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Alkyne PEG (Polyethylene Glycol Alkyne) is a versatile reagent with a terminal alkyne and a functional group, perfect for bioconjugation, click chemistry, and drug delivery. The alkyne easily reacts with azides to form stable triazole bonds, while the PEG spacer boosts water solubility and provides precise control over linker length. Researchers use it widely in ADC research, surface modification, and cell imaging.

Cat# Name Structure M.W. Purity Pricing
Ald-benzyl-PEG-propargyl
AP10112Ald-benzyl-PEG4-propargyl292.33≥95% Pricing
AP10111Ald-benzyl-amide-PEG3-propargyl319.35≥95% Pricing
AP10113Ald-benzyl-PEG5-propargyl336.38≥95% Pricing
AP10110Ald-benzyl-amide-PEG4-propargyl363.4≥95% Pricing
Ald-CH2-PEG-propargyl
AP10043Ald-CH2-PEG3-propargyl186.21≥95% Pricing
AP10044Ald-CH2-PEG4-propargyl230.26≥95% Pricing
Alkyne-ethyl-PEG-t-Butyl ester
AP12294Alkyne-ethyl-PEG1-t-Butyl ester198.26≥95% Pricing
Alkyne-PEG-Iodide
AP13844Iodoacetamide Alkyne223.01≥95% Pricing
AP13745Alkyne-PEG3-Iodide298.12≥95% Pricing
AP15235Alkyne-PEG4-iodide342.17≥95 Pricing
Aminooxy-PEG-Propargyl
AP12335Aminooxy-PEG1-propargyl HCl salt115.13≥95% Pricing
AP12336Aminooxy-PEG3-propargyl203.24≥95% Pricing
AP12337Aminooxy-PEG4-propargyl247.29≥95% Pricing
Aminooxyacetamide-PEG-propargyl
AP10084Aminooxyacetamide-PEG3-propargyl260.29≥95% Pricing
AP10082Aminooxyacetamide-PEG4-propargyl304.34≥95% Pricing
APN-C3-PEG-alkyne
AP12342APN-C3-PEG4-alkyne469.54≥95% Pricing
Bis-Propargyl-PEG
AP12308Bis-propargyl-PEG2138.17≥95% Pricing
AP12309Bis-propargyl-PEG3182.22≥95% Pricing
AP12310Bis-propargyl-PEG4226.27≥95% Pricing
AP12311Bis-propargyl-PEG5270.33≥95% Pricing
AP12321N-Me-N-bis(PEG2-propargyl)283.37≥95% Pricing
AP12312Bis-propargyl-PEG6314.38≥95% Pricing
AP12313Bis-propargyl-PEG7358.43≥95% Pricing
AP12314Bis-propargyl-PEG8402.48≥95% Pricing
AP12315Bis-propargyl-PEG9446.54≥95% Pricing
AP12316Bis-propargyl-PEG10490.59≥95% Pricing
AP12317Bis-propargyl-PEG12578.7≥95% Pricing
AP12318Bis-propargyl-PEG13622.75≥95% Pricing
AP12319Bis-propargyl-PEG14666.8≥95% Pricing
AP12320Bis-propargyl-PEG18843.01≥95% Pricing
Boc-Aminooxyacetamide-PEG-propargyl
AP10085Boc-Aminooxyacetamide-PEG3-propargyl360.4≥95% Pricing
AP10083Boc-Aminooxyacetamide-PEG4-propargyl404.46≥95% Pricing
Boc-HyNic-PEG-Propargyl
AP10604Boc-HyNic-PEG2-Propargyl378.42≥95% Pricing
AP10605Boc-HyNic-PEG3-Propargyl422.48≥95% Pricing
Branched Propargyl PEG
AP12368NH-bis(PEG2-propargyl)269.34≥95% Pricing
AP12362Hydroxy-Amino-bis(PEG2-propargyl)313.39≥95% Pricing
AP12363N-(Propargyl-PEG2)-N-bis(PEG1-alcohol)319.4≥95% Pricing
AP12366N-(Amino-PEG1)-N-bis(PEG2-propargyl) HCl salt356.46≥95% Pricing
AP10791N,N-Bis(PEG2-propargyl)-N-4-oxo-butanoic acid369.41≥95% Pricing
AP12369N-Boc-N-bis(PEG2-propargyl)369.46≥95% Pricing
AP12365N-(Acid-PEG2)-N-bis(PEG2-propargyl)429.51≥95% Pricing
