Narrow Molecular Weight Distribution Research Articles

Living cationic polymerization of isobutylene in seconds based on microflow system

An initiating system composed by ethylaluminum sesquichloride (Et3Al2Cl3)//2,6-dimethylpyridine (DMP) is established for fast living cationic polymerization of isobutylene. The polymerization can be accelerated at utmost to finish in only 4 s with extremely narrow molecular weight distribution in microflow system. Investigations on influence factors reveal that mixing intensity that dependent on Re affects the molecular weight distribution significantly. DMP has two-stage effect, corresponding to its interactions with protonic impurity and Lewis acid respectively. Polymerization rate can be facilely adjusted by changing the volume ratio of mixed solvent of n-hexane and CH2Cl2 from 8:2 to 7:3 with the assurance of livingness. This polymerization system can even operate at elevated temperature as high as −55 °C without the loss of livingness, which is of great beneficial to the commercialization of polyisobutylene-based functional polymer materials.

Porphyrinic covalent organic framework designed by molecular engineering as heterogeneous photocatalyst for aerobic mediated RAFT polymerization

Covalent organic frameworks (COFs) have gained increasing attention as heterogeneous materials for their prominent applications in photocatalytic processes. The already tailored structure endows COFs with ordered dimensional channels for the separation and migration of the electro-hole pairs and improves their photocatalytic properties. In this contribution, oxygen-mediated RAFT polymerization was achieved by using M-TCPP-DHTA-COFs (M = H2 or Zn) as photocatalysts with the assistance of TEA as co-catalyst producing polymers with accurate molecular weight and narrow molecular weight distribution under visible light irradiation. The control experiments revealed excellent dual control behavior of light and gas toward polymerization processes. Notably, porphyrinic COFs can be straightforwardly separated and recycled for recycling experiments and exhibit remarkable compatibility features of controllable polymerization for functional monomers under aerobic conditions. This study offers a promising pathway for the construction of an efficient heterogeneous catalyst of oxygen-mediated RAFT polymerization and extends the novel applications of porphyrin-based COF materials.

Synthesis of Optically Active Helical Polycarbenes through Helix-Sense-Selective Polymerization Strategy and Their Application in Chiral Separation.

In this work, helical polycarbenes with optical activity were designed and facilely synthesized through the helix-sense-selective polymerization (HSSP) of the diazoacetate monomer with a dimethylbenzyl ester pendant catalyzed by π-allylPdCl with chiral phosphine ligands at room temperature. The polymerization was carried out in a living and controlled style, and a range of helical polycarbenes with the desired number-average molecular weights and narrow molecular weight distributions were obtained. Circular dichroism and UV-vis analyses revealed that these polycarbenes exhibited a stable helical conformation with a preferred handedness, and their helical directions were dependent on the chirality of the chiral phosphine ligands. Further studies showed that the helical conformation of the obtained polycarbenes was from the polymeric backbone rather than the intermolecular aggregation in the solutions. Moreover, the prepared, optically active, helical polycarbenes possessed excellent enantioselective crystallization ability for threonine racemates. The enantiomeric excess (e.e.) of the induced crystals could be up to 83% via utilizing the prepared helical polycarbenes as a chiral separation agent.

Dimensional Optimization for ZnO-Based Mechano-ATRP with Extraordinary Activity.

Various piezoelectric nanomaterials were utilized in ultrasound-mediated atom transfer radical polymerization (ATRP), owing to their outstanding piezoelectric effect. However, the relationship between the morphology of those piezocatalysts and polymerization has not been clearly established. Herein, we employed different piezoelectric zinc oxide (ZnO) nanomaterials to achieve novel mechano-induced ATRP (mechano-ATRP). Based on the synergistic effect of piezoelectric properties and specific surface area, the catalytic activity of 1D ZnO nanorods (1D-ZnO NRs) with increased aspect ratio outperformed that of 0D ZnO nanoparticles (0D-ZnO NPs). Compared to the conventional ATRP system, this system exhibited extraordinary activity toward the less activated monomer acrylonitrile (67% conversion after 6 h), with a narrow molecular weight distribution (polydispersity index ∼ 1.19). Furthermore, implications of ZnO loading, copper salt amount, degree of polymerization, monomer, and solvent were also studied for the highly efficient mechano-ATRP.

