Preparation of ultrahigh‐molecular‐weight syndiotactic poly(vinyl pivalate) monodisperse microspheres by low‐temperature suspension polymerization of vinyl pivalate

The particle size distributions of poly(vinyl pivalate) (PVPi) produced from low‐temperature suspension polymerization of vinyl pivalate (VPi) with 2,2′‐azobis(4‐methoxy‐2,4‐dimethylvaleronitrile) (AMDMVN) as an initiator have been studied. By controlling various synthesis parameters, near‐monodisperse PVPi microspheres from 100 to 400 μm were obtained that are expected to be precursors of near‐monodisperse syndiotactic poly(vinyl alcohol) (PVA) microspheres for biomedical embolic applications. The mean particle diameter follows the relationship: the volume average diameter, D_vad ∝ Y^0.26[VPi]^0.52[AMDMVN]^?0.25[PVA]^0.40T^?8.35Rpm^?0.67, where Y, [VPi], [AMDMVN], [PVA], T, and Rpm are the fractional conversion, concentrations of VPi, AMDMVN, and suspending agent, polymerization temperature, and agitation speed during the polymerization of VPi, respectively. The polydispersity of the particle size distribution of PVPi decreased with decreasing conversion, [AMDMVN], T, and Rpm and with increasing [VPi]. In the case of [PVA], optimization of the suspension stability led to a narrow particle size distribution. Ultrahigh‐molecular‐weights PVPi and PVA (number‐average degrees of polymerization of PVPi (25,000–32,000) and PVA (14,000–17,500), of high syndiotactic diad content (63%), and of high ultimate conversion of VPi into PVPi (85–95%) were obtained by suspension polymerization at 10 °C, followed by saponification. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 789–800, 2005     

Seeded polymerization of vinyl acetate using monodisperse poly(vinyl acetate) latex particles

The seeded polymerizations of vinyl acetate, using monodisperse poly(vinyl acetate) latex particles prepared in the absence of emulsifiers with potassium persulfate, have been investigated at 70°C with potassium persulfate as an initiator. New small particles were formed in the system containing a small amount of seed particles, but were not observed in the system containing a large amount of seed particles. The size of the secondary particles increased, and their number decreased, with an increase in the seed particle number. The minimum diameter of PVAc particles, which are stabilized by the sulfate ion groups bound at the end of polymer chains during polymerization, was determined to be 0.12 μm diameter from the limiting total surface area of seed particles which prevented further secondary nucleation. The minimum diameter of the particles increased as the speed of the stirrer increased. The new small particle number calculated using this value agreed well with that formed in the seeded polymerization.     

Synthesis of monodisperse poly(divinylbenzene) microspheres

Highly crosslinked monodisperse poly(divinylbenzene) microspheres were produced by precipitation polymerization with acetonitrile as solvent. The radical initiators AIBN, BPO, and ADVN were used. The process does not require stabilizers of any type, and produces monodisperse particles with diameters between 2 and 5 μm, depending on the conditions. These microspheres do not swell or dissolve in any common solvent, and have clean, stabilizer-free surfaces. The particle formation and growth mechanism is proposed to resemble that of dispersion polymerization, except that the particles are stabilized against coagulation by their rigid, crosslinked surfaces rather than by added stabilizers. Spherical particles were formed only at effective crosslinker/monomer or divinyl/monovinyl ratios larger than 1 : 2. © 1993 John Wiley & Sons, Inc.     

Preparation of monodisperse poly(St‐co‐PETEA) microspheres by precipitation polymerization using ethanol as solvent

Precipitation polymerization of styrene (St) was carried out with pentaerythritol tetraacrylate (PETEA) to produce monodisperse crosslinked microspheres. A much safer ethanol replaced acetonitrile as a solvent in precipitation polymerization of monodisperse crosslinked poly(St‐co‐PETEA) microspheres. Monodisperse crosslinked microspheres with high monomer conversion were achieved within 4 hr. Uniform and well‐separated monodisperse were obtained in ethanol when PETEA concentration varied from 30 vol% to 90 vol% and the particle diameter decreased from 0.75 to 0.58 µm. The particle yield increased from 36.51 to 64.38% by increase in the initiator loading from 1 to 8 wt%. No coagulum occurred between particles when the polymerization time varied from 2 to 10 hr. Copyright © 2010 John Wiley & Sons, Ltd.     

