Star‐shaped polystyrenes with glycoconjugated periphery and interior: Synthesis and entrapment of hydrophilic molecule

Star‐shaped polystyrenes with acetyl glucose in the periphery and interior were synthesized via two‐steps, 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO)‐mediated living radical polymerizations. In the first step, styrene (St) was polymerized with 4‐[1′‐(2″,2″,6″,6″‐tetramethyl‐1″‐piperidinyloxy)ethyl]phenyl 2,3,4,6‐tetra‐O‐acetyl‐β‐D ‐glucopyranoside, 1 , at 120 °C to afford a TEMPO‐terminated polystyrene with acetyl glucose in the chain‐end, arm‐polymer 2 . Similarly, St was polymerized with 1‐phenyl‐1‐(2′,2′,6′,6′‐tetramethyl‐1′‐piperidinyloxy)ethane, 3 , to obtain a TEMPO‐terminated polystyrene, arm‐polymer 4 . In the second step, the coupling reaction of arm‐polymer 2 was performed using divinylbenzene (DVB) as a linking agent in m‐xylene at 138 °C, giving a star‐shaped polystyrene with acetyl glucose in the periphery, 5 . The coupling reaction of arm‐polymer 4 with DVB was carried out in the presence of 1 , which produced a star‐shaped polystyrene with acetyl glucose in the interior, 6 . Dynamic laser light scattering (DLS) measurements indicated that 5 and 6 existed as the particles in toluene with the average diameters ranging from 12–40 nm. The numbers of the arm (N_arm) were 12–23 and 6–64 for 5 and 6 , respectively, which were determined by their isolated yields and static laser light scattering (SLS) measurements. The numbers of the acetyl glucose units (N₁) were 12–23 and 9–104 for 5 and 6 , respectively, which were determined from specific rotation ([α]₃₆₅). Finally, 5 and 6 were modified by deacetylation using sodium methoxide, producing star‐shaped polystyrenes with glucose in the periphery and interior, 7 and 8 , respectively. The final architectures were found to entrap a hydrophilic molecule at their glycoconjugated periphery or interior in good solvents for polystyrene such as chloroform. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4373–4381, 2005     

A Facile Strategy for Preparation of α‐Heterobifunctional Polystyrenes with Well‐Defined Molecular Weight

A facile strategy for synthesis of α‐heterobifunctional polystyrenes is reported. The novel functional polystyrenes have been successfully synthesized via a combination of atom transfer radical polymerization (ATRP) and chemical modification of end‐functional groups. First, ε‐caprolactone end‐capped polystyrenes with controlled molecular weight and low polydispersity were prepared by ATRP of styrene using α‐bromo‐ε‐caprolactone (αBrCL) as an initiator. Then, removal of the terminal bromine atom was performed with iso‐propylbenzene in the presence of CuBr/PMDETA. Finally, ring‐opening modifications of the caprolactone group were carried out with amines, n‐butanol and H₂O to produce novel polystyrenes containing two different functional groups at one end.

     

Large‐scale synthesis of arborescent polystyrenes

A method was developed for the large (100 g) scale synthesis of arborescent polystyrenes using acetyl coupling sites. Successive generations of dendritic graft polymers were obtained from cycles of polystyrene substrate acetylation with acetyl chloride and coupling in the presence of LiCl with “living” polystyryllithium chains capped with 2‐vinylpyridine units. The grafting yield for the synthesis of a generation zero (G0 or comb‐branched) arborescent polystyrene under the conditions previously reported for the 10 g scale reactions decreased from 95 to 75% when scaled up to 100 g. The lowered yield was linked to side chain dimerization and deactivation of the macroanions. The modified 100 g scale procedure, using end‐capping of the polystyryllithium with 1,1‐diphenylethylene and the addition of 3–6 equivalents per living end of 2‐vinylpyridine as a dilute solution, eliminated side chain dimerization and provided grafting yields of up to 95%. The large‐scale procedure developed was applied to the synthesis of arborescent polystyrenes of generations up to G2 with low polydispersity indices (M_w/M_n ≤ 1.04) and molecular weights increasing in an approximately geometric fashion for each cycle. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5742–5751, 2008     

