Tandem Mass Spectrometry Characteristics of Silver-Cationized Polystyrenes: Backbone Degradation via Free Radical Chemistry

The [M + Ag]+ ions of polystyrene (PS) oligomers are formed by matrix-assisted laser desorption/ionization, and their fragmentation characteristics are determined by tandem mass spectrometry experiments in a quadrupole/time-of-flight mass spectrometer. Collisionally activated dissociation (CAD) of [M + Ag]+ starts with random homolytic CC bond cleavages in the PS chain, which generate radical ions carrying either the initiating (an•, bn•) or the terminating (yn•, zn•) chain end and primary (an•, yn•) or benzylic (bn•, zn•) radical centers. The fragments ultimately observed arise by consecutive, radical-induced dissociations. The primary radical ions mainly decompose by monomer evaporation and, to a lesser extent, by β-H• loss. The benzylic radical ions primarily decompose by 1,5-H rearrangement (backbiting) followed by β C−C bond scissions; this pathway leads to either closed-shell fragments with CH2 end groups, internal fragments with 2−3 repeat units, or truncated benzylic bn•/zn• radical ions that can undergo anew backbiting. The same internal fragments are produced in all backbiting steps; hence, these fragments and small benzylic radical ions (which cannot undergo backbiting) dominate the low-mass region of the CAD spectra, while the less abundant closed-shell fragments with CH2 end groups (an/yn) dominate the medium- and high-mass regions. The latter fragments are suitable for determining the individual initiating and terminating end groups, whereas the internal ions could be valuable in sequence analyses of styrene copolymers

Tandem Mass Spectrometry Characteristics of Silver-Cationized Polystyrenes: Internal Energy, Size, and Chain End versus Backbone Substituent Effects

The Ag+ adducts of polystyrene (PS) oligomers with different sizes (6−19 repeat units) and initiating (α) or terminating (ω) end groups mainly decompose via free radical chemistry pathways upon collisionally activated dissociation. This reactivity is observed for ions formed by matrix-assisted laser desorption/ionization as well as electrospray ionization. With end groups lacking weak bonds (robust end groups), dissociation starts with random homolytic C−C bond cleavages along the PS chain, which lead to primary and benzylic radical ions containing either of the chain ends. The primary radical ions mainly depolymerize by successive β C−C bond scissions. For the benzylic radical ions, two major pathways are in competition, namely, depolymerization by successive β C−C bond scissions and backbiting via 1,5-H rearrangement followed by β C−C bond scissions. The extent of backbiting decreases with internal energy. With short PS chains, the primary radical ions also undergo backbiting involving 1,4- and 1,6-H rearrangements; however, this process becomes negligible with longer chains. If the polystyrene contains a labile substituent at a chain end, this substituent is eliminated easily and, thus, not contained in the majority of observed fragments. Changes in the PS backbone structure can have a dramatic effect on the resulting dissociation chemistry. This is demonstrated for poly(α-methylstyrene), in which backbiting is obstructed due to the lack of benzylic H atoms; instead, this backbone connectivity promotes 1,2-phenyl shifts in the primary radical ions formed after initial C−C bond homolyses as well as H atom transfers between the incipient primary and benzylic radicals emerging from these homolyses

Precision Synthesis of ω‑Branch, End-Functionalized Comb Polystyrenes Using Living Anionic Polymerization and Thiol–Ene “Click” Chemistry

A combination of living anionic polymerization and thiol–ene “click” chemistry provides an efficient and convenient method for synthesis of well-defined comb polystyrenes with precisely controlled architecture details and a wide selection of functionalities. ω-(p-Vinylbenzyl)­polystyrene macromonomer was synthesized by sec-butyllithium-initiated polymerization of styrene followed by termination with 4-vinylbenzyl chloride (VBC). For the synthesis of α-4-pentenyl-ω-(p-vinylbenzyl)­polystyrene macromonomer, an unsaturated initiator, 4-pentenyllithium, was used followed by termination with VBC. To ensure successful living anionic polymerization of macromonomers, impurities present in the macromonomers and glass reactors were readily removed by titration with excess sec-butyllithium initiator right before initiation, resulting in polymacromonomers with controlled Mn (74 000, 130 000 g/mol) and narrow Mw/Mn. Living anionic copolymerization of mixtures of both types of macromonomers yielded a well-defined comb-shape precursor with controlled fractions of ω-vinyl branch end groups, which were subsequently subjected to facile and efficient functionalization by photoinitiated thiol–ene “click” reactions with diverse functional groups (−OH, −CO2H, and −C8F17). Characterization by NMR, SEC, and MALDI-TOF mass spectrometry established their chemical structures and chain-end functionalities, which indicates precisely defined comb polystyrenes with controlled degrees of functionalization