Characterization of poly(vinyl alcohol) during the emulsion polymerization of vinyl acetate using poly(vinyl alcohol) as emulsifier

During the emulsion polymerization of vinyl acetate (VAc) using poly(vinyl alcohol) (PVA) as stabilizer and potassium persulfate as initiator, the VAc reacts with PVA forming PVA-graft-PVAc. When the grafted polymer reaches a critical size it becomes water-insoluble and precipitates from the aqueous phase contributing to the formation of polymer particles. Since particle formation and therefore the properties of the final latex will depend on the degree of grafting, it is important to quantify and to characterize the grafted PVA. In this work, the quantitative separation and characterization of the grafted water-insoluble PVA was carried out by a two-step selective solubilization of the PVAc latex, first with acetonitrile to separate PVAc homopolymer, followed by water to separate the water-soluble PVA from the remaining acetonitrile-insoluble material. After the separation, the water-soluble and water-insoluble PVA were characterized by Fourier Transform Infrared (FTIR) spectroscopy and ¹H and ¹³C nuclear magnetic resonance (NMR) analyses, from which the details of the PVA-graft-PVAc structure were obtained. © 1996 John Wiley & Sons, Inc.     

Preparation of syndiotacticity-rich high molecular weight poly(vinyl alcohol) microfibrillar fiber by photoinitiated bulk polymerization and saponification

Vinyl pivalate (VPi) was polymerized in bulk by ultraviolet-ray initiation at low temperatures using 2,2′-azobis(2,4-dimethylvaleronitrile) (ADMVN) and 2,2'-azobis(isobutyronitrile) (AIBN) as photoinitiators. High molecular weight (HMW) poly(vinyl pivalate) (PVPi), having a number-average degree of polymerization (P_n) of 13,000–28,000, was obtained at conversions below 30% and converted by saponification to a syndiotacticity-rich HMW poly(vinyl alcohol) (PVA) microfibrillar fiber with P_n of 7300–18,300, syndiotactic diad (S-diad) and triad contents of ∼ 64% and ∼ 39%, respectively, and crystal melting temperature (T_m) of ∼ 249°C. ADMVN gave higher P_n than AIBN. On the other hand, conversion was smaller with the former than with the latter, and it was found that the initiation rate of ADMVN was lower than that of AIBN. P_n of PVA was constant while P_n of the precursor PVPi increased with increasing conversion. The syndiotacticity, T_m and thermal stability of PVA obtained from PVPi were much superior to those of PVA derived from poly(vinyl acetate) prepared under the same polymerization conditions. Polymerization of VPi at lower temperatures gave PVA with higher syndiotacticity. © 1997 John Wiley & Sons, Inc.     

Preparation and biodegradation of sugar-containing poly(vinyl acetate) emulsions

To accelerate the biodegradability of poly(vinyl acetate)-based emulsions, emulsion copolymerizations of vinyl sugars, including triacetylated N-acetyl-D-glucosamine (GlcNAc)-substituted 2-hydroxyethyl methacrylate (GlcNAc(Ac)3-substituted HEMA), glucose-substituted HEMA (GEMA) and 6-O-vinyladipoyl-D-glucose (6-O-VAG) with vinyl acetate (VAc), were carried out using poly(vinyl alcohol) as an emulsifying agent in the presence of poly[(butylene succinate)-co-(butylene adipate)] [poly(BS-co-BA)]. Copolymerization with GEMA produced a stable emulsion and that with 6-O-VAG also produced a homogeneous emulsion. Their biodegradation tests indicated that PVAc main chain scission was accelerated by copolymerization with vinyl sugars.     

