Undesirable Crosslinking Research Articles

Enhancing the Kinetics of Vapor-based Polymerization by Pulsed Filament Approach.

Initiated chemical vapor deposition is a versatile technique for synthesizing conformal polymer films on both planar and porous surfaces. It can retain functional groups and avoid undesired cross-linking. However, there is still room for enhancing its performance without altering the feed parameters. Here, we investigate a pulsed iCVD approach to improve the deposition process, achieved by switching on and off the resistively heated filament periodically. By strategically switching off the filament, a shortage of thermally activated primary radicals was created, which allowed uninterrupted chain propagation with fewer termination reactions and potentially increased monomer conversion rates. This has caused significantly faster deposition kinetics with a higher molecular weight and longer chain length for poly(glycidyl methacrylate) compared to continuous deposition. Spectra analyses confirmed that the functionality and stoichiometry ratios remained intact throughout the pulsed deposition process. The pulsed iCVD method is therefore a competitive and sustainable tool, demonstrating fast deposition kinetics and a well-preserved functionality.

Sterically Selective [3 + 3] Cycloaromatization in the On-Surface Synthesis of Nanographenes.

Surface-catalyzed reactions have been used to synthesize carbon nanomaterials with atomically predefined structures. The recent discovery of a gold surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituted arenes has enabled the on-surface synthesis of arylene-phenylene copolymers, where the surface activates the isopropyl substituents to form phenylene rings by intermolecular coupling. However, the resulting polymers suffered from undesired cross-linking when more than two molecules reacted at a single site. Here we show that such cross-links can be prevented through steric protection by attaching the isopropyl groups to larger arene cores. Upon thermal activation of isopropyl-substituted 8,9-dioxa-8a-borabenzo[fg]tetracene on Au(111), cycloaromatization is observed to occur exclusively between the two molecules. The cycloaromatization intermediate formed by the covalent linking of two molecules is prevented from reacting with further molecules by the wide benzotetracene core, resulting in highly selective one-to-one coupling. Our findings extend the versatility of the [3 + 3] cycloaromatization of isopropyl substituents and point toward steric protection as a powerful concept for suppressing competing reaction pathways in on-surface synthesis.

Overcoming thermal instability of polymeric Core-Shell-Particles: Advanced processing options for the preparation of Stimuli-Responsive structural color materials

Core-shell particles (CSP) represent one of the most promising building blocks for the artificial production of stimuli-responsive materials with iridescent structural colors. Despite tremendous efforts in the past two decades, scalable processing options for CSP are overall rare and mostly limited to 2D structures in the form of films and foils. This work investigates why common large-scale polymer-processing techniques, such as injection molding or fused filament fabrication, are hardly applicable to state-of-the-art CSP: While rheological prerequisites are fulfilled, thermal stability is proven to be surprisingly poor. Temperature instability during processing is caused by a thermally induced cross-linking reaction of residual reactive moieties. This undesired cross-linking reaction can be efficiently suppressed via optimizations of the particle architecture, in terms of adjusting the core-to-shell ratio and number of grafting anchors. Thermal stability can be further increased upon incorporation of primary antioxidants. The theoretical framework and the feasibility of the developed solution strategies are verified by a variety of thermoanalytical methods, including differential scanning calorimetry, thermogravimetric analysis, as well as rheological and mechanical measurements. Finally, advanced CSP are developed and demonstrated to be conveniently processable at elevated temperatures of up to 250 °C. These next-generation CSP formulations are advantageous for established processing techniques and may further pave the way for the design of new large-scale methods suitable for industrial use. Potential applications are smart sensors, advanced display technologies, or anti-counterfeiting materials.