Characterization of a series of phenyl‐capped vinyl‐terminated oligomers of styrene

Low molecular weight (MW) polystyrenes were synthesized by radical polymerization in the presence of catalytic chain‐transfer agents. Synthetic conditions are controlled to produce molecules containing one methyl group at one end as well as a double bond at the other end, capped with a phenyl group. Individual oligomers were separated by liquid chromatography, and the properties were analyzed using NMR, ultraviolet–visible (UV–vis) spectroscopy, and size exclusion chromatography with light scattering. The UV–vis spectra, proton NMR spectra, and differential refractive‐index increments exhibit an MW dependence of up to six–eight monomer units. The obtained dependencies can be used for precise characterization of the molecular weight distribution of polystyrene obtained by catalytic chain transfer. The double‐bonded end groups were found to be exclusively in the transconfiguration for all oligomers. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1099–1105, 2001     

Synthesis and characterization of amphiphilic copolymer of linear poly(ethylene oxide) linked with [poly(styrene‐co‐2‐hydroxyethyl methacrylate)‐graft‐poly(ε‐caprolactone)] using sequential controlled polymerization

A well‐defined amphiphilic copolymer of ‐poly(ethylene oxide) (PEO) linked with comb‐shaped [poly(styrene‐co‐2‐hydeoxyethyl methacrylate)‐graft‐poly(ε‐caprolactone)] (PEO‐b‐P(St‐co‐HEMA)‐g‐PCL) was successfully synthesized by combination of reversible addition‐fragmentation chain transfer polymerization (RAFT) with ring‐opening anionic polymerization and coordination–insertion ring‐opening polymerization (ROP). The α‐methoxy poly(ethylene oxide) (mPEO) with ω,3‐benzylsulfanylthiocarbonylsufanylpropionic acid (BSPA) end group (mPEO‐BSPA) was prepared by the reaction of mPEO with 3‐benzylsulfanylthiocarbonylsufanyl propionic acid chloride (BSPAC), and the reaction efficiency was close to 100%; then the mPEO‐BSPA was used as a macro‐RAFT agent for the copolymerization of styrene (St) and 2‐hydroxyethyl methacrylate (HEMA) using 2,2‐azobisisobutyronitrile as initiator. The molecular weight of copolymer PEO‐b‐P(St‐co‐HEMA) increased with the monomer conversion, but the molecular weight distribution was a little wide. The influence of molecular weight of macro‐RAFT agent on the polymerization procedure was discussed. The ROP of ε‐caprolactone was then completed by initiation of hydroxyl groups of the PEO‐b‐P(St‐co‐HEMA) precursors in the presence of stannous octoate (Sn(Oct)₂). Thus, the amphiphilic copolymer of linear PEO linked with comb‐like P(St‐co‐HEMA)‐g‐PCL was obtained. The final and intermediate products were characterized in detail by NMR, GPC, and UV. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 467–476, 2006     

Synthesis of mikto‐topology star polymer containing one cyclic arm

Well‐defined mikto‐topology star polystyrene composed of one cyclic arm and four linear arms was synthesized by a combination of atom transfer radical polymerization (ATRP) and Cu‐catalyzed azide‐alkyne cycloaddition (CuAAC) click reaction. First, the bromine‐alkyne α,ω‐linear polystyrenes containing four hydroxyl groups protected with acetone‐based ketal groups were synthesized by ATRP of styrene using a designed initiator. Then, the bromine end‐group was converted to the azide and the linear polystyrene was cyclized intra‐molecularly by the CuAAC reaction. The four hydroxyl groups were released by deprotection and then esterified with 2‐bromoisobutyryl bromide to produce a cyclic polymer bearing four ATRP initiating units. By subsequent ATRP of styrene to grow linear polymers with the cyclic polystyrene as a macroinitiator, the mikto‐topology star polymers were prepared. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Copper(0)‐mediated living radical polymerization of acrylonitrile at room temperature