Core‐shell microspheres with surface grafted poly(vinyl alcohol) as drug carriers for the treatment of hepatocellular carcinoma

Core(polyvinyl neodecanoate‐ethylene glycol dimethacrylate)‐shell(polyvinyl alcohol) (core (P(VND‐EGDMA))‐shell(PVA)) microspheres were developed by seeded polymerization with the use of conventional free radical and RAFT/MADIX mediated polymerization. Poly(vinyl pivalate) PVPi was grafted onto microspheres prepared via suspension polymerization of vinylneodecanoate and ethylene glycol dimethacrylate. The amount of grafted polymer was found to be independent from the technique used with conventional free radical polymerization and MADIX polymerization resulting into similar shell thicknesses. Both systems—grafting via free radical polymerization or the MADIX process—were found to follow slightly different kinetics. While the free radical polymerization resulted in a weight gain linear with the monomer consumption in solution the growth in the MADIX controlled system experienced a delay. The core‐shell microspheres were obtained by hydrolysis of the poly(vinyl pivalate) surface grafted brushes to form poly(vinyl alcohol). During hydrolysis the microspheres lost a significant amount of weight, consistent with the hydrolysis of 40–70% of all VPi units. Drug loading was found to be independent of the shell layer thickness, suggesting that the drug loading is governed by the amount of bulk material. The shell layer does not appear to represent an obstacle to the drug ingress. Cell testing using colorectal cancer cell lines HT 29 confirm the biocompatibility of the empty microspheres whereas the clofazimine loaded particles lead to 50% cell death, confirming the release of the drug. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3256–3272, 2007     

Thermal curing and water re‐exposure of silane–PVA/PVAc complex film on glass fiber surface

Because glass fiber reinforced composites are in industrial demand, chemistry and topography of the glass fiber sizing are of interest. Silane–PVA/PVAc (polyvinyl alcohol/polyvinyl acetate) complex film on the glass fiber surface is studied during thermal curing and water re‐exposure by using atomic force microscopy. The complex film consists of silane with the honeycomb structure film and PVA/PVAc with the hexagonal close pack structure of ellipsoidal shaped microspheres (270 × 620 nm). The thermal curing at 100 °C is leading to the evaporation of water contained in the microspheres. Because of water evaporation, the average roughness value of 1‐min thermal curing decreases from initial 7.3 to only 2.7 nm. Such a collapse of microsphere is followed by an intermixing between silane film and PVA/PVAc microspheres leading to a change of silane honeycomb structure along with silane tips. The average value of the silane honeycomb structure wall width decreases from 144 nm to 54 nm, for curing times of 15 and 30 min, respectively. A re‐exposure to an aqueous environment after 100 °C curing leads to almost completely restored microspheres regarding shape and size. The average complex film thickness increases from 180 nm for thermal curing for 30 min to 225 nm for water re‐exposed film. Interestingly, the pits in the microsphere structure are observed presumably because of the tips from intermixing. The thermal curing at 200 °C enhances the intermixing, and after 15 min, an intramixing is suggested to occur between PVAc core and PVA shell of the microsphere. The water re‐exposure after 15 min of 200 °C curing leads to a re‐containing of water but without restored microsphere structure; Because of the intramixing, leaving the silane–PVA/PVAc film is not complex anymore. Copyright © 2014 John Wiley & Sons, Ltd.     