Tuning the Glass Transition Temperature of a Core-Forming Block during Polymerization-Induced Self-Assembly: Statistical Copolymerization of Lauryl Methacrylate with Methyl Methacrylate Provides Access to Spheres, Worms, and Vesicles.

A series of poly(lauryl methacrylate)–poly(methyl methacrylate-stat-lauryl methacrylate) (PLMAx–P(MMA-stat-LMA)y) diblock copolymer nanoparticles were synthesized via RAFT dispersion copolymerization of 90 mol % methyl methacrylate (MMA) with 10 mol % lauryl methacrylate (LMA) in mineral oil by using a poly(lauryl methacrylate) (PLMA) precursor with a mean degree of polymerization (DP) of either 22 or 41. In situ1H NMR studies of the copolymerization kinetics suggested an overall comonomer conversion of 94% within 2.5 h. GPC analysis confirmed a relatively narrow molecular weight distribution (Mw/Mn ≤ 1.35) for each diblock copolymer. Recently, we reported an unexpected morphology constraint when targeting PLMA22–PMMAy nano-objects in mineral oil, with the formation of kinetically trapped spheres being attributed to the relatively high glass transition temperature (Tg) of the PMMA block. Herein we demonstrate that this limitation can be overcome by (i) incorporating 10 mol % LMA into the core-forming block and (ii) performing such syntheses at 115 °C. This new strategy produced well-defined spheres, worms, or vesicles when using the same PLMA22 precursor. Introducing the LMA comonomer not only enhances the mobility of the core-forming copolymer chains by increasing their solvent plasticization but also reduces their effective glass transition temperature to well below the reaction temperature. Copolymer morphologies were initially assigned via transmission electron microscopy (TEM) studies and subsequently confirmed via small-angle X-ray scattering analysis. The thermoresponsive behavior of PLMA22–P(0.9MMA-stat-0.1LMA)113 worms and PLMA22–P(0.9MMA-stat-0.1LMA)228 vesicles was also studied by using dynamic light scattering (DLS) and TEM. The former copolymer underwent a worm-to-sphere transition on heating from 20 to 170 °C while a vesicle-to-worm transition was observed for the latter. Such thermal transitions were irreversible at 0.1% w/w solids but proved to be reversible at 20% w/w solids.

Boron-Based Lewis Pairs Catalyzed Living, Regioselective, and Topology-Controlled Polymerization of (E, E)-Alkyl Sorbates.

It remains as a great challenge to realize living and controlled polymerization of renewable monomers by the boron-based Lewis pairs. Here, strong nucleophilic N-heterocyclic olefins (NHOs) or N-heterocyclic carbenes (NHCs) as Lewis bases, and boron-based compounds as Lewis acids, are employed to construct LPs for polymerization of alkyl sorbates, including (E, E)-methyl sorbate and (E, E)-ethyl sorbate. Systematic investigation reveals that the combinations of B(C6 F5 )3 with appropriate acidity and steric hindrance, and strong nucleophilic NHOs promote living and controlled polymerization of alkyl sorbates in 100% 1,4-addition manner, furnishing polymers with predicted molecular weight (Mw up to 56.6kgmol-1 ) and narrow molecular weight distribution (Đ as low as 1.12). Furthermore, topology analysis shows that NHC1/B(C6 F5 )3 LP produce PMS possessing cyclic structure.

Synthesis of sulfur-containing polymers via metal-free cross coupling polymerization of tosylhydrazones and thiols

An efficient protocol for synthesizing sulfur-containing polymers through cross coupling polymerization of N -tosylhydrazones and thiols without transition-metal catalyst has been exploited. Various aryl and heteroaryl N -tosylhydrazones with different pendant alkyls, as well as ferrocenyl N -tosylhydrazones, are suitable substrates giving the corresponding polymers. In the presence of K 2 CO 3 , a diversity of sulfur-containing polymers with moderate number-average molecular weight and narrow molecular weight distribution can be produced in decent to good yields. Moreover, one-pot, two-step conversion from corresponding ketones to ultimate polymers offers an alternative, economic and convenient route to form C–S linkages. The cross coupling polymerization of N -tosylhydrazones and thiols has been developed through a transition-metal free protocol. The 16 sulfur-containing polymers have obtained with moderate number-average molecular weight and narrow molecular weight distribution in decent to good yields. • An efficient synthesis of sulfur-containing polymers process with N -tosylhydrazones and thiols has been developed. • The reaction was conducted through a transition-metal free protocol. • The reaction had broad substrate scope with readily accessible materials. • Economic one-pot conversion from carbonyl compounds generated N -tosylhydrazones to polymers was realized.

