ), 2,2,2-trifluoro-1-trifluoromethylethyl acrylate ( 2a ), and 2,2,2-trifluoro-1-trifluoromethylethyl methacrylate ( 2b ) were examined with triethylaluminium and diethylaluminium active methylene chelate compounds as initiators. 2b was polymerized with triethylaluminium. Diethyl(ethyl cyanoacetato)aluminium was found to produce polymers of 1a and 1b . Diethyl(acetylacetonato)aluminium and diethyl (dimethyl malonato)aluminium, however, show low reactivity towards these four monomers. The resulting polymers show unimodal molecular weight distribution with number-average molecular weights of 10⁴?10⁵, measured by GPC. The initiation reaction and the copolymerization reactivity with styrene support the anionic polymerization mechanism.     

Synthesis of poly(methyl methacrylate) macromonomers with phenolic functionality by group transfer polymerization and their use in polycondensation reactions

Methyl 1-trimethylsiloxy-2-[2,5-bis(trimethylsiloxy)phenyl]vinyl ether (6) was used as the initiator for the group transfer polymerization (GTP) of methyl methacrylate (MMA) to give macromonomers with aromatic hydroxyl functions. Quantitative removal of the silyl-protective groups was achieved by acidic or fluoride-catalyzed cleavage to give the deprotected macromonomer 9 . The reactivity of this macromonomer in polycondensation reactions was shown by the synthesis of a liquid-crystalline main-chain polyester with poly(methyl methacrylate) (PMMA) side chains (PES-graft-PMMA) (15) and a 4,4′-isopropylidenediphenol-based polycarbonate (PC-graft-PMMA) ( 19 ).     

Nanostructured hybrid hydrogels prepared by a combination of atom transfer radical polymerization and free radical polymerization

A new method to prepare nanostructured hybrid hydrogels by incorporating well-defined poly(oligo (ethylene oxide) monomethyl ether methacrylate) (POEO₃₀₀MA) nanogels of sizes 110–120 nm into a larger three-dimensional (3D) matrix was developed for drug delivery scaffolds for tissue engineering applications. Rhodamine B isothiocyanate-labeled dextran (RITC-Dx) or fluorescein isothiocyanate-labeled dextran (FITC-Dx)-loaded POEO₃₀₀MA nanogels with pendant hydroxyl groups were prepared by activators generated electron transfer atom transfer radical polymerization (AGET ATRP) in cyclohexane inverse miniemulsion. Hydroxyl-containing nanogels were functionalized with methacrylated groups to generate photoreactive nanospheres.¹H NMR spectroscopy confirmed that polymerizable nanogels were successfully incorporated covalently into 3D hyaluronic acid-glycidyl methacrylate (HAGM) hydrogels after free radical photopolymerization (FRP). The introduction of disulfide moieties into the polymerizable groups resulted in a controlled release of nanogels from cross-linked HAGM hydrogels under a reducing environment. The effect of gel hybridization on the macroscopic properties (swelling and mechanics) was studied. It is shown that swelling and nanogel content are independent of scaffold mechanics. In-vitro assays showed the nanostructured hybrid hydrogels were cytocompatible and the GRGDS (Gly–Arg–Gly–Asp–Ser) contained in the nanogel structure promoted cell–substrate interactions within 4 days of incubation. These nanostructured hydrogels have potential as an artificial extracellular matrix (ECM) impermeable to low molecular weight biomolecules and with controlled pharmaceutical release capability. Moreover, the nanogels can control drug or biomolecule delivery, while hyaluronic acid based-hydrogels can act as a macroscopic scaffold for tissue regeneration and regulator for nanogel release.     

NMR evidence of the block structure of poly(methyl methacrylate)-block-poly(ethyl acrylate) prepared by group transfer polymerization

The structure of poly(methyl methacrylate)-block-poly(ethyl acrylate) prepared by group transfer polymerization was studied by ¹H and ¹³C 1 D and 2D NMR methods including SINEPT, COSY, LR H-H-C RELAY and COLOC using model homopolymers of methyl methacrylate (MMA) and ethyl acrylate (EA) of a length equal to that of the blocks and prepared under the same conditions. The ¹H and ¹³C spectra of the copolymer are shown to be a superposition of the respective spectra of the homopolymers, with the exception that the copolymer lacks the terminal group present in the MMA homopolymer and the initiating group of the EA polymer. Moreover, a new minor signal is found in the CH₂ region of the copolymer which is shown to belong to the link of the blocks. The existence of a direct link between the blocks is further supported by the results of 1D and 2D coherence transfer methods, especially, those using the newly modified DS INEPT and H-C-C RELAY pulse sequences.     

Atom transfer radical polymerization of styrene catalyzed by copper carboxylate complexes

The atom transfer radical polymerization (ATRP) of styrene has been studied using complexes of copper carboxylates. Compared with systems employing copper halides as the catalyst, the use of the copper carboxylates resulted in faster polymerization rates and higher polydispersities. A ligand (dNbpy) to Cu(I) carboxylate ratio of 1 was sufficient to achieve the maximum rate of polymerization. The addition of a small amount of either Cu(II) or Cu(I) halide to the copper carboxylate system yielded polymers with better controlled molecular weights and lower polydispersities yet the polymerization still remained relatively fast.