AP10800N,N-Bis(PEG2-propargyl)-N-amido-PEG2-oxyamine444.52≥95% Pricing
AP10799N,N-Bis(PEG2-propargyl)-N-amido-PEG2-thiol445.57≥95% Pricing
AP12370N-(Boc-PEG1)-N-bis(PEG2-propargyl)456.58≥95% Pricing
AP10792N,N-Bis(PEG2-propargyl)-4-oxo-butanoic NHS ester466.48≥95% Pricing
AP14940Tri(propargyl-NHCO-ethyloxyethyl)amine476.6≥95% Pricing
AP12373N-(t-butyl ester-PEG2)-N-bis(PEG2-propargyl)485.62≥95% Pricing
AP12374N-(Propargyl-PEG2)-N-Boc-PEG3-t-butyl ester503.63≥95% Pricing
AP12371N-(Aminooxy-PEG2)-N-bis(PEG3-propargyl)504.62≥95% Pricing
AP10794N,N-Bis(PEG2-propargyl)-N-amido-PEG2-maleimide508.56≥95% Pricing
AP12375N-(Propargyl-PEG4-carbonyl)-N-bis(PEG1-methyl ester)519.59≥95% Pricing
AP12367N-(Amino-PEG2)-N-bis(PEG3-azide)550.66≥95% Pricing
AP12372N-(t-Boc-Aminooxy-PEG2)-N-bis(PEG3-propargyl)604.74≥95% Pricing
AP12364N-(Propargyl-PEG4)-N-bis(PEG4-acid) HCl salt727.84≥95% Pricing
AP14941Tri(propargyl-PEG2-NHCO-ethyloxyethyl)amine740.90≥95% Pricing
AP13713N-bis(PEG2-propargyl)-N-(PEG2-amidoPEG1)-N-(bis(PEG2-propargyl)768≥98% Pricing
AP14942Tri(propargyl-PEG5-NHCO-ethyloxyethyl)amine1137.4≥95% Pricing
Bromoacetamide-PEG-propargyl
AP10952Bromoacetamide-PEG3-propargyl308.17≥95% Pricing
AP13903Bromoacetamido-PEG5-Propargyl396.27≥95% Pricing
DNP-PEG-propargyl
AP10119DNP-PEG4-propargyl397.38≥95% Pricing
DSPE-PEG-propargyl
AP10051DSPE-PEG5-propargyl1034.39≥95% Pricing
AP15079DSPE-PEG-Alkyne ,MW 1K1000≥95% Pricing
AP15080DSPE-PEG-Alkyne ,MW 2K2000≥95% Pricing
AP15081DSPE-PEG-Alkyne ,MW 3.4K3400≥95% Pricing
AP15082DSPE-PEG-Alkyne ,MW 5K5000≥95% Pricing
AP15083DSPE-PEG-Alkyne ,MW 10K10000≥95% Pricing
AP15084DSPE-PEG-Alkyne ,MW 20K20000≥95% Pricing
Folate-PEG-Propargyl
AP10009Folate-PEG3-Propargyl610.62≥95% Pricing
AP15264Propargyl Folate478.469≥95% Pricing
AP15251Folate-PEG4-alkyne654.67≥95% Pricing
HyNic-PEG-Propargyl
AP10594HyNic-PEG2-Propargyl318.38≥95% Pricing
M-PEG-Propargyl
AP10196m-PEG3-Propargyl202.25≥95% Pricing
AP10197m-PEG4-Propargyl246.30≥95% Pricing
AP12338m-PEG8-Propargyl378.46≥95% Pricing
AP10198m-PEG9-Propargyl422.51≥95% Pricing
AP12339m-PEG12-Propargyl554.67≥95% Pricing
AP12340m-PEG17-Propargyl774.94≥95% Pricing
Non-fluorescent alkynes
AP13778Alkyne hydrazide126.16≥95% Pricing
AP13779Alkyne maleimide234.25≥95% Pricing
AP14900F-ara-EdU ((2’S)-2’-Deoxy-2’-fluoro-5-ethynyluridine)270.22≥95% Pricing
AP14246Pentynoic acid STP ester348.20≥95% Pricing
AP14245Hexynoic acid STP ester362.23≥95% Pricing
Norbornene-PEG-Propargyl
AP10566Norbornene-PEG2 Propargyl263.33≥95% Pricing
AP10565Norbornene-PEG3 Propargyl307.39≥95% Pricing
AP10572Norbornene-PEG4 Propargyl351.44≥95% Pricing
AP10568Norbornene-PEG8 Propargyl527.65≥95% Pricing
AP10567Norbornene-PEG12 Propargyl703.86≥95% Pricing
Other Alkyne
AP12600Alkynyl Myristic Acid244.32≥95% (H NMR) Pricing
AP15173Alkyne-PEG4-hydrazide274.31≥95% Pricing