Interaction of water in polymers: Poly(ethylene‐co‐vinyl acetate) and poly(vinyl acetate)

We studied the interaction of water in poly(ethylene‐co‐vinyl acetate) of various vinyl acetate compositions and poly(vinyl acetate), on the basis of the infrared spectrum of the water dissolved therein. The spectrum shows a very sharp and distinct band at about 3690 cm^?1 (named as A), and less‐sharp two bands around 3640 (B) and 3550 cm^?1 (C), the A band being outstanding especially at a low vinyl acetate composition. As the vinyl acetate composition increases, the A band decreases in intensity relative to the C band, whereas the B band increases contrarily. Analysis of the spectral change has elucidated that one‐bonded water (of which one OH is hydrogen‐bonded to the C?O of an ester group and the other OH is free) and two‐bonded water (each OH of which is hydrogen‐bonded to one C?O) coexist in the copolymer and that two‐bonded water increases in relative population with increasing vinyl acetate composition. Dissolved water is entirely two‐bonded in poly(vinyl acetate), in which C?O groups are densely distributed in the matrix. We proved that dissolved water in polymers is hydrogen‐bonded through one or two OH groups to the possessed functional groups but does not cluster. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 777–785, 2005     

Dynamic mechanical and dielectrical properties of poly(vinyl alcohol) and poly(vinyl alcohol)‐based nanocomposites

In this article we report on the investigation of the dynamics of poly(vinyl alcohol) (PVA) and PVA‐based composite films by means of dielectric spectroscopy and dynamic mechanical thermal analysis. Once the characterization of pure PVA was done, we studied the effect of a nanostructured magnetic filler (nanosized CoFe₂O₄ particles homogeneously dispersed within a sulfonated polystyrene matrix) on the dynamics of PVA. Our results suggest that the α‐relaxation process, corresponding to the glass transition of PVA, is affected by the filler. The glass‐transition temperature of PVA increases with filler content up to compositions of around 10 wt %, probably as a result of polymer–filler interactions that reduce the polymer chain mobility. For filler contents higher than 10 wt %, the glass‐transition temperature of PVA decreases as a result of the absorption of water that causes a plasticizing effect. The β‐ and γ‐relaxation processes of PVA are not affected by the filler as stated from both dynamic mechanical thermal analysis and dielectric spectroscopy. Nevertheless, both relaxation processes are greatly affected by the moisture content. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1968–1975, 2001     

Low temperature suspension polymerization of vinyl acetate using 2,2′-azobis(2,4-dimethylvaleronitrile) for the preparation of high molecular weight poly(vinyl alcohol) with high yield

 To obtain high molecular weight (HMW) poly(vinyl acetate) (PVAc) with high conversion and high linearity for a precursor of HMW poly(vinyl alcohol) (PVA), vinyl acetate (VAc) was suspension-poly-merized using a low-temperature initiator, 2,2′-azobis (2,4-dimethyl-valeronitrile) (ADMVN), and the effects of polymerization conditions on the polymerization behavior and molecular structures of PVAc and PVA prepared by saponifying PVAc were investigated. On the whole, the experimental results well corres-ponded to the theoretically predicted tendencies. Suspension polymerization was slightly inferior to bulk polymerization in increasing molecular weight of PVA. In contrast, the former was absolutely superior to the latter in increasing conversion of the polymer, which indicated that the suspension polymerization rate of VAc was faster than the bulk one. These effects could be explained by a kinetic order of ADMVN concentration calculated by initial-rate method and an activation energy difference of polymerization obtained from the Arrhenius plot. Suspension polymerization at 30 °C by adopting ADMVN proved to be successful in obtaining PVA of HMW (number-average degree of polymerization (P _n)): (4200–5800) and of high yield (ultimate conversion of VAc into PVAc: 85–95%) with diminishing heat generated during polymerization. In the case of bulk polymerization of VAc at the same conditions, maximum P _n and conversion of 5200–6200 and 20–30% was obtained, respectively. The P _n, lightness, and syndiotacticity were higher with PVA prepared from PVAc polymerized at lower temperatures. Received: 10 February 1998 Accepted: 15 April 1998     

A comparative study of UV aging characteristics of poly(ethylene‐co‐vinyl acetate) and poly(ethylene‐co‐vinyl acetate)/carbon black mixture