Synthesis of Cyclic Polystyrenes Using Living Anionic Polymerization and Metathesis Ring-Closure

A combination of living anionic polymerization and metathesis ring-closure provides an efficient method for synthesis of well-defined, macrocyclic polymers over a broad molecular weight range. A series of well-defined, α,ω-divinylpolystyrene precursors (Mn = 2800, 8600, 17000, and 38000 g/mol) were synthesized by 4-pentenyllithium-initiated polymerization of styrene followed by termination with 4-chloromethylstyrene. Efficient cyclization of these α,ω-divinylpolystyrene precursors was effected in CH2Cl2 and CH2Cl2/cyclohexane mixtures using a Grubb’s catalyst, bis(tricyclohexylphosphine)benzylidine ruthenium(IV) chloride. As the precursor Mn increased, more cyclohexane was added and the concentration of the precursor was decreased from 1.41 × 10–4 to 2.15 × 10–6 M. The macrocyclic polymers were uniquely characterized by MALDI–TOF mass spectrometry in terms of peaks that appeared characteristically 28 m/z units lower than those of the corresponding open-chain precursor peaks, corresponding to the loss of an ethylene unit. Relative to linear analogues, the macrocycles exhibited longer SEC retention volumes, lower intrinsic viscosities, and higher Tgs at the lower Mn values

Temperature-Induced Reversible Morphological Changes of Polystyrene-<i>block</i>-Poly(ethylene Oxide) Micelles in Solution

Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles with degrees of polymerization of 962 for the PS and 227 for the PEO blocks (PS962-b-PEO227) in N,N-dimethylformamide (DMF)/water, in which water is a selective solvent for the PEO block, were observed. For a system with 0.2 wt % copolymer concentration and 4.5 wt % water concentration in DMF/water, the micelle morphology observed in transmission electron microscopy changed from vesicles at room temperature to worm-like cylinders and then to spheres with increasing temperature. Mixed morphologies were also formed in the intermediate temperature regions. Cooling the system back to room temperature regenerated the vesicle morphology, indicating that the morphological changes were reversible. No hysteresis was observed in the morphological changes during heating and cooling. Dynamic light scattering revealed that the hydrodynamic radius of the micelles decreased with increasing temperature. Combined static and dynamic light scattering results supported the change in morphology with temperature. The critical micellization temperatures and critical morphological transition temperatures were determined by turbidity measurements and were found to be dependent on the copolymer and water concentrations in the DMF/water system. The morphological changes were only possible if the water concentration in the DMF/water system was low, or else the mobility of the PS blocks would be severely restricted. The driving force for these morphological changes was understood to be mainly a reduction in the free energy of the corona and a minor reduction in the free energy of the interface. Morphological observations at different time periods of isothermal experiments indicated that in the pathway from one equilibrium morphology to another, large compound micelles formed as an intermediate or metastable stage

Polymer Dynamics of Well-Defined, Chain-End-Functionalized Polystyrenes by Dielectric Spectroscopy

A novel strategy is described to study polymer dynamics by using a combination of dielectric spectroscopy and functionalized polymers. The first results are presented using various well-defined, chain-end-functionalized polystyrenes (PS) synthesized using a combination of modern anionic polymerization techniques and hydrosilylation chemistry. The end-functionalized polystyrenes investigated contain the cyano (−CN), hydroxyl (−OH), acetyl (−OCOCH3, −Ac), or ethyl ether (−OCH2CH3, −OEt) groups. By applying broadband dielectric spectroscopy (BDS) over an extensive temperature range (approximately 50−413 K), it was possible to fully characterize the polymer dynamics associated with the segmental α-relaxation as well as the local secondary process related to the specific movement of the functional groups themselves. Combining these data with the results from differential scanning calorimetry (DSC), it is shown that for rather large functional groups the overall polymer matrix properties are altered, giving rise to a decrease in the glass transition temperature. The trend can be rationalized in terms of free volume effects caused by the bulky functional groups and points toward matrix plasticization effects. However, for cyano-functionalized PS the inclusion of this group does not significantly affect the matrix properties. By taking advantage of the strong dipole moment of the CN group, a clear dielectric signal can be obtained that can be used to selectively study the specific dynamics where the group is located. In other words, by appropriately attaching cyano groups at different parts of the chains, these can be exploited as in situ dielectric probes that allow determination of specific contributions to dynamical processes in polymers