AP12602Alkynyl Stearic Acid280.45≥95% (H NMR) Pricing
AP12656Alkyne Sphinganine297.46≥95% (H NMR) Pricing
AP12652Alkyne Cholesterol368.61≥95% (H NMR) Pricing
AP12654E-Cholesterol Alkyne396.66≥95% (H NMR) Pricing
AP1265327-Alkyne Cholesterol396.66≥95% (H NMR) Pricing
AP12597N-(4-pentynoyl)-galactosamine tetraacylated (Ac4GalNAl)427.4≥90% Pricing
AP12595N-(4-pentynoyl)-mannosamine-tetraacylated (Ac4ManNAl)427.4≥90% Pricing
AP12661O-Alkyne-Trehalose (O-AlkTMM)464.46≥97% (H NMR) Pricing
Propargyl PEG NHS carbonate
AP10097Propargyl PEG4 NHS carbonate329.30≥95% Pricing
AP10096Propargyl PEG5-NHS carbonate373.36≥95% Pricing
Propargyl-PEG-(CH2)3-acid
AP12274Propargyl-PEG4-(CH2)3-acid274.31≥95% Pricing
Propargyl-PEG-(CH2)3-methyl ester
AP12341Propargyl-PEG4-(CH2)3-methyl ester288.34≥95% Pricing
AP10586Propargyl-PEG1-BocNH Tyrosine Methyl Ester377.44≥95% Pricing
Propargyl-PEG-5-nitrophenyl carbonate
AP12327Propargyl-PEG4-5-nitrophenyl carbonate397.38≥95% Pricing
Propargyl-PEG-acid
AP12267propargyl-PEG1-acid128.13≥95% Pricing
AP10290Propargyl-PEG2-acid172.18≥95% Pricing
AP10291Propargyl-PEG3-acid216.23≥95% Pricing
AP10292Propargyl-PEG4-acid260.28≥95% Pricing
AP10293Propargyl-PEG5-acid304.34≥95% Pricing
AP10294Propargyl-PEG6-acid348.39≥95% Pricing
AP12268propargyl-PEG7-acid392.45≥95% Pricing
AP12269propargyl-PEG8-acid436.5≥95% Pricing
AP12270propargyl-PEG10-acid524.6≥95% Pricing
AP10295Propargyl-PEG13-acid656.74≥95% Pricing
AP12271propargyl-PEG14-acid700.82≥95% Pricing
AP10296Propargyl-PEG25-acid1185.34≥95% Pricing
Propargyl-PEG-Acrylate
AP12343Propargyl-PEG2-acrylate154.17≥95% Pricing
Propargyl-PEG-alcohol
AP10272Propargyl-PEG1-alcohol100.12≥95% Pricing
AP10273Propargyl-PEG2-alcohol144.17≥95% Pricing
AP10274Propargyl-PEG3-alcohol188.22≥95% Pricing
AP10275Propargyl-PEG4-alcohol232.27≥95% Pricing
AP10276Propargyl-PEG5-alcohol276.33≥95% Pricing
AP10277Propargyl-PEG6-alcohol320.38≥95% Pricing
AP10994Propargyl-PEG8-alcohol364.44≥95% Pricing
AP12295Propargyl-PEG9-alcohol408.49≥95% Pricing
AP12296Propargyl-PEG10-alcohol452.54≥95% Pricing
AP12297Propargyl-PEG13-alcohol584.7≥95% Pricing
AP12298Propargyl-PEG14-alcohol628.75≥95% Pricing
AP12299Propargyl-PEG18-alcohol804.97≥95% Pricing
Propargyl-PEG-amine
AP12300Propargyl-PEG1-amine99.13≥95% Pricing
AP10278Propargyl-PEG2-amine143.18≥95% Pricing
AP13826Propargyl-PEG3-amine187.2≥95% Pricing
AP10280Propargyl-PEG4-amine231.29≥95% Pricing
AP12301Propargyl-PEG5-amine275.35≥95% Pricing
AP10281Propargyl-PEG6-amine319.39≥95% Pricing
AP10282Propargyl-PEG8-amine407.50≥95% Pricing
AP12302Propargyl-PEG9-amine451.56≥95% Pricing
AP12303Propargyl-PEG10-amine495.61≥95% Pricing
AP10283Propargyl-PEG12-amine583.71≥95% Pricing
AP10284Propargyl-PEG24-amine1112.34≥95% Pricing
propargyl-PEG-azide
AP10131Propargyl-PEG2-azide169.18≥95% Pricing
AP10129Propargyl-PEG3-azide213.23≥95% Pricing