The copper(0)‐catalyzed living radical polymerization of acrylonitrile (AN) was investigated using ethyl 2‐bromoisobutyrate as an initiator and 2,2′‐bipyridine as a ligand. The polymerization proceeded smoothly in dimethyl sulphoxide with higher than 90% conversion in 13 h at 25 °C. The polymerization kept the features of controlled radical polymerization. ¹H NMR spectra proved that the resultant polymer was end‐capped by ethyl 2‐bromoisobutyrate species. Such polymerization technique was also successfully introduced to conduct the copolymerization of styrene (St) and AN to obtain well‐controlled copolymers of St and AN at 25 °C, in which the monomer conversion of St could reach to higher than 90%. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Liquid crystal alignment property of n‐alkylthiomethyl‐ or n‐alkylsulfonylmethyl‐substituted polystyrenes

We synthesized a series of n‐alkylthiomethyl‐substituted polystyrenes (#T‐PS, # = 4, 8, 12, and 16) and n‐alkylsulfonylmethyl‐substituted polystyrenes (#S‐PS, # = 4, 8, 12, and 16), where # is the number of carbon atoms in the n‐alkyl side group of the polymers, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a longer n‐alkyl side group, a thioether linkage group, and a higher molar content of n‐alkyl side group showed homeotropic LC alignment behavior with a pretilt angle of about 90°. The homeotropic alignment behavior was well correlated with the surface energy of the polymer films; when the positive dielectric anisotropic LC (ZLI‐5900‐000 from Merck) and negative dielectric anisotropic LC (MLC‐7026‐000 from Merck) were used to fabricate the LC cells, homeotropic alignment was observed when the surface energy values of the polymer were smaller than about 25 and 32 mJ/m², respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

Macromonomer synthesis using α‐(2‐methyl‐2‐phenylpropyl)acrylates as addition–fragmentation chain‐transfer agents expelling the cumyl radical

α‐(2‐Methyl‐2‐phenylpropyl)acrylate (RS‐2) was examined as a C? C bond‐cleavage type addition–fragmentation chain transfer (AFCT) agent in the benzene solution polymerizations of styrene (St), ethyl methacrylate (EMA), and cyclohexyl acrylate (CHA) with the objective of achieving efficient macromonomer synthesis by radical polymerization. The AFCT efficiency was evaluated in terms of the decrease in the number‐average molecular weight (M_n) upon the addition of the AFCT agent and the number of unsaturated end groups introduced per chain (f). The AFCT efficiency was rationalized by the consideration of the relative importance of AFCT as an end‐forming event and the competition between ‐fragmentation and crosspropagation as adduct radical reaction pathways. In St and EMA polymerizations at 60 °C, RS‐2 resulted in higher f values and lower M_n values than methyl α‐(2‐methyl‐2‐carbomethoxypropyl)acrylate (MMA‐2), and this suggested the facilitation of ‐fragmentation due to the expulsion of the more stable cumyl radical from the RS‐2 adduct radical. Higher f values were observed for MMA‐2 than for RS‐2 in CHA polymerization because of unsaturated end group formation by ‐fragmentation of midchain radicals. However, RS‐2 resulted in lower M_n values for poly(CHA) than MMA‐2 because of a smaller contribution of crosspropagation. Retardation in the presence of the AFCT agents was affected by the balance between b‐fragmentation and crosspropagation and by the addition rate of the propagating radical to the AFCT agent. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6021–6030, 2004     

Radical ring‐opening copolymerization behavior of vinyloxirane: Synthesis of copolymer from 2‐isopropenyl‐3‐phenyloxirane and styrene and its functionalization