Aziridine-derived poly(sulfonamide thioether)s: Synthesis and the self-healing elastomeric properties

We introduce a straightforward polymerization methodology for synthesizing poly(sulfonamide thioether)s by the step-growth polyaddition of dithiols to chiral bis(2-monosubstituted N-sulfonyl aziridine)s. Undesired cross-linking reactions do not occur when employing the nucleophilic catalyst tributylphosphine. The resulting polymers exhibit both optical activity and impressive thermal stability, with a decomposition temperature (Td,5%) up to 328.1 °C. Moreover, one of the synthesized polymers, incorporating the diethylene glycol unit, demonstrates remarkable mechanical properties, presenting a tensile strength exceeding 2.0 MPa and an extraordinary strain tensile rate exceeding 2,000%. Most notably, this elastomer exhibits rapid self-healing capabilities at room temperature without external stimuli. These findings not only contribute to the synthetic methods for aziridine-derived polymeric materials but also offer promising avenues for developing advanced elastomers with exceptional mechanical and self-healing attributes.

Customizing ultraviolet curable polymer‐ink properties via reactive diluent modification and viscosity control

AbstractIn inkjet 3D printing, material selection plays a crucial role in shaping both printing performance and material characteristics. Achieving the desired properties relies on precisely formulated compositions. In this context, ultraviolet (UV) curable polymers require specific viscosities, typically ranging from 3700 to 5700 mPa·s, to function effectively in 3D printing's material jetting technique, preventing complications during material dispensing. This study formulates UV‐curable polymers by blending four distinct monomers: Genomer 4247 Aliphatic Urethane Dimethacrylate, 1,10‐Decanediol Dimethacrylate, Isobornyl Acrylate, and Methyl Acrylate, within a controlled dark room environment at room temperature to prevent undesired crosslinking reactions during mixing. The selection of these monomers and their compositions is meticulous, considering their capacity to achieve specified viscosities and enhance overall material performance. This formulation process yields a wide range of UV‐curable polymer viscosities. The study employs comprehensive methodologies, including rheometry, Fourier transform infrared analysis, Soxhlet extraction, thermal analysis, and tensile testing, for rigorous evaluation of viscosity, curing efficiency, thermal characteristics, and mechanical performance. Notably, mechanical and chemical performance exhibits marginal differences within the viscosity range, attributed to UV‐curable polymer crosslinking, consistently exceeding 99% for all samples. However, the polymer composed of 98% v/v oligomer and methyl acrylate (MA) demonstrates notably better thermal and mechanical properties due to its 99.91% gel content crosslinking. Remarkable polymer fabrication thus occurs within the desired viscosity range.

Enhanced oil recovery in fractured low-permeability reservoirs by a novel gel system prepared by sustained-release crosslinker and water-soluble thixotropic polymer

Polymer gels have been widely used in improving oil recovery and decreasing excessive water production in heterogeneous reservoirs after the long-term water flooding process. However, the high initial viscosity of polymer and the short gelation time still restricts the performance of polymer-based in situ cross-linked gels for in-depth conformance control. Herein, a novel gel system formed by in-situ crosslinking of water-soluble thixotropic polymer (WTP) and sustained-release crosslinker (SRC) was proposed. The WTP was synthesized by acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, and N-[3-(dimethylamino)propyl] methacrylamide. It exhibits low viscosity under high shear rates and becomes thick under low shear rates, which ensures good injectability during the injection process and high gelation strength after cross-linking. The SRC was prepared by coating poly(ethyleneimine) (PEI) into the W/O/W multiple emulsions. The sustained-release mechanisms were well elaborated by confocal laser scanning microscopy (CLSM) observations. Only after the demulsification of multiple emulsions, the PEI would be released from the internal aqueous phase to the external aqueous phase. Therefore, these emulsions can be employed to deliver crosslinkers to specified targets in deep regions and prolong the release of crosslinkers to prevent undesirable crosslinking reactions near the injecting wells. According to the environmental scanning electron microscope (ESEM) observations and viscosity measurements, the low viscosity and an incompact spatial network could be observed in SRC/WTP gel system in the first 17 d, after which the PEI molecules were released to the external aqueous phase and cross-linked with WTP, forming the solid gel with small pores and yielding a high viscosity of 8793 mPa⋅s. Rheology tests have demonstrated that the elastic modules and the liner viscoelastic region of the novel gel system was nearly two times and twenty times than the traditional polymer gel, indicating higher gelation strength and better shear resistance. The core flooding tests showed that the SRC/WTP gel system had good injectability and could effectively plug the millimeter and submillimeter-sized cracks in low-permeability cores. 0.6 pore volume (PV) of SRC/WTP gel system with 3 gel slugs and a low injection rate can yield the best plugging performance in fractured core samples. And the gelation time shortens with the decrease of fracture apertures. Most importantly, the field tests in Ordos Basin verified the desirable EOR performance of the SRC/WTP gel system in fractured low-permeability reservoirs.