In this comparative study, the effect of carbon black (CB) on the UV aging characteristics of poly(ethylene‐co‐vinyl acetate) (EVA) was investigated. EVA, containing 13% vinyl acetate (VA), and poly(ethylene‐co‐vinyl acetate)/carbon black mixture (EVA/CB), containing 13% VA and 1% CB, were aged by means of UV light with a wavelength in the vicinity of 259 nm, in air, up to 400 hr. Sol‐gel analyses were made to determine the percentage gelation of both virgin and aged samples. FT‐IR measurements were performed to follow the chemical changes which took place in the samples during aging. Dynamic and isothermal thermogravimetry studies were performed for determination of the thermal stabilities of virgin and aged samples. Sol‐gel analysis results showed that EVA itself has a tendency to form a gel under UV irradiation. EVA/CB, however, becomes a gel to a smaller extent, comparatively, under the same conditions. As a result of FT‐IR measurements, some oxidation products such as ketone, lactone and vinyl species were observed through UV ageing of EVA and EVA/CB. Thermal analysis experiments exhibited that the thermal stabilities of EVA and EVA/CB decreased, to a similar extent through UV aging. Copyright © 2004 John Wiley & Sons, Ltd.     

A comparative study of thermal ageing characteristics of poly(ethylene‐co‐vinyl acetate) and poly(ethylene‐co‐vinyl acetate)/carbon black mixture

In this comparative study, the effect of carbon black (CB) on the thermal ageing characteristics of poly(ethylene‐co‐vinyl acetate) (EVA) was investigated. EVA, containing 13% vinyl acetate (VA), and poly(ethylene‐co‐vinyl acetate)/carbon black mixture (EVA/CB) containing 13% VA and 1% CB were aged at 85°C in air up to 30 weeks. Sol‐gel analysis experiments were made to determine the percentage gelation of both virgin and aged samples. FT‐IR measurements were performed to follow the chemical changes which took place in the samples during ageing. Dynamic and isothermal thermogravimetric studies were performed for determination of the thermal stabilities of virgin and aged samples. Sol‐gel analysis results showed that EVA itself has a tendency to form a gel under thermal treatment, whereas EVA/CB never becomes a gel when being thermally aged under the same conditions. As a result of FT‐IR measurements, some oxidation products such as ketone, lactone and vinyl species were observed through thermal ageing of EVA. It is also clear that these kind of oxidation products did not appear to a considerable extent in EVA/CB. Thermal analysis experiments exhibit that thermal stability of EVA decreased through thermal ageing; whereas that of EVA/CB remained almost unchanged. Copyright © 2004 John Wiley & Sons, Ltd.     

Photocrosslinked poly(ester‐anhydride) microspheres with macroporous structure

In this work, the new method of preparation biodegradable microspheres with macroporous structure is presented. Typical methods used for generation of porous structures in microspheres obtained from preformed polymers require the use of additional substances acting as porogens. In this study, the porosity was achieved as the effect of photocrosslinking, without porogens. Microspheres were prepared using emulsion solvent evaporation technique from functional poly(ester‐anhydride)s with different amount of allyl groups in the side chains. The crosslinking was carried out by UV irradiation during the solvent evaporation (photoinitiator was introduced to polymer solution). The size of microspheres obtained was in the range of 1.7 – 4 µm (small microspheres) or 31 – 50 µm (large ones) and depended on the conditions used in emulsion formulation process. Effectiveness of the crosslinking was characterized by the content of insoluble part of samples, and it was in the range of 42–89%. The content of insoluble part of sample of microspheres and their porosity were dependent on functionality of poly(ester‐anhydride)s, the amount of photoinitiator used, and also on size of microparticles. The small particles were always more crosslinked than the large ones, but the latter were more porous than the small ones. Crosslinked microparticles indicated higher loading efficiency of model compound and appeared to degrade faster than uncrosslinked ones, probably due to their high porosity. The high porosity of microspheres obtained would enable their eventual use in pulmonary drug delivery systems or in construction of porous scaffolds for tissue engineering. Copyright © 2013 John Wiley & Sons, Ltd.     