The radical ring‐opening copolymerization of 2‐isopropenyl‐3‐phenyloxirane (1) with styrene (St) was examined to obtain the copolymer [copoly(1‐St)] with a vinyl ether moiety in the main chain. The copolymers were obtained in moderate yields by copolymerization in various feed ratios of 1 and St over 120 °C; the number‐average molecular weights (M_n) were estimated to be 1800–4200 by gel permeation chromatography analysis. The ratio of the vinyl ether and St units of copoly(1‐St) was estimated with the ¹H NMR spectra and varied from 1/7 to 1/14 according to the initial feed ratio of 1 and St. The haloalkoxylation of copoly(1‐St) with ethylene glycol in the presence of N‐chlorosuccinimide produced a new copolymer with alcohol groups and chlorine atoms in the side group in a high yield. The M_n value of the haloalkoxylated polymer was almost the same as that of the starting copoly(1‐St). The incorporated halogen was determined by elemental analysis. The analytical result indicated that over 88% of the vinyl ether groups participated in the haloalkoxylation. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3729–3735, 2000     

Synthesis of liquid crystalline–amorphous block copolymers by the combination of atom transfer and photoinduced radical polymerization mechanisms

The synthesis of ABA‐type block copolymers, involving liquid‐crystalline 6‐(4‐cyanobiphenyl‐4′‐oxy)hexyl acrylate (LC6) and styrene (St) monomer with copper‐based atom transfer radical polymerization (ATRP) and photoinduced radical polymerization (PIRP), was studied. First, photoactive α‐methylol benzoin methyl ether was esterified with 2‐bromopropionyl bromide, and it was subsequently used for ATRP of LC6 in diphenylether in conjunction with CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine as a catalyst. The obtained photoactive functional liquid‐crystalline polymer, poly[6‐(4‐cyanobiphenyl‐4′‐oxy)hexyl acrylate] (PLC6), was used as an initiator in PIRP of St. Similarly, photoactive polystyrenes were also synthesized and employed for the block copolymerization of LC6 in the second stage. The spectral, thermal, and optical measurements confirmed a full combination of ATRP and PIRP, which resulted in the formation of ABA‐type block copolymers with very narrow polydispersities. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1892–1903, 2003     

Branched poly(styrene‐b‐tert‐butyl acrylate) and poly(styrene‐b‐acrylic acid) by ATRP from a dendritic poly(propylene imine)(NH2)64 core

With the aim of creating highly branched amphiphilic block copolymers, the primary amine end groups of the poly(propylene imine) dendrimers DAB‐dendr‐(NH₂)₈ and DAB‐dendr‐(NH₂)₆₄ were converted to 2‐bromoisobutyramide groups. Poly (styrene‐b‐tert‐butyl methacrylate) (PS‐b‐PtBMA) was synthesized by ATRP from the eight end group initiator, and poly(styrene‐b‐tert‐butyl acrylate) (PS‐b‐PtBA) was synthesized from the 64 end group initiator. The tert‐butyl groups were removed to produce poly(styrene‐b‐methacrylic acid) (PS‐b‐PMAA) and poly(styrene‐b‐acrylic acid) (PS‐b‐PAA). Comparison of size exclusion chromatography (SEC) absolute molecular weight analyses of the polystyrenes with calculated molecular weights showed that the eight end group initiator produced a polystyrene with about eight branches, and that the 64 end group initiator produced polystyrene with many fewer than 64 branches. The PS‐b‐PtBA materials also have many fewer than 64 branches. The PS‐b‐PAA samples dissolved molecularly in DMF but formed aggregates in water even at pH 10. AFM images of the PS‐b‐PtBAs spin coated from THF and DMF onto mica showed aggregates. AFM images of the PS‐b‐PAAs spin coated from various mixtures of DMF and water at pH 10 showed flat disks and worm‐like images similar to those observed with linear PS‐b‐PAAs. Use of a PS‐b‐PAA and a PS‐b‐PMAA as templates for emulsion polymerization of styrene produced latexes 100–200 nm in diameter. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4623–4634, 2007     

Synthesis of Well‐Defined Figure‐of‐Eight‐Shaped Polymers by a Combination of ATRP and Click Chemistry

Well‐defined figure‐of‐eight‐shaped (8‐shaped) polystyrene (PS) with controlled molecular weight and narrow polydispersities has been prepared by the combination of atom transfer radical polymerization (ATRP) and click chemistry. The synthesis involves two steps: 1) Preparation of a linear tetrafunctional PS with two azido groups, one at each end of the polymer chain, and two acetylene groups at the middle of the chain. 2) Intramolecular cyclization of the linear tetrafunctional PS at a very low concentration by a click reaction to produce the 8‐shaped polystyrenes. The resulting intermediates and the target polymers were characterized by ¹H NMR and FT‐IR spectroscopy, and gel permeation chromatography. The glass transition temperatures (T_gs) were determined by differential scanning calorimetry and it was found that the decrease in chain mobility by cyclization resulted in higher T_gs for 8‐shaped polystyrenes as compared to their corresponding precursors.