A host-vector toolbox for improved secretory protein overproduction in Bacillus subtilis

Target proteins in biotechnological applications are highly diverse. Therefore, versatile flexible expression systems for their functional overproduction are required. In order to find the right heterologous gene expression strategy, suitable host-vector systems, which combine different genetic circuits, are useful. In this study, we designed a novel Bacillus subtilis expression toolbox, which allows the overproduction and secretion of potentially toxic enzymes. This toolbox comprises a set of 60 expression vectors, which combine two promoter variants, four strong secretion signals, a translation-enhancing downstream box, and three plasmid backbones. This B. subtilis toolbox is based on a tailor-made, clean deletion mutant strain, which is protease and sporulation deficient and exhibits reduced autolysis and secondary metabolism. The appropriateness of this alternative expression platform was tested for the overproduction of two difficult-to-produce eukaryotic model proteins. These included the sulfhydryl oxidase Sox from Saccharomyces cerevisiae, which forms reactive hydrogen peroxide and undesired cross-linking of functional proteins, and the human interleukin-1β, a pro-inflammatory cytokine. For the best performing Sox and interleukin, overproducing and secreting variants of these new B. subtilis toolbox fermentation strategies were developed and tested. This study demonstrates the suitability of the prokaryotic B. subtilis host-vector system for the extracellular production of two eukaryotic proteins with biotechnological relevance.Key points• Construction of a versatile Bacillus subtilis gene expression toolbox.• Verification of the toolbox by the secretory overproduction of two difficult-to-express proteins.• Fermentation strategy for an acetoin-controlled overproduction of heterologous proteins.

Isolation of Low Dispersity Fractions of Acetone Organosolv Lignins to Understand their Reactivity: Towards Aromatic Building Blocks for Polymers Synthesis.

Two organosolv lignins extracted during pilot runs of the Fabiola process were analyzed, fractionated and chemically modified with ethylene carbonate (EC) to produce building blocks suitable for polymer synthesis. Isolation of low dispersity fractions relied on the partial solubility of the lignins in organic solvents. Lignins solubility was first evaluated and analyzed with Hansen and Kamlet‐Taft solubility parameters, showing a good correlation with the solvents dipolarity/polarizability parameter π*. The results were then used to select a sequence of solvents able to fractionate the lignins into low dispersity fractions of increasing molar masses, which were analyzed by 31P NMR, SEC and DSC. The lignins were then reacted with EC, to convert the phenolic OH groups into primary aliphatic OH groups. The reactivity of the organosolv lignins was high, and milder reaction conditions than previously reported were sufficient to fully convert the phenolic OH groups. A gradual reduction in reactivity with increasing molar mass was evidenced and attributed to reduced solubility of high molar mass fragments in EC. Undesirable crosslinking side reactions were evidenced by SEC, but were efficiently limited thanks to a fine control of the reaction conditions, helping to maximize the benefits of the developed lignin modification with EC.