Phosphonate‐terminated poly(vinyl acetate) synthesized by RAFT/MADIX polymerization

Four different xanthates containing either phosphonate or bisphosphonate moieties were synthesized with high degree of purity. These xanthates were used as chain transfer agents (CTA) in the RAFT/MADIX polymerization of vinyl acetate (VAc) to prepare end‐capped poly(VAc). The rate of VAc polymerization in the presence of these new CTAs was shown to be similar to that obtained with conventional xanthate, that is, (methyl ethoxycarbonothioyl) sulfanyl acetate. Good control of VAc polymerization was also obtained since the molecular weight increased linearly with monomer conversion for each phosphonate‐containing xanthate. Low‐PDI values were obtained, ascribed to efficient exchange during RAFT/MADIX polymerization. C_ex value was therefore calculated to about 25, based on RAFT/MADIX of VAc in the presence of rhodixan A1/VAc adduct. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Characterization of polysuccinate and hydroxyapatite‐based nanocomposites containing poly(ester‐anhydride) microspheres

Repair and regeneration of bone defects with particular shape may be enhanced by in situ forming biomaterials which can be used in minimal invasive surgery. This study is aimed to prepare novel in situ forming biodegradable nanocomposites based on poly(3‐allyloxy‐1,2‐propylene) succinate (PSAGE) and nanosized hydroxyapatite (HA). These nanocomposite materials contain poly(ester‐anhydride) (PEA) microspheres embedded in a polyester matrix prepared by crosslinking PSAGE with oligo(1,2‐propylene maleate) and methacrylic monomers. Methyl methacrylate and one of hydrophilic oligo(ethylene glycol) methacrylates with different functionality and various length of oligooxyethylene chains were used as polymerizable diluents. Incorporation of microspheres which degrade faster than crosslinked polyester matrices enables formation of porous structure in situ. The obtained materials are liquid before curing and harden in several minutes with moderate exothermic effect. The effect of the composition of nanocomposite materials on selected properties, such as water sorption, mechanical strength, porosity and hydrolytic degradation process, was investigated. Rheological behavior and injectability of liquid formulations were studied. Analysis by energy dispersive spectroscopy confirmed the presence of characteristic features of HA in the nanocomposite materials. The morphology of the cured nanocomposites subjected to hydrolytic degradation was evaluated by scanning electron microscopy. The MTS cytotoxicity assay was carried out for extracts from crosslinked materials using hFOB1.19 cells. It was found that the extracts exhibit a dose‐dependent cytotoxic response. Copyright © 2014 John Wiley & Sons, Ltd.     

Use of specific polysaccharide‐immobilized monodisperse poly(glycidyl methacrylate) core–silica shell microspheres for affinity purification of lectins

Immobilization of polysaccharides (yeast mannan and gum arabic) on the macroporous poly(glycidyl methacrylate) monodisperse microspheres coated with silica (SiO₂)‐containing amino groups on the surface was used to prepare affinity sorbents for lectin purification. The efficiency of isolating mannose specific Pisum sativum lectin was demonstrated on sorbent with immobilized yeast mannan and that of galactose specific Glycine hispida lectin on sorbent with immobilized gum arabic. The microspheres with immobilized polysaccharides can be used for selecting an affinity sorbent for purification of other mannose‐ and galactose‐specific lectins. In contrast to yeast mannan, the gum arabic immobilized on the microspheres possesses much narrower specificity and is suitable for purification of only those galactose specific lectins which interact well with l ‐rhamnose or l ‐arabinose. The synthesized macroporous particles are capable of immobilizing 50 mg of polysaccharide per 1 g of the matrix, which is 10 times higher than the capacity of epoxy‐activated Sepharose 6B. That makes it possible to obtain the same lectin quantity using a column of 10 times smaller volume. Another advantage of novel affinity sorbents comparing corresponding Sepharose gels is the possibility of sorbent drying after use. Copyright © 2014 John Wiley & Sons, Ltd.