     

End‐functionalized polystyrene by ATRP: A facile approach to primary amino and carboxylic acid terminal groups

Monoamino‐terminated and monocarboxylic acid‐terminated polystyrenes containing active halogenated end groups were prepared by atom transfer radical polymerization (ATRP) using the so‐called initiator method and protective group chemistry. α‐Chloropropionates were synthesized and utilized as initiators containing the tert‐butoxycarbonyl (t‐BOC)‐protected amino and the tert‐butyl (t‐Bu)‐protected carboxylic acid function, respectively. Optimum polymerization conditions were attained using CuCl/N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (PMDETA) as catalyst and 10 vol % n‐butanol as homogenizing agent at 110 °C. However, targeting larger quantities an alternative route was established employing 50 vol % N,N‐dimethylformamide (DMF). Subsequent hydrolysis of the ω‐tert‐butoxycarbonyl polystyrenes afforded well‐defined polymers with quantitative deprotection of the functional groups. Comparatively, thermolytic cleavage of the protective sites was studied. ¹H NMR verified the quantitative removal of the t‐BOC‐protecting groups. Furthermore, the resulting α‐amino‐ω‐chloro polystyrenes were reacted with Sanger reagent to confirm the existence of the thereby converted primary amino groups. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3845–3859, 2009     

Lipase‐catalyzed ring‐opening polymerization of 6(S)‐methyl‐morpholine‐2,5‐dione

Lipase‐catalyzed ring‐opening bulk polymerizations of 6(S)‐methyl‐morpholine‐2,5‐dione (MMD) were investigated. Selected commercial lipases were screened as catalysts for MMD polymerization at 100 °C. Polymerizations catalyzed with 10 wt % porcine pancreatic lipase type II crude (PPL), lipase from Pseudomonas cepacia, and lipase type VII from Candida rugosa resulted in MMD conversions of about 75% in 3 days and in molecular weights ranging from 8200 to 12,100. Poly(6‐methyl‐morpholine‐2,5‐dione) [poly(MMD)] had a carboxylic acid group at one end and a hydroxyl group at the other end. However, lipase from Mucor javanicus showed lower catalytic activity for the polymerization. During the polymerization, racemization of the lactate residue took place. PPL was selected for further studies. The rate of polymerization increased with increasing PPL concentration under otherwise identical conditions. When the PPL concentration was 5 or 10 wt % with respect to MMD, a conversion of about 70% was reached after 6 days or 1 day, respectively, whereas for a PPL concentration of 1 wt %, the conversion was less than 20% even after 6 days. High concentrations of PPL (10 wt %) resulted in high number‐average molecular weights (<3 days); with a lower concentration of PPL, lower molecular weight poly(MMD) was obtained. The concentration of water was an important factor that controlled not only the conversion but also the molecular weight. With increasing water content, enhanced polymerization rates were achieved, whereas the molecular weight of poly(MMD) decreased. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3030–3039, 2005     

Preparation and spectroscopic characterization of surface‐enriched (with active sites) polymer‐supported phase‐transfer catalysts and their efficiency in organic addition reactions: A kinetic study