Chloromethylation and Quaternization of Poly(aryl ether ketone sulfone)s with Clustered Electron-rich Phenyl Groups for Anion Exchange Membranes

Ion segregation is critically important for achieving high ion conductivity for anion exchange membranes (AEMs). Herein, a new bisphenol monomer bearing ten electron-rich phenyl groups was designed and polymerized with various amounts of electron-deficient 4,4′-dihydroxydiphenylsulfone and 4,4′-difluorobenzophenone to yield dense and selective reaction sites for chloromethylation and quaternization. As the most challenging step, chloromethylation was optimized by tuning the reaction temperature, reaction time, and reactant ratios. Ion exchange capacity, water uptake, anion conductivity, mechanical stability, and alkaline stability of the resulting AEMs were characterized in detail. It is found that chloromethylation reaction needed to be carried out at low equivalent of chloromethylation agents to avoid undesirable crosslinking. The QA-PAEKS-20 sample with an IEC of 1.19 mmol·g−1 exhibited a Cl– conductivity of 11.2 mS·cm−1 and a water uptake of 30.2% at 80 °C, which are promising for AEM applications.

Mechanisms and Characterization of the Pulsed Electron-Induced Grafting of Styrene onto Poly(tetrafluoroethylene-co-hexafluoropropylene) to Prepare a Polymer Electrolyte Membrane.

During the pulsed-electron beam direct grafting of neat styrene onto poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) substrate, the radiolytically-produced styryl and carbon-centered FEP radicals undergo various desired and undesired competing reactions. In this study, a high-dose rate is used to impede the undesired free radical homopolymerization of styrene and ensure uniform covalent grafting through 125-μm FEP films. This outweighs the enhancement of the undesired crosslinking reactions of carbon-centered FEP radicals and the dimerization of the styryl radicals. The degree of uniform grafting through 125-μm FEP films increases from ≈8%, immediately after pulsed electron irradiation to 33% with the subsequent thermal treatment exceeding the glass transition temperature of FEP of 39°C. On the contrary, steady-state radiolysis using 60Co gamma radiolysis, shows that the undesired homopolymerization of the styrene has become the predominant reaction with a negligible degree of grafting. Time-resolved fast kinetic measurements on pulsed neat styrene show that the styryl radicals undergo fast decays via propagation homopolymerization and termination reactions at an observed reaction rate constant of 5 × 108 l · mol-1 · s-1. The proton conductivity of 25-μm film at 80°C is 0.29 ± 0.01 s cm-1 and 0.007 s cm-1 at relative humidity of 92% and 28%, respectively. The aims of this work are: 1. electrolyte membranes are prepared via grafting initiated by a pulsed electron beam; 2. postirradiation heat-treated membranes are uniformly grafted, ideal for industry; 3. High dose rate is the primary parameter to promote the desired reactions; 4. measurement of kinetics of undesired radiation-induced styrene homopolymerization; and 5. The conductivity of prepared membranes is on par or higher than industry standards.

