The ring opening polymerization of ethylene oxide cyclic monomer obtains polyethylene glycol or polyethylene oxide. Ethylene oxide or various epoxides, and other cyclic ethers can be polymerized with anionic, cationic, and coordination type catalyst. For the commercial production of polymer of such type, the most effective catalyst found were (CH3)3N, Na, K, and SnCl4, CaCO3, FeCl3. The other compounds showing catalytic activity were NaNH2, ZnO, SrO and CaO. The resulting polymer bears the following chemical structure: 
The control polymerization of ethylene oxide can be achieved by aqueous alkali as catalyst. The monomer conversion increase linearly with time and that the degree of polymerization (and molecular weight) increased as the reaction proceeded.
Based on the number of repeat units the polymer normally known as poly(ethylene glycol) (PEG) or poly(ethylene oxide) (PEO). If ‘m’ is lower than 455 and the terminal ends of the polymer chain bears hydroxyl group than the polymer term as polyethylene glycol. This formula can be represented in brief as: or if the chemical structure are as than it is called polyethylene glycol mono methylether, or mPEG in brief, in which one terminus is the relatively inert bearing methoxy group (OCH3), while the other terminus is a hydroxyl group can be modified to other functional group such as amino, halogen, sulfonic acid, carboxylic acid etc. Similarly, other alkyloxy groups such as benzyloxy (CH2—C6H5), tert-butoxy (C(CH3)3 ethoxy (OCH2CH3) or any long chain branch or linear alkyloxy can be substituted for methoxy in the above formula.
When a linear or branched polymer's chain ends are functionalized that participate in further chain extension referred as Macromonomers and Telechelics. They are normally low molecular weight polymers and can be distinguished from one another by the functionality of their chain ends, and by the nature of the products resulting from the reactions of their chains ends. The term macromonomer or macromer, (i.e., macromolecular monomer) was introduced by Milkovich in 1974, and can be defined as oligomers or polymers having at least one homopolymerizable end group. Such groups may be either vinylic, acetylenic, acrylic or heterocyclic. The term telechelic is derived from the Greek word ‘tele’=distant and ‘chelos’=claw. This term was proposed by Uraneck, and can be defined as relatively low molar mass macromolecules possessing two reactive end groups. A polymer or oligomer can be considered to be a telechelic if it contains at least one reactive end group that can react selectively to give a bond with another molecule. Depending on functionality, telechelics can be classified as mono, di-, tri-, or poly-telechelic. Similarly when a difunctional carboxylic or sulfonic acid terminated polymer chains are converted to a salt form by neutralization with metal alkoxides in appropriate solvents, the polymers are refer to halato-telechelics as reported by Teyssie. These functionalized polymer bearing functional group at both terminus are normally termed as telechelics and if the one end of the polymer chain is functionalized, it is term as monochelics.
The telechelic polymers can be obtained by “living” anionic or cationic polymerization, or even by stable free-radical process. The living polymerization techniques are unequivocally preferred to other methods in order to control their molecular parameters: molecular weight, homogeneity of each chain length, i.e. low molecular dispersity, microstructure of the polymer backbone, and finally, nature of the end group of the chain. By controlling such parameters, these properly tailored macromolecules can then be used to design new polymeric materials. The end functionalization can be achieved by two different strategies: (1) Either by deactivation of the living species with a suitable electrophile or chain transfer reagent or (2) By initiation of the living process with an organic anionic species that bears the protected functionalized group. However, a disadvantage in the former route to prepare functionalized polymer is that any polymer chain that has been terminated during the propagation will not react with the electrophile, therefore, impairing quantitative functionalization. If such conventional procedures are not available, end functionalized polymer can however be obtained in high yield by appropriate chemical modification of a existing reactive functional group available on the polymer chain. In any event, the functionalization with functional group must be as quantitative as possible for further use of the products. Polymerization of ethylene oxide can be carried out in solvent with high polarity solvent such as tetrahydrofuran (THF), N,N′-dimethylformamide (DMF), methyl sulfoxide (DMSO) etc. The initiator can be alkali metal based compounds such as Na, K, Cs etc. However one can achieved the polymerization of ethylene oxide using lithium based initiator if the polymerization is carried out in the presence of phosphazene base t-BuP4 at 40° C. polymerization temperature.
Branched PEGs architecture are also known. The branched architecture can be synthesized by addition of ethylene oxide to various multifunctional initiator such as potassium salt of polyols that includes glycerol, pentaerythritol, dipentaerythritol, sorbitol or multifunctional polyols etc. These polyols generate three-, four-, six-, eight-, or multi-arm branched PEG respectively. The architecture of these branched polymer can be illustrated as shown below:Core-(—CH2CH2O)n—H)xin which core represents a central core based on the initiator polyol molecule and x represents the number of arms which can range from 3, 4, 6, 8 or more. The terminal hydroxyl groups are readily subject to further chemical modification as desired. Wherein core is a branching core moiety and x is from 3 to about 100 or more. Star like polymers are generally described in U.S. Pat. No. 5,171,264 to Merrill. A branched form of PEG and related polymers is also described in recent patent application U.S. Ser. No. 08/443,383. The branched form has a single terminus per branch that can be chemically modified to other various functional group such as NH2 or COOH, SO3H etc.
The copolymers of ethylene oxide and propylene oxide are closely related to PEG in their chemistry, and they can be substituted for PEG in many of its applications. 