We prepared two batches of surface‐enriched (with active sites) polymer‐supported phase‐transfer catalysts (SE‐PSPTC) by fixing the crosslinking monomer divinylbenzene (DVB) at 2% (first batch) and 6% (second batch) through a free‐radical suspension copolymerization method with vinylbenzyl chloride (VBC; 25%) as a functionality and with styrene (St) as a supporting monomer, followed by the quaternization of the resulting terpolymer beads with triethylamine. The enrichment of the active sites on the surfaces of the beads was accomplished by a surface‐grafting technique through the delayed addition of the functional monomer (VBC) to the partially polymerized copolymer beads of poly(St/DVB). To bring the active sites fully onto the surfaces, we prepared six different types of terpolymer beads in each batch by varying the partial polymerization time (PPT) of St/DVB—0 h [0 VBC (conventional)], 3 h (3 VBC), 6 h (6 VBC), 9 h (9 VBC), 12 h (12 VBC), and 15 h (15 VBC)—and then gradually adding the functional monomer (VBC) to the partially polymerized poly(St/DVB) system. The resulting terpolymer beads, containing different concentrations of pendant benzyl chloride (? CH₂Cl) on the surface in each batch, underwent facile quaternization [? CH₂N⁺(C₂H₅)₃Cl^?] with an increase in the PPT of St/DVB and remained constant at 12 VBC and 15 VBC. To asses the superiority of the catalysts according to the surface enrichment of the active sites, particularly between conventional (0 VBC) catalysts and other PPT‐based SE‐PSPTCs, we characterized all the catalysts by estimating the chloride‐ion concentration, by using Fourier transform infrared (FTIR), scanning electron microscopy (SEM), EDAX, and ESCA, and by carrying out the dichlorocarbene addition to olefins. The chloride‐ion concentration by the Volhard method and the peak intensity of the C? N stretching absorbance concentration, that is, the quaternary onium group in the FTIR spectra of both batches, increased with the PPT of St/DVB in both batches of catalysts. In particular, the chloride concentration of a first‐batch catalyst of a representative mesh size (?120 + 140) had a twofold enhancement between the conventional catalyst (0 VBC; 1.88 m equiv g^?1) and 9 VBC/SE‐PSPTC (3.74 m equiv g^?1), although the same amount of the functional monomer was added in both preparations. These results showed the higher enrichment of the active site on the surface of 9 VBC, and the same trend was also maintained for second‐batch catalysts, regardless of the catalyst mesh size. SEM images of both batches showed that there was a higher concentration of nodules [due to the grafting of poly(VBC)] on the surfaces of the beads of 9 VBC/SE‐PSPTC and the aforementioned PPT catalysts than on the surfaces of the conventional catalysts (0 VBCs), which exhibited smooth surfaces (because of the simultaneous addition of all three monomers). This observation confirmed the enrichment of active sites on the surfaces. In the EDAX analysis, up to a depth of 0.5–1 μm, the surface chloride concentration increased from 0 VBC to 9 VBC/SE‐PSPTC and remained constant in 12 VBC and 15 VBC, first‐batch catalysts of a representative mesh size (?120 + 140). The same trend was also observed in second‐batch catalysts, indicating the enrichment of the onium group more on the surface in 9 VBC/SE‐PSPTCs. The ESCA analysis, to a depth of about 20–30Å, proved that the concentration of covalent chloride on the surface had increased from 0 VBC (15%) to 9 VBC/SE‐PSPTCs (29%) and remained constant thereafter in first‐batch catalyst; the trend was the same for second‐batch catalysts, also confirming the strong evidence of surface enrichment of the active sites. Similarly, the rate constants of different olefin addition reactions catalyzed by both batches of catalysts also increased from 0 VBC to 9 VBC and remained constant with 12 VBC and 15 VBC catalysts. The twofold increase of the rate constants, regardless of the olefins, for conventional catalysts to 9 VBC/SE‐PSPTCs confirmed the enrichment of the active sites on the surfaces. All these experimental observations proved that 50% of the active sites were successfully brought out from inside the poly(St/DVB) networks to the exterior surfaces, although same amount of VBC was added for the preparation of all the catalyst types. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 347–364, 2003     

Influence of the catalyst and polymerization conditions on the long‐chain branching of metallocene‐catalyzed polyethenes