Screening Cysteine Mutants for Site‐Specific PEGylation of L‐asparaginase II

L‐Asparaginase II is a therapeutically important enzyme that has been used for treating certain types of cancer since the 1970s, i.e., acute lymphoblastic leukemia, acute myeloid leukemia, and non‐Hodgkin's lymphoma. During chemotherapy, asparaginase is injected into the bloodstream to deplete the pool of circulating asparagine, an essential amino acid for lymphoblastic cells, which consequently affects protein biosynthesis in the malignant cells causing them to undergo apoptosis without affecting normal cells.[1,2] Some drawbacks limiting the efficiency of asparaginase are the rapid serum clearance and immune inactivation due to anti‐asparaginase antibodies developed by the host. This has been addressed by decorating the enzyme with biocompatible polymers. Specifically, a commercial formulation of E. coli L‐asparaginase II randomly conjugated with mono‐methoxy polyethylene glycol, reduced the incidence of neutralizing antibodies and exhibited longer blood half‐life activity than the non‐conjugated enzyme. Unfortunately, random modification also impacts negatively the drug's pharmacodynamics leading to drastically reduced biological activity (85–0%) and substrate affinity (KM).[3–6]In our previous work, we site‐selectively modified asparaginase with bi‐maleimide‐PEGs by linking the polymers to cysteines which were introduced by site‐specific mutagenesis. In addition, in order to reduce the number of polymers around the enzyme, we successfully cross‐linked the asparaginase subunits to generate a tetrameric and enzymatically active 1kDa‐PEG‐conjugate (~140 kDa by SDS‐Page) using a 1000 Da PEG polymers, and a higher molecular weight 5kDa‐PEG‐conjugate (~400 kDa by size exclusion chromatography) using 5000 Da PEGs. The last one exhibited superior enzymatic activity over the non‐conjugated asparaginase.[7] This novel idea represents a solution to increase the hydrodynamic size of the drug thus reducing glomerular filtration and affording prolong blood half‐life, while at the same time retaining full biological activity. However, this approach has a bottleneck which is the recombinant expression of asparaginase‐cysteine mutants. We observed that our double‐mutation (A38C‐T263C) reduces the expression of this variant to about 33% of the native recombinant asparaginase, tested in E. coli BL21(DE3) cells. In addition, purification was also challenging, since only 70% purity (by SDS‐Page) was achieved for this double‐mutant, in contrast to >95% for the native recombinant asparaginase.[7]To overcome these limitations, we designed a new expression system in which a His‐tag is introduced, along with deletion of the two natural Cys amino acids present in asparaginase. The last modification seeks to reduce undesired crosslinking during the PEGylation reaction, while the first one should easy the purification. We evaluated this system in terms of extracellular expression and final purity (after affinity chromatography). Our results show that this mutant can be purified relatively easy with high purity (>95% by SDS‐Page), but expression was almost inexistent. This let us to believe that the natural cysteine(s) in E. coli L‐asparaginase II play a key role in the extracellular secretion mechanism.[8]Support or Funding Information Research facilities at the UPR ‐ Chemistry Department, Facundo Bueso Research facilities at the UPR‐Molecular Sciences Research Center RISE Program at the University of Puerto Rico ‐ Rio Piedras Campus This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Cure and scorch in the processing of ethylene‐propylene‐diene terpolymer (EPDM)

ABSTRACTThe objective of this work is to ascertain the characteristics of desirable (cure) and especially undesirable (scorch) crosslinking when carbon black filled ethylene propylene diene terpolymer (EPDM) is processed using different peroxide initiators. The mixing temperature and the nature of the peroxide initiator are crucial parameters affecting scorch (undesirably premature crosslinking) in this rubber. Processability and properties of EPDM prepared using various mixer set temperatures have been investigated. Dicumyl peroxide (Luperox DC), di(t‐butylperoxy) diisopropylbenzene (Luperox F), and 2,5‐dimethyl‐2,5‐di(t‐butylperoxy) hexane (Luperox 101) were used as crosslinking initiators. Higher mixing temperatures give shorter scorch times, greater scorch magnitudes, greater heterogeneities in crosslink spatial distribution and poorer tensile properties. However, extreme localization of the unwanted crosslinking at the rubber‐filler interface does have a beneficial effect. Luperox DC offers poorer processability and poorer resulting properties than do Luperox F and Luperox 101, due to its shorter half‐life and greater solubility in the rubber phase. This is the first time that the spatial heterogeneity of crosslinking and scorch has been related to the basic thermodynamics of 3‐component 2‐phase systems. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44523.