AP10130Propargyl-PEG4-azide257.29≥95% Pricing
Propargyl-PEG-beta-D-glucose
AP12955Propargyl-PEG3-beta-D-glucose306.31≥95% Pricing
AP12956Propargyl-PEG5-beta-D-glucose394.42≥95% Pricing
AP12958Propargyl-PEG5-beta-D-galactose394.42≥95% Pricing
AP12596N-(4-pentynoyl)-glucosamine tetraacylated (Ac4GlcNAl)427.4≥90% Pricing
AP12957Propargyl-PEG5-tetra-Ac-beta-D-glucose562.57≥95% Pricing
AP12959Propargyl-PEG5-tetra-Ac-beta-D-galactose562.57≥95% Pricing
Propargyl-PEG-bromide
AP10961Propargyl-PEG2-bromide207.07≥95% Pricing
AP12304Propargyl-PEG3-bromide251.12≥95% Pricing
AP10986Propargyl-PEG4-bromide295.17≥95% Pricing
AP12305Propargyl-PEG8-bromide471.39≥95% Pricing
AP12306Propargyl-PEG9-bromide515.44≥95% Pricing
AP12307Propargyl-PEG13-bromide691.65≥95% Pricing
Propargyl-PEG-CH2CO2-NHS
AP12280Propargyl-PEG4-CH2CO2-NHS343.33≥95% Pricing
AP12281Propargyl-PEG5-CH2CO2-NHS387.39≥95% Pricing
Propargyl-PEG-CH2CO2-NHS ester
AP10919Propargyl-PEG2-CH2CO2-NHS ester255.22≥95% Pricing
AP10920propargyl-PEG3 NHS acetate299.28≥95% Pricing
AP10921propargyl-PEG6 NHS acetate431.44≥95% Pricing
AP10922propargyl-PEG7 NHS acetate475.49≥95% Pricing
Propargyl-PEG-CH2CO2H
AP13825Propargyl-PEG3-CH2CO2H202.2≥95% Pricing
AP12272Propargyl-PEG4-CH2CO2H246.26≥95% Pricing
AP12273Propargyl-PEG5-CH2CO2H290.31≥95% Pricing
Propargyl-PEG-CH2CO2tBu
AP12292Propargyl-PEG4-CH2CO2tBu302.37≥95% Pricing
AP12293Propargyl-PEG5-CH2CO2tBu346.42≥95% Pricing
Propargyl-PEG-Maleimide
AP13859N-Propargyl-maleimide130.12≥95% Pricing
AP10161Mal-PEG2-propargyl223.23≥95% Pricing
AP13779Alkyne maleimide234.25≥95% Pricing
AP10049Mal-PEG3-propargyl267.28≥95% Pricing
AP10053Mal-PEG4-Propargyl311.33≥95% Pricing
Propargyl-PEG-methyl ester
AP11682Propargyl-PEG3-methyl ester230.26≥95% Pricing
Propargyl-PEG-methylamine
AP12322Propargyl-PEG2-methylamine157.21≥95% Pricing
AP12323Propargyl-PEG4-methylamine245.32≥95% Pricing
Propargyl-PEG-Ms
AP12324Propargyl-PEG3-Ms222.26≥95% Pricing
AP12325Propargyl-PEG6-Ms354.41≥95% Pricing
Propargyl-PEG-NHS ester
AP13740Alkyne NHS ester (hexynoic acid NHS ester)209.20≥95% Pricing
AP13852Propargyl-NHS Ester225.20≥95% Pricing
AP11026Propargyl-PEG1 NHS ester225.2≥95% Pricing
AP10918Propargyl-PEG2-NHS ester269.25≥95% Pricing
AP12275Propargyl-PEG3-NHS ester313.31≥95% Pricing
AP12276Propargyl-PEG4-NHS ester357.36≥95% Pricing
AP12277Propargyl-PEG5-NHS ester401.41≥95% Pricing
AP10998Propargyl-PEG6-NHS ester445.47≥95% Pricing
AP12278Propargyl-PEG7-NHS ester489.52≥95% Pricing
AP12279Propargyl-PEG8-NHS ester533.57≥95% Pricing
AP15078Propargyl-PEG13-NHS ester753.83≥95% Pricing
Propargyl-PEG-phosphonic acid
AP11812Propargyl-PEG3-phosphonic acid252.2≥95% Pricing
Propargyl-PEG-phosphonic acid ethyl ester
AP11813Propargyl-PEG3-phosphonic acid ethyl ester308.31≥95% Pricing
Propargyl-PEG-silane
AP11834Propargyl-PEG3-triethoxysilane419.59≥95% Pricing