The scale-up of electrochemically mediated atom transfer radical polymerization without deoxygenation

The scale-up of Atom Transfer Radical Polymerization (ATRP) from 15 mL to 15 L (scale-up factor 1000) by galvanostatic simplified electrochemically mediated ATRP (seATRP) is disclosed. An electrochemical O2 scavenging cycle was embedded in the ATRP equilibrium to allow self-degassing of the mixture and avoid impractical deoxygenation procedures. Low volume seATRP of acrylamide in water was optimized by studying various reaction parameters, including the concentrations of scavenger, supporting electrolyte, type of initiator and ligand, and catalyst loading. The target reaction was scaled up from 0.015 L to 15 L. At all volumes and up to 15 L, polymerizations were relatively fast and produced well-defined polyacrylamide with narrow molecular weight (MW) distributions. All reactions were powered by renewable electricity, obtained from a photovoltaic plant. Several water-soluble monomers, including (meth)acrylic acid and methacrylamide, were polymerized at pH ranging from 1.0 to 7.4, without compromising the self-degassing mechanism. These results show a feasible scale-up of seATRP and could foster its adoption by polymer industries promoting a more widespread use of controlled radical polymerizations.

Well-Controlled Living Polymerization of Phenylacetylenes in Water: Synthesis of Water-Soluble Stereoregular Telechelic Poly(phenylacetylene)s.

Well-controlled living polymerization of water-soluble phenylacetylene derivatives in water was achieved for the first time using a multicomponent catalytic system consisting of [Rh(nbd)Cl]2 , an aryl boronic acid, diphenylacetylene having carboxy groups, a tetraalkylammonium hydroxide, and a water-soluble triphenylphosphine. This catalytic system enables a direct synthesis of various water-soluble cis-stereoregular poly(phenylacetylene)s having a narrow molecular weight distribution, the molecular weight of which can be controlled by the initial feed ratio of the monomer to the catalyst. Moreover, the syntheses of water-soluble telechelic poly(phenylacetylene)s having various functional groups at both chain ends as well as a water-soluble block copolymer were achieved.

Well‐Controlled Living Polymerization of Phenylacetylenes in Water: Synthesis of Water‐Soluble Stereoregular Telechelic Poly(phenylacetylene)s

AbstractWell‐controlled living polymerization of water‐soluble phenylacetylene derivatives in water was achieved for the first time using a multicomponent catalytic system consisting of [Rh(nbd)Cl]2, an aryl boronic acid, diphenylacetylene having carboxy groups, a tetraalkylammonium hydroxide, and a water‐soluble triphenylphosphine. This catalytic system enables a direct synthesis of various water‐soluble cis‐stereoregular poly(phenylacetylene)s having a narrow molecular weight distribution, the molecular weight of which can be controlled by the initial feed ratio of the monomer to the catalyst. Moreover, the syntheses of water‐soluble telechelic poly(phenylacetylene)s having various functional groups at both chain ends as well as a water‐soluble block copolymer were achieved.

A Study on the Dual Thermo- and pH-Responsive Behaviors of Well-Defined Star-like Block Copolymers Synthesize by Combining of RAFT Polymerization and Thiol-Ene Click Reaction.