Controlled grafting of vinylic monomers on polyolefins: a robust mathematical modeling approach

Experimental and mathematical modeling analyses were used for controlling melt free-radical grafting of vinylic monomers on polyolefins and, thereby, reducing the disturbance of undesired cross-linking of polyolefins. Response surface, desirability function, and artificial intelligence methodologies were blended to modeling/optimization of grafting reaction in terms of vinylic monomer content, peroxide initiator concentration, and melt-processing time. An in-house code was developed based on artificial neural network that learns and mimics processing torque and grafting of glycidyl methacrylate (GMA) typical vinylic monomer on high-density polyethylene (HDPE). Application of response surface and desirability function enabled concurrent optimization of processing torque and GMA grafting on HDPE, through which we quantified for the first time competition between parallel reactions taking place during melt processing: (i) desirable grafting of GMA on HDPE; (ii) undesirable cross-linking of HDPE. The proposed robust mathematical modeling approach can precisely learn the behavior of grafting reaction of vinylic monomers on polyolefins and be placed into practice in finding exact operating condition needed for efficient grafting of reactive monomers on polyolefins.

Versatility of Microbial Transglutaminase

Although microbial transglutaminases (mTGs) were initially discovered to offset the cost of producing mammalian transglutaminases for food applications, they have quickly become important tools in research and biotechnology. Today, mTGs are utilized for a large number of applications to conjugate proteins and peptides to small molecules, polymers, surfaces, and DNA, as well as to other proteins. It is important to know how to maximize the advantages of the enzymatic approach and avoid undesired cross-linking. This review focuses on the versatility of transglutaminases in the field of bioconjugation and covers recent developments in utilizing mTG for generating antibody drug conjugates (ADCs) for therapeutic applications.

The structure of dual-variable-domain immunoglobulin molecules alone and bound to antigen

A dual-specific, tetravalent immunoglobulin G-like molecule, termed dual variable domain immunoglobulin (DVD-Ig™), is engineered to block two targets. Flexibility modulates Fc receptor and complement binding, but could result in undesirable cross-linking of surface antigens and downstream signaling. Understanding the flexibility of parental mAbs is important for designing and retaining functionality of DVD-Ig™ molecules. The architecture and dynamics of a DVD-Ig™ molecule and its parental mAbs was examined using single particle electron microscopy. Hinge angles measured for the DVD-Ig™ molecule were similar to the inner antigen parental mAb. The outer binding domain of the DVD-Ig™ molecule was highly mobile and three-dimensional (3D) analysis showed binding of inner antigen caused the outer domain to fold out of the plane with a major morphological change. Docking high-resolution X-ray structures into the 3D electron microscopy map supports the extraordinary domain flexibility observed in the DVD-Ig™ molecule allowing antigen binding with minimal steric hindrance.

N-Acetoxy-phthalimide (NAPI) as a new H-abstracting agent at high temperature: application to the melt functionalization of polyethylene

A new H-abstracting agent, N-acetoxy-phthalimide (NAPI), was designed that cleaves mainly homolytically into acyloxyl and aminyl radicals at 190 °C. Melt PE grafting experiments confirmed the H-abstraction efficiency of NAPI. It was also shown that the grafting of maleic anhydride increased and at the same time undesired crosslinking lowered compared to peroxides.

Studies on the Rheological Behavior of Polycarbosilane Part I: Effect of Time, Temperature and Atmosphere

Silicon carbide based fibers are generally prepared by the melt spinning of ceramic material precursors like polycarbosilane (PCS) and the hetero-metal substituted derivatives (M-PCS). Therefore, the rheological behaviour of PCS in the melt with respect to time, temperature, atmosphere, molecular weight and polydispersity becomes very important for successful processing and for preparing green fibres. In the present study PCSs having different softening points were synthesized via the thermal backbone rearrangement of polydimethylsilane (PDMS) and their melt viscosity variation and thermosetting behavior were studied. Various conditions like, atmosphere (inert & air), temperature, time and shear rate were chosen as variables. In an inert atmosphere, the viscosity of all the PCSs decreases with increasing temperature then under isothermal conditions it remains almost constant for the low softening point samples (high polydispersity) whereas it rises for the high softening point samples (low polydispersity). It has been observed that invariably all of the PCSs crosslinked rapidly in air as compared to in an inert atmosphere of argon. The effect of temperature was found to be prominent with the increase in SiH/SiCH3 ratio and the processing time reduces with an increase in processing temperature. The high molecular weight PCS thermosets easily due to enhanced crosslinking of the polymer as compared to low molecular weight PCS irrespective of the atmosphere. Thus, for processing of PCS it is essential to have a polydisperse sample and to maintain an inert atmosphere to prevent undesired crosslinking.