A study was made on the effects of polymerization conditions on the long‐chain branching, molecular weight, and end‐group types of polyethene produced with the metallocene‐catalyst systems Et[Ind]₂ZrCl₂/MAO, Et[IndH₄]₂ZrCl₂/MAO, and (n‐BuCp)₂ZrCl₂/MAO. Long‐chain branching in the polyethenes, as measured by dynamic rheometry, depended heavily on the catalyst and polymerization conditions. In a semibatch flow reactor, the level of branching in the polyethenes produced with Et[Ind]₂ZrCl₂/MAO increased as the ethene concentration decreased or the polymerization time increased. The introduction of hydrogen or comonomer suppressed branching. Under similar polymerization conditions, the two other catalyst systems, (n‐BuCp)₂ZrCl₂/MAO and Et[IndH₄]₂ZrCl₂/MAO, produced linear or only slightly branched polyethene. On the basis of an end‐group analysis by FTIR and molecular weight analysis by GPC, we concluded that a chain transfer to ethene was the prevailing termination mechanism with Et[Ind]₂ZrCl₂/MAO at 80 °C in toluene. For the other catalyst systems, β‐H elimination dominated at low ethene concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 376–388, 2000     

Star‐branched polystyrenes by nitroxide living free‐radical polymerization

Star‐branched polystyrenes, with polydispersity indices of 1.15–1.56 and 4–644 equal arms, were synthesized by the reaction of 2,2,6,6‐tetramethylpiperidin‐1‐yloxy (TEMPO)‐capped polystyrene (PS‐T) with divinylbenzene (DVB). The characterization of PS‐T and the final star polymers was carried out by size exclusion chromatography, low‐angle laser light scattering, and viscometry. The degree of branching of the star polymers depended on the DVB/PS‐T ratio and the PS‐T molecular weight. An asymmetric (or miktoarm) star homopolymer of the PS_nPS′_n type was made by the reaction of the PS_n symmetric star, which had n TEMPO molecules on its nucleus and consisted of a multifunctional initiator, with extra styrene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 320–325, 2001     

A novel approach to the characterization of end groups in styrene–methyl methacrylate copolymers by pyrolysis–gas chromatography

The end groups of styrene–methyl methacrylate (St‐MMA) copolymers polymerized radically with 2,2′‐azobisisobutyronitrile (AIBN) as an initiator, which are difficult to characterize even by NMR, were investigated by pyrolysis–gas chromatography. On the resulting pyrograms, characteristic products that formed from the end‐group moiety due to AIBN, such as 2‐cyanopropane, 2‐cyanopropen, and various compounds consisting of an isobutyronitrile group and a monomer unit, were observed together with those from the main chain, such as St and MMA monomers and various dimeric and trimeric products. The relative abundance between the recombination and disproportionation termination reactions in the copolymerization process was estimated from the relative intensities between the characteristic peaks of the end group and those of the main chain. Thus, the estimated abundance for the termination reactions suggested that the polymerization process for this particular copolymer system terminated preferentially by recombination rather than by disproportionation. Furthermore, the relative abundance between the monomer units adjacent to the chain‐end AIBN residues was estimated on the basis of the peak intensities of the products consisting of an isobutyronitrile group and either monomer unit, which reflected the penultimate neighboring structure of the end group in the polymer chain. Thus, the observed results suggested that the isobutyronitrile radical formed by the dissociation of AIBN in the initiation reaction was predominantly adjoined by St monomer rather than by MMA monomer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1880–1888, 2000     

A Second‐Coordination‐Sphere Strategy to Modulate Nickel‐ and Palladium‐Catalyzed Olefin Polymerization and Copolymerization

Transition‐metal‐catalyzed copolymerization reactions of olefins with polar‐functionalized comonomers are highly important and also highly challenging. A second‐coordination‐sphere strategy was developed to address some of the difficulties encountered in these copolymerization reactions. A series of α‐diimine ligands bearing nitrogen‐containing second coordination spheres were prepared and characterized. The properties of the corresponding nickel and palladium catalysts in ethylene polymerizations and copolymerizations were investigated. In the nickel system, significant reduction in polymer branching density was observed, while lower polymer branching densities, as well as a wider range of polar monomer substrates, were achieved in the palladium system. Control experiments and computational results reveal the critical role of the metal−nitrogen interaction in these polymerization and copolymerization reactions.