Poly(ethylene glycol) is used in biological applications because it has properties that are highly desirable and is generally approved for biological or biotechnical applications. PEG is clear, colorless, odorless, soluble in water, stable to heat, inert to many chemical agents, does not hydrolyze or deteriorate, and is nontoxic. Poly(ethylene glycol) is considered to be bio-compatible, which is to say that PEG is capable of coexistence with living tissues or organisms without causing harm. More specifically, PEG is not immunogenic, which is to say that PEG does not tend to produce an immune response in the body. When attached to a moiety having some desirable function in the body, the PEG tends to mask the moiety and can reduce or eliminate any immune response so that an organism can tolerate the presence of the moiety.
End-reactive poly(ethylene glycol)s (PEG) are very important class of material in a variety of fields such as biology, biomedical science, and surface chemistry, due to their unique properties such as solubility and flexibility of the chains and basicity of the ether oxygens in the main chain. It is a non-ionic water-soluble polymer. Non-toxic, it has been proven to resist recognition by immune system as well as to display resistance to protein and cell adsorption. These unique properties conjugated to chain flexibility and basicity of the ether oxygen atoms made it eligible for use in a variety of fields such as biology, biomedical science, and surface chemistry.
The effective functionalization of poly(ethylene oxide) chain-end leading to end-reactive polymer become more and more important due to the high versatility of the introduced end groups. One of the most important utilizations of PEG is the construction of polymer brushes, a densely packed layer of tethered polymers anchored on the surface utilizing the end-functionality of the polymer chain. Such a PEG brush significantly changes the surface properties. For example, a PEGylated surface, which means that the poly(ethylene glycol) chains are densely packed on a surface and attached by the end of the polymer chain, shows effective rejection of protein adsorption resulting in a good blood compatibility. In general, commercially available methoxy-ended PEGs having a hydroxyl group at the another terminus are utilized as the starting material for the monochelic PEG preparations. Thus, these PEG surface brushes possess inert free end groups (OCH3 terminal ends). If certain reactive groups can be introduced to the free ends of the brush, an opportunity for the PEG brush will be expanded. For example, the introduction of an affinity ligand to the brush free end changes the surface to be utilized for affinity separation, keeping a low nonspecific adsorption. When PEG is chemically attached to a water insoluble compound, the resulting conjugate generally is water soluble as well as soluble in many organic solvents. When the molecule to which PEG is attached is biologically active, such as a drug, this activity is commonly retained after attachment of PEG and the conjugate may display altered pharmacokinetics. For example, it has been demonstrated that the water insoluble antimalarial, artemisinin, becomes water soluble and exhibits increased antimalarial activity when coupled to PEG. (for example see the report published by Bentley et al., Polymer Preprints, 38(1):584 (1997). Furthermore, U.S. Pat. No. 4,179,337 to Davis et al. discloses that proteins coupled to PEG have enhanced blood circulation lifetime because of reduced kidney clearance and reduced immunogenicity. The lack of toxicity of the polymer and its rapid clearance from the body are advantageous for pharmaceutical applications.
To couple PEG to a molecule such as a protein or a small drug molecule, it is necessary to use an “activated derivative” of the PEG having a functional group at the terminus suitable for reaction with a group on the other molecule. For example, the hydroxyl group of CH3O-PEG-OH can be converted to an aldehyde group, and this aldehyde group can then be covalently linked to a molecule or surface bearing one or more amine groups using the method of reductive amination. An example of this approach is described in U.S. Pat. No. 5,252,714 to Harris and Herati. Detailed investigation for the functionalization of polyethylene glycol reported by Zalipsky for preparation of conjugates, the use of PEG acetaldehyde has been limited by its high reactivity, which leads to condensation side reactions. (See reference published in Bioconjugate Chemistry, 6:150 (1995). Recently the synthesis of PEG bearing at one end aldehyde and other end free hydroxyl group has been reported by Nagasaki et.al published in Macromolecules 1998, 31, 1473 using the initiator bearing protected group such as 3,3-diethoxy propanol (acetal group) that can be converted to aldehyde by hydrolysis at pH 2–3. This procedure allow to synthesize quantitative functionalization PEG bearing one end with aldehyde and other end hydroxyl group.
Preparation of PEG baring carboxylic acid and a hydroxyl group can be achieved by the oxidation of aldehyde group. The selective oxidatation reaction of the aldehyde group are problematic when working with polyethylene oxide polymer bearing free hydroxyl group. Oxidation of such polymer results in a number of impurities and some time degradation of polymer chain, therefore, destroying the architecture. Zalipsky and Barany in J. of Bioactive and Compatible Polymers, 5, 227–231 (1990) describe preparation of the following heterofunctional PEG in which the polymer has a carboxymethyl group at one terminus and a hydroxyl group at the other terminus. There are several other reports on the synthesis of Heterofunctional PEGs using homotelechelic PEG as the starting material for example see U.S. Pat. No. 5,672,662. The synthetic methods, however, are complicated because they have to use several reaction steps to derivatize the PEG terminus. In addition, the efficiency for derivatizations is not very high, meaning that the resulting PEG is a partial mixture of starting homotelechelics and the resulting heterotelechelics.
To date, use of living anionic polymerization has been a successful method for the preparation of end-reactive polymers with theoretical functionalities, narrow molecular weight distribution, and controlled molecular weight. Reactions of the living polymer end-chain with a variety of electrophiles have been carried out to generate different functional groups. This is a particularly suitable way to effect chain end functionalization, given it generates stable polymer end chains once all the monomer is consumed. The existence of the functional end groups renders the polymer important for use in biological and pharmaceutical applications.