Therapeutic IgG4 antibodies engage in Fab-arm exchange with endogenous human IgG4 in vivo

Two humanized IgG4 antibodies, natalizumab and gemtuzumab, are approved for human use, and several others, like TGN1412, are or have been in clinical development. Although IgG4 antibodies can dynamically exchange half-molecules, Fab-arm exchange with therapeutic antibodies has not been demonstrated in humans. Here, we show that natalizumab exchanges Fab arms with endogenous human IgG4 in natalizumab-treated individuals. Gemtuzumab, in contrast, contains an IgG4 core-hinge mutation that blocks Fab-arm exchange to undetectable levels both in vitro and in a mouse model. The ability of IgG4 therapeutics to recombine with endogenous IgG4 may affect their pharmacokinetics and pharmacodynamics. Although pharmacokinetic modeling lessens concerns about undesired cross-linking under normal conditions, unpredictability remains and mutations that completely prevent Fab-arm exchange in vivo should be considered when designing therapeutic IgG4 antibodies.

Comparative analysis of radical‐mediated polyethylene modifications: Vinyltriethoxysilane versus mercaptopropyltriethoxysilane addition

AbstractThe efficiency and selectivity of two approaches for introducing alkoxysilane functionality to polyethylene (PE) are examined along with the moisture‐curing performance of the resulting products. Although the peroxide‐initiated grafting of vinyltriethoxysilane to PE is accompanied by undesirable crosslinking, comparable silane contents can be introduced without affecting the melt viscosity through the addition of mercaptopropyltriethoxysilane (MPTES) to the unsaturation within the polymer. Rapid hydrogen atom donation by thiols underlies this unique selectivity for grafting versus molecular weight alteration, and gives rise to a remarkable tolerance of MPTES additions to phenolic antioxidants. Direct comparisons of the moisture‐curing efficiencies provided by the two functionalization techniques reveal few significant differences in crosslink yields or composition distributions. POLYM. ENG. SCI., 46:480–485, 2006. © 2006 Society of Plastics Engineers.

Oxidation and epoxidation of poly(1,3‐cyclohexadiene)

AbstractPoly(1,3‐cyclohexadiene) (PCHD) derivatives were synthesized via facile chemical modification reactions of the residual double bond in the repeat unit. The oxidation and degradation of PCHD was investigated to enable subsequent controlled epoxidation reactions. PCHD exhibited a 15% weight loss at 110 °C in the presence of oxygen. The oxidative degradation, demonstrated by gel permeation chromatography (GPC) and 1H NMR spectroscopy, was attributed to main‐chain scission. Aldehyde and ether functional groups were introduced into the polymer during the oxidation process. PCHD was quantitatively epoxidized in the absence of deleterious oxidation with meta‐chloroperoxybenzoic acid. 1H and 13C NMR spectroscopy confirmed that polymers with controlled degrees of epoxidation were reproducibly obtained. Epoxidized PCHD exhibited a glass‐transition temperature at 154 °C, which was slightly higher than that of a PCHD precursor of a nearly equivalent molecular weight. Moreover, GPC indicated the absence of undesirable crosslinking or degradation, and the molecular weight distributions remained narrow. The thermooxidative stability of the fully epoxidized polymer was compared to that of the PCHD precursor, and the epoxidized PCHD exhibited an initial weight loss at 250 °C in oxygen, which was 140 °C higher than the temperature for PCHD. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 84–93, 2003