A series of stimuli-responsive star-like block copolymers are synthesized via the combination of reversible addition, fragmentation chain transfer (RAFT) polymerization, and photo-initiated thiol-ene (PITE) click reaction. The controllable block ratio and block sequence, narrow distribution of molecular weight, and customized arm numbers of the star-shaped copolymers reveal the feasibility and benefits of combination of RAFT polymerization and PITE click reaction for synthesis of well-defined star-like (co)polymers. A clear insight into the relationship among the arm number, block sequence, and block ratio of the star-like block copolymers and their stimuli-responsive aggregation behavior was achieved via dynamic light scattering and UV-vis spectroscopy study. Notably, the star-like poly(acrylic acid)-b-poly(2-(dimethylamino) ethyl methacrylate) (star-PAA-b-PDMAEMA) shows higher lower critical solution temperature (LCST) compared to star-PDMAEMA-b-PAA with the same arm number and block ratio due to the inner charged PAA segments at pH > IEP. In addition, for star-like PAA-b-PDMAEMA, higher PAA content enhances the hydrophilicity of the polymer in basic solution and leads to the LCST increase, except for star-PAA1-b-PDMAEMA4 at pH = 9.0 (IEP). For star-PDMAEMA-b-PAA, the PAA content shows minimal effect on their LCSTs, except for the polymer in solution with pH = 9.5, which is far from their IEP. The star-like block copolymers with well-defined structure and tunable composition, especially the facile-controlled block sequence, bring us a challenging opportunity to control the stimuli-responsive properties of star-like block copolymers.

Multimodal stimuli-responsive behaviors of photochromic spiropyran-bearing telechelic poly(2-isopropyl-2-oxazoline)

Spiropyran-bearing telechelic poly (2-isopropyl-2-oxazoline) (SP-PiPrOx-SP) was designed as a multimodal stimuli-responsive polymer. Formed via living cationic ring-opening polymerization and ester coupling of spiropyran units, the resultant SP-PiPrOx-SP had an average molecular weight of 8.0 kDa with a narrow molecular weight distribution. SP-PiPrOx-SP exhibited a sharp hydrophilic-hydrophobic thermal transition at a specific temperature. Moreover, SP-PiPrOx-SP showed reversible photochromic changes between the colorless SP-PiPrOx-SP and purple MC-PiPrOx-MC states upon UV or visible light irradiation. The addition of DBU to MC-PiPrOx-MC resulted in the formation of a stable yellow complex that did not exhibit photochromic behavior. When CO2 was added to the yellow DBU adduct of MC-PiPrOx-MC, the solution immediately changed to purple due to the dissociation of DBU from MC-PiPrOx-MC. The addition of DBU to MC-PiPrOx-MC induced dramatic changes to the thermal transition temperature. Because SP-PiPrOx-SP simultaneously shows thermal transition, photochromic, and chemoresponsive properties, the introduction of spiropyran units has great potential for the design of functional materials.

Nanosphere size control by varying the ratio of poly(ester amide) block copolymer blends

HypothesisBlending amphiphilic triblock (A-B-A) and diblock (A-B) copolymers comprised of the same hydrophobic tyrosine-derived oligomeric B-block and hydrophilic poly(ethylene glycol) methyl ether (mPEG) A-block can provide highly tunable self-assembled nanosphere particle sizes suitable for biomedical applications. ExperimentTriblock and diblock copolymers were synthesized via carbodiimide chemistry and were characterized by nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). The amount of free PEG present in the purified copolymers was determined using a standard addition calibration curve and GPC peak deconvolution methods. Nanospheres were prepared by co-precipitation of each copolymer and of copolymer blends over a range of mole ratios. Nanospheres were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM) and % polymer recovery post-preparation. FindingPrecise synthesis control produced triblock and diblock copolymers with narrow molecular weight distributions and minimal residual reactants. Self-assembled nanosphere particle sizes were 33 nm for the triblock and 129 nm for the diblock, and the size of their blends increased continuously as a function of mole ratio within that biomedically relevant range. Addition of unreacted PEG had minimal impact on either triblock or diblock nanosphere particle sizes whereas addition of unreacted oligomeric B-block increased nanosphere sizes.

1,1,3,3-Tetramethylguanidine-Mediated Zwitterionic Ring-Opening Polymerization of Sarcosine-Derived N-Thiocarboxyanhydride toward Well-Defined Polysarcosine.

Zwitterionic ring-opening polymerization (ZROP) of sarcosine-derived N-thiocarboxyanhydrides (Me-NNTAs) can be induced by using 1,1,3,3-tetramethylguanidine (TMG) initiators in CH2Cl2 at 25 °C, rapidly producing well-defined polysarcosine polymers with controlled molecular weights (Mn = 1.9–37 kg/mol) and narrow molecular weight distributions (Đ = 1.01–1.12). The reaction exhibits characteristics of a living polymerization, evidenced by pseudo-first-order polymerization kinetics, the linear increase of polymer molecular weight (Mn) with conversion, and the successful chain extension experiments. The polymerization is proposed to proceed via propagating macro-zwitterions bearing a cationic 1,1,3,3-tetramethylguanidinium and an anionic thiocarbamate chain end. The TMG not only initiates the polymerization but also serves to stabilize the thiocarbamate chain end where the monomer addition occurs. Because of the enhanced hydrolytic stability of Me-NNTA, the polymerization can be conducted without the rigorous exclusion of moisture, further enhancing the appeal of the method to access well-defined polysarcosine.