Rapid and Efficient Anionic Synthesis of Well-Defined Eight-Arm Star Polymers Using OctavinylPOSS and Poly(styryl)lithium

A new approach has been developed for the preparation of well-defined, eight-arm star polymers via the addition of poly­(styryl)lithium to octavinylPOSS in benzene. The reaction proceeds rapidly to completion (within 5 min for molecular weight of each arm up to 33 kg/mol), forming predominantly eight-arm star polymers. The products were purified by fractionation and fully characterized by ¹H NMR, ¹³C NMR, ²⁹Si NMR, FT-IR, MALDI-TOF mass spectrometry, and size exclusion chromatography. Compared to conventional coupling approaches, this process is found to be less sensitive to the stoichiometry of the reactants and the molecular weight of each arm. A mechanism based on cross-association and intra-aggregate addition is invoked to account for this unusual observation. As evidence, when a polar solvent, tetrahydrofuran, or a strongly coordinating and disassociating Lewis base, tetramethylethylenediamine, was used to dissociate the living polymer chains, star polymers with lower average arm numbers than those of the products synthesized in pure benzene were formed at the same stoichiometry of the reactants. The method has general implications in the understanding of the reactive nature of the living anionic polymerization and may find practical application in the synthesis of functional star polymers of diverse compositions and architectures

Anionic Synthesis of Mono- and Heterotelechelic Polystyrenes via Thiol–Ene “Click” Chemistry and Hydrosilylation

A series of precisely defined, mono- and heterotelechelic polystyrenes have been facilely synthesized by combining living anionic polymerization with other efficient chemical transformations, such as thiol–ene “click” chemistry and hydrosilylation reactions, leading to a versatile and general functionalization methodology for chain-end-functionalized anionic polymers. Specifically, α-vinyl-ended poly(styryl)lithiums, which were prepared using 4-pentenyllithium as an initiator under high-vacuum conditions, were reacted with different end-capping reagents using living functionalization methods to afford various chain-end functionalities quantitatively, namely, α-vinylpolystyrene, α-vinyl-ω-hydroxylpolystyrene, and α-vinyl-ω-hydrosilylpolystyrene. Subsequent functionalizations using photoinitiated thiol–ene “click” chemistry and hydrosilylation reactions allow facile and efficient installation of diverse functionalities onto the α- and ω-chain ends of these polymers, respectively, including amine groups, carboxylic acid groups, hydroxyl groups, and perfluorinated alkyl chains. It was found that the heterofunctionalization should be carried out in the sequence of hydrosilylation and then thiol–ene reaction to achieve precisely defined products, probably due to the side products associated with the reaction between silyl hydrides and radical intermediates. The polymers have been thoroughly characterized by 1H NMR, 13C NMR, FT-IR, SEC, and MALDI-TOF mass spectrometry to establish their chemical structures and chain-end functionalities, which indicates precisely defined mono- and heterotelechelic polystyrenes with 100% functionalities. These polymers serve as important model compounds in the study of their bulk properties as well as self-assembling behaviors

Synthesis of Shape Amphiphiles Based on POSS Tethered with Two Symmetric/Asymmetric Polymer Tails via Sequential “Grafting-from” and Thiol–Ene “Click” Chemistry

A series of shape amphiphiles based on functionalized polyhedral oligomeric silsesquioxane (POSS) head tethered with two polymeric tails of symmetric or asymmetric compositions was designed and synthesized using sequential “grafting-from” and “click” surface functionalization. The monofunctionalization of octavinylPOSS was performed using thiol–ene chemistry to afford a dihydroxyl-functionalized POSS that was further derived into precisely defined homo- and heterobifunctional macroinitiators. Polymer tails, such as polycaprolactone and polystyrene, could then be grown from these POSS-based macroinitiators with controlled molecular weight via ring-opening polymerization and atom transfer radical polymerization (ATRP). The vinyl groups on POSS were found to be compatible with ATRP conditions. These macromolecular precursors were further modified by thiol–ene chemistry to install surface functionalities onto the POSS cage. The polymer chain composition and POSS surface chemistry can thus be tuned separately in a modular and efficient way