Propargyl-PEG-sulfonic acid
AP12326Propargyl-PEG4-sulfonic acid296.33≥95% Pricing
Propargyl-PEG-t-butyl ester
AP12282Propargyl-PEG1-t-butyl ester184.24≥95% Pricing
AP12283Propargyl-PEG2-t-butyl ester228.29≥95% Pricing
AP12284Propargyl-PEG3-t-butyl ester272.34≥95% Pricing
AP12285Propargyl-PEG4-t-butyl ester316.39≥95% Pricing
AP12286Propargyl-PEG5-t-butyl ester360.45≥95% Pricing
AP12287Propargyl-PEG6-t-butyl ester404.5≥95% Pricing
AP12288Propargyl-PEG7-t-butyl ester448.55≥95% Pricing
AP12289Propargyl-PEG8-t-butyl ester492.61≥95% Pricing
AP12290Propargyl-PEG10-t-butyl ester580.71≥95% Pricing
AP13827Propargyl-PEG13-t-butyl ester712.9≥95% Pricing
AP12291Propargyl-PEG14-t-butyl ester756.92≥95% Pricing
Propargyl-PEG-thiol
AP10285Propargyl-PEG3-thiol204.29≥95% Pricing
AP10286Propargyl-PEG4-thiol248.34≥95% Pricing
AP10287Propargyl-PEG6-thiol336.45≥95% Pricing
AP10288Propargyl-PEG8-thiol424.55≥95% Pricing
AP10289Propargyl-PEG12-thiol600.76≥95% Pricing
S-acetyl-PEG-Propargyl
AP11429S-acetyl-PEG4-propargyl290.37≥95% Pricing
t-Boc-aminooxy-PEG-Propargyl
AP12328t-Boc-aminooxy-PEG1-propargyl215.25≥95% Pricing
AP12329t-Boc-aminooxy-PEG2-propargyl259.3≥95% Pricing
AP12330t-Boc-aminooxy-PEG3-propargyl303.36≥95% Pricing
AP12331t-Boc-aminooxy-PEG4-propargyl347.41≥95% Pricing
AP12332t-Boc-aminooxy-PEG5-propargyl391.46≥95% Pricing
AP12333t-Boc-aminooxy-PEG6-propargyl435.51≥95% Pricing
t-Boc-N-Amido-PEG-Propargyl
AP11632Boc-Amine-PEG2-propargyl243.30≥95% Pricing
AP11633t-Boc-N-Amido-PEG3-propargyl287.36≥95% Pricing
AP11634t-Boc-N-Amido-PEG4-propargyl331.41≥95% Pricing
AP11635Boc-Amine-PEG5-propargyl375.46≥95% Pricing
AP11636t-Boc-N-Amido-PEG6-propargyl419.52≥95% Pricing
AP11637t-Boc-N-Amido-PEG7-propargyl463.57≥95% Pricing
AP11638t-Boc-N-Amido-PEG8-propargyl507.62≥95% Pricing
AP11639t-Boc-N-Amido-PEG9-propargyl551.67≥95% Pricing
AP14094t-Boc-N-Amido-PEG10-propargyl595.70≥95% Pricing
AP11640t-Boc-N-Amido-PEG12-propargyl683.83≥95% Pricing
Tetrazine-PEG-Alkyne
AP10718Methyltetrazine-PEG5-Alkyne402.45≥95% Pricing
AP12561Methyltetrazine-PEG4-Alkyne487.56≥95% Pricing
AP11848Methyltetrazine-amido-PEG5-alkyne487.56≥95% Pricing
Thalidomide-O-PEG-Propargyl
AP13003Thalidomide-O-PEG2-Propargyl400.39≥95% Pricing
AP13002Thalidomide-O-PEG3-Propargyl444.44≥95% Pricing
AP12334Thalidomide-O-PEG4-Propargyl488.49≥95% Pricing
AP12127Thalidomide-O-amido-PEG4-propargyl545.55≥95% Pricing
Tos-PEG-alkyne
AP11900Propargyl-PEG2-Tos254.3≥95% Pricing
AP11901Propargyl-PEG3-Tos298.35≥95% Pricing
AP11902Propargyl-PEG4-Tos342.41≥95% Pricing
AP11903Propargyl-PEG5-Tos386.46≥95% Pricing
AP13878Propargyl-PEG6-Tos430.5≥95% Pricing
Trifluoroethyl-PEG-Propargyl
AP116831,1,1-Trifluoroethyl-PEG3-Propargyl226.2≥95% Pricing
AP116841,1,1-Trifluoroethyl-PEG5-Propargyl314.3≥95% Pricing

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    Alkyne PEG (Polyethylene Glycol Alkyne)