The Largest Quasicrystalline Tiling with Dodecagonal Symmetry from a Single Pentablock Quarterpolymer of the AB1CB2D Type.

A quasicrystalline tiling pattern with tile size of ca. 60 nm has been discovered in the bulk state of a four-component pentablock polymer molecule of the AS1IS2P type, where A, S, I, and P denote poly(4-vinylbenzyldimethylamine), polystyrene, polyisoprene, and poly(2-vinylpyridine), respectively. The polymer samples used were prepared by anionic polymerizations and have narrow molecular weight distribution. The sample films were obtained by an extremely slow solvent-cast process from dilute solutions of tetrahydrofuran for 14 days. It has been found by TEM observation that the quarterpolymer, AS1IS2P-4 (Mn = 149 kg/mol, ϕA/ϕS1/ϕI/ϕS2/ϕP = 0.12/0.27/0.20/0.29/0.13), reveals two final stable structures, i.e., a 3.3.4.3.4 periodic tiling pattern as a minor component and a quasicrystalline (QC) tiling with dodecagonal symmetry as a major component, where the former includes a triangle/square number ratio of 2 and the latter has one of approximately 2.28, which is close enough to the ideal ratio, 4/ 2.31, for the triangle/square random tiling of the dodecagonal QC tiling (DDQC). Two structures were also clearly proved by SAXS diffraction patterns. Here, it should be noted this QC structure having a tile side length of ca. 60 nm was created with a single block polymer molecule.

Suppression of Chain Transfer and Promotion of Chain Propagation in Neutral Anilinotropone Nickel Polymerization Catalysis

Both suppression of chain transfer and promotion of chain propagation are highly important but greatly challenging to enhance desirable turnover frequency (TOF) and molecular weight (MW) in olefin polymerization, which is one of the most important chemical reactions. A transition-metal catalyst is the most important key to controlling these fundamental steps. In this contribution, we report the neutral, single-component anilinotropone nickel catalyst Ni6 via an alternative synthetic pathway, which is the isomer of the milestone salicylaldiminato nickel catalyst Ni6-iso. Compared to the classic Ni6-iso, Ni6 produces polyethylenes with elevated TOF (five times), much higher MW (102 times), narrower molecular weight distribution (PDI = 1.05, living fashion at 40 °C), and lower branching density (substantially linear microstructure) in ethylene polymerization. In particular, in the copolymerization of ethylene with a polar comonomer, Ni6 concurrently gives 4.5 times higher TOF, 34 times higher MW, and 1.6 times higher comonomer incorporation relative to Ni6-iso. Mechanistic insights using density functional theory (DFT) calculation reveal both the suppression of chain transfer and promotion of chain propagation. This evidences the anilinotropone nickel catalyst as a promising candidate.

Well‐Controlled Living Polymerization of N‐Propargylamides and Their Derivatives by Rhodium Catalysis

AbstractA substantially improved method for living polymerization of N‐propargylamides and their derivatives has been developed. Rhodium(I) complexes bearing an aryl‐substituted 1,3,5‐hexatriene chain can work as excellent initiators of the polymerization of such non‐conjugated terminal alkynes to give the corresponding cis‐stereoregular polymers having a narrow molecular weight distribution. The typical living nature has been confirmed by investigating the effects of initial feed ratios of the monomer to the initiator on the molecular weight of the resulting polymers as well as multistage polymerization. Moreover, we demonstrated that the present method enables functionalization of both polymer chain ends and synthesis of novel block copolymers consisting of poly(N‐propargylamide) and poly(phenylacetylene) blocks with a narrow molecular weight distribution.

Well-Controlled Living Polymerization of N-Propargylamides and Their Derivatives by Rhodium Catalysis.

A substantially improved method for living polymerization of N-propargylamides and their derivatives has been developed. Rhodium(I) complexes bearing an aryl-substituted 1,3,5-hexatriene chain can work as excellent initiators of the polymerization of such non-conjugated terminal alkynes to give the corresponding cis-stereoregular polymers having a narrow molecular weight distribution. The typical living nature has been confirmed by investigating the effects of initial feed ratios of the monomer to the initiator on the molecular weight of the resulting polymers as well as multistage polymerization. Moreover, we demonstrated that the present method enables functionalization of both polymer chain ends and synthesis of novel block copolymers consisting of poly(N-propargylamide) and poly(phenylacetylene) blocks with a narrow molecular weight distribution.

Organosolv fractionation of a lignocellulosic biomass feedstock using a pilot scale microwave-heating reactor

Bamboo is among the short-rotation and dedicated energy crops which serve as lignocellulosic feedstock for future bio-refinery. A pilot scale microwave fractionation system (PMFS) was developed and used for the organosolv fractionation of bamboo sawdust. The key component of the system, a 1000 L microwave-heating reactor, was designed with unique features for high efficiency of microwave energy utilization and uniform temperature distribution. More than 800 kg reactant can be heated at 65% energy efficiency with temperature differences between top and bottom reactor positions of less than 4 °C. The operational parameters of the PMFS were investigated, and the fractionation products from the PMFS process were characterized and compared with those obtained from lab scale experiments. Higher lignin (93.2%) and hemicellulose (84.7%) removal rates without serious cellulose degradation (cellulose retention 89.1%) were achieved with the PMFS. Recovered lignin has relatively narrow molecular weight distributions (polydispersity index ≈ 2). The mass balance of the fractionation process showed that 81.5% utilization efficiency of the bamboo feedstock was obtained by organosolv fractionation with the PMFS. The results from this study provide insightful information for the industrial implementation of microwave-assisted thermochemical conversion of lignocellulosic biomass.

Olefin polymerization cocatalysts: Development, applications, and prospects

<p indent="0mm">The annual global production of polyolefin products exceeds 100 million tons, and they are widely used in various fields. In contrast to heterogeneous polymerization that produces polyolefins with broad dispersity, single-site transition-metal-catalyzed homogeneous olefin polymerization enables the synthesis of polyolefins with narrow molecular weight distribution, controllable chain segment length, and uniform insertion ratio. It can flexibly tune the molecular structure of polyolefins to provide polyolefin materials with the required properties, which can be applied to high-end applications such as medical packaging and medical equipment. It is important to choose a suitable combination of a catalyst and cocatalyst to obtain tailor-made polyolefins through high-efficiency homogeneous polymerization. The main function of the cocatalyst is to react with the catalyst precursor to generate and stabilize active metal cations, the latter being the center of catalyzing olefin polymerizations. This review summarizes the most important work regarding cocatalysts in the past two decades, including novel structures: Mono/dinuclear boron-, aluminum-, and polymer-based promoters. This review illustrates the importance of the weakly coordinative nature of the counter anion, providing the design principle for the development of a highly efficient cocatalyst. Activation mechanism: The use of an industrial catalysis-related pyridylamido hafnium complex is an example that showcases the complicated reactions of this catalyst with different cocatalysts, shedding light on the importance of choosing the pair of catalyst and cocatalyst. Applications in polymerization: Binuclear cocatalysts and the cocatalysts combined with coordination chain transfer polymerizations are used to synthesize novel structural polyolefins, indicating that the cocatalyst plays a crucial role in alternating polyolefin structures. Cocatalyst-involved catalyst deactivation reactions: This part emphasizes the reaction pathways of cocatalysts that lead to catalyst deactivation, including the replacement of metal and ligand. Further, this review proposes the possible future development of cocatalysts by investigating the reported compounds with high Lewis acidity and the development of novel functionalized cocatalysts. For instance, the development of external stimuli-responsive cocatalyst structures may be one of the future development directions. Combining cocatalysts with existing catalytic methods, such as the coordinative chain transfer polymerization for synthesizing polyolefins with a novel structure, may be the second development direction in the future. With the advancement of computer hardware and algorithms together with experimental data, computational chemistry helps understand the effect of cocatalysts on polymerization. Predicting the structure of new and efficient cocatalysts through big data and artificial intelligence is the third possible future development direction.