    Overview Alkyne PEG is a versatile reagent with a triple-bond alkyne at one end and functional groups at the other. The alkyne makes it perfect for click chemistry and bioconjugation, while the PEG backbone adds flexibility and water solubility. Researchers use Alkyne PEG for drug delivery, surface modification, and advanced research. Key Features
    • Terminal Alkyne Group: The alkyne reacts easily with azides to form stable triazoles under mild conditions. This click chemistry reaction is highly efficient and produces very few side products.
    • Bioisostere Advantage: The triazole bond mimics an amide bond, with minimal impact on the biological activity of linked molecules.
    • Dual Functional Groups: Alkyne PEG has an alkyne on one end and another functional group on the other, allowing for further modifications.
    • Defined PEG Spacer: The PEG spacer controls the length between functional groups and boosts water solubility. This helps in bioconjugation, ADC development, and cell imaging.
    Applications
    • Bioconjugation: Links proteins, peptides, or other molecules with minimal effect on their function.
    • ADC Research: Enables precise linker lengths in Antibody-Drug Conjugates for better therapeutic performance.
    • Cell Imaging: Used in labeling and tracking cells due to the stable triazole bond.
    • Surface Modification: Improves biocompatibility and reduces protein sticking on surfaces.
    • Drug Delivery: PEGylates drugs to increase solubility and reduce immune response.
    Product Specifications
    • Chemical Formula: H-(OCH2CH2)n-OCH2C≡CH
    • Functional Groups: Alkyne at one end, customizable group at the other
    • Purity: 95% or higher
    • Molecular Weight Options: Ranges from 500 to 20,000 Da
    • Forms Available: Liquid or powder
    • Storage: Store at -20°C, away from light and moisture
    Handling and Safety
    • Safety Tips: Use gloves and eye protection. Work in a well-ventilated space.
    • Storage Instructions: Keep in a cool, dry, dark place.
    • Disposal: Follow local regulations for chemicals.
    Ordering Information
    • Catalog Details: Specify the desired weight and functional group.
    • Packaging Options: Available in 1g, 5g, 10g, 25g, or bulk.
    • Custom Orders: Different PEG lengths, weights, and modifications available on request.
    FAQs Q: How does the PEG spacer improve solubility? It adds water solubility, making Alkyne PEG ideal for use in biological settings. Q: What makes the triazole linkage special? The triazole acts like an amide bond, but it has less effect on the function of linked molecules. Related Products
    • Azide PEG: Pairs well with Alkyne PEG for click chemistry.
    • Carboxyl PEG: For further functionalization with amines.
    • Amine PEG: Useful for coupling with carboxyl derivatives.