Research: Science and Education
Nonclassical or Reactivation Chain Polymerization: A General Scheme of Polymerization Yen Wei Department of Chemistry, Drexel University, Philadelphia, PA 19104;
[email protected] Background Nature is most skilled in the art of biological polymerization to make macromolecules of intricate structure such as nucleic acids, proteins, polysaccharides, and cis-polyisoprene. The foundation of the synthetic polymer chemistry was laid in 1929 by W. H. Carothers, who established two classical polymer growth processes (1), namely, chain and step polymerizations, in modern terminology (2, 3). In classical chain polymerization (eq 1), polymer growth is achieved when a reactive species (Mm*) reacts with a monomer (M) to form a reactive species one unit longer (Mm+1*). Depending on the type of the reactive species, radical (4), ionic (5), group-transfer (6 ), Ziegler–Natta (7), and other metal-mediated or catalyzed (8) polymerizations have been developed. Mm* + M → Mm+1*
(1)
In the classical step polymerization (eq 2), polymer growth is accomplished by reactions of functional groups between two multifunctional reactants of any length (Mm and Mn) to generate a compound of higher molecular weight (Mm+n). Mm + Mn → Mm+n
(12–15), and biological polymerization for the synthesis of nucleic acids and proteins (16 ). Oxidative polymerization is an important method for the synthesis of electrically conductive polymers such as polyaniline, polypyrrole, and polythiophene (17, 18). The mechanism of polyaniline chain growth has been established through detailed kinetic studies (9, 10). As illustrated in Scheme I, the process starts with a dormant chain of aniline oligomer or polymer (i.e., Mm in eq 3). In the presence of an oxidant or applied electric potential, this dormant chain could be oxidized (reactivated) to reactive species such as iminium or nitrenium ions (Mm*). The reactive Mm* attacks a neutral aniline monomer (M); this is followed by deprotonation to afford a new dormant species of higher molecular weight (Mm+1). The process repeats, leading to the formation of higher oligomers and eventually polymers (10). This mechanism is clearly a nonclassical chain polymerization rather than a classical step polymerization as previously believed (3, 17 ). R
Reactivation
A New Scheme of Polymerization
+M
Mm → M m* → Mm+1 reactivation
chain growth
(3)
Again the dormant Mm+1 is reactivated to a reactive Mm+1*, which reacts with the monomer to complete another chain growth step yielding a dormant Mm+2. Such a process may continue to repeat to afford a polymer. The reactivation can be achieved chemically (e.g., oxidation), physically (e.g., heating or radiation), or biologically (e.g., enzymatic catalysis). This polymerization process is neither a classical chain nor a classical step polymerization. However, its characteristic is closer to that of the classical chain polymerization (hence, the process can also be named “nonclassical chain polymerization”). Under proper conditions, the polymerization could be stopped at desired stages by not reactivating the dormant chains, and the process becomes a living polymerization. Major examples of the nonclassical chain polymerization include the oxidative polymerization of aniline (9, 10) and probably other aromatic monomers (11), living radical polymerization
NH2 - 2e-
(2)
Here, a new general scheme of polymerization is proposed—namely, reactivation chain polymerization, which integrates many synthetic and biological polymer growth processes. In this theory (eq 3), the polymer chain propagation is accomplished by reactivating a nonreactive or dormant chain (Mm) to a reactive species (Mm*), which reacts with a monomer (M), leading to formation of a dormant product with higher molecular weight (Mm+1).
NH
R
[O]
+ N H
+ NH2 - H+
R
•• NH +
HN
NH2
Chain growth
- H+
R
HN
HN
NH2
Scheme I R = propagating chain or initiator such as Ph (PPDA) or H (PDA)
Application of the New Scheme We have successfully applied this new theory in the synthesis of aniline oligomers of well-defined structures with controllable end-groups and molecular weights. As predicted by chain characteristics of the nonclassical chain polymerization, the molecular weight of the products should decrease as the [M]/[Mm] ratio is decreased (2, 3). Therefore, various amounts of aromatic amine additives such as N-phenyl-1,4-phenylenediamine (PPDA, an aniline dimer) were introduced into the aniline polymerization system in aqueous HCl solution with (NH4)2S2O8 as oxidant at ᎑5 °C (Scheme I, R = Ph). The reaction conditions were identical to those for conventional aniline polymerization (18) except for the presence of the additives.
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P(St)m–TEMPO
∆
reactivation
552
xST (x ≥ 1)
P(St)mⴢ + TEMPOⴢ → P(St)m+x–TEMPO (4) chain growth
4.5
PPDA
a
PDA
log (Mn )
4.0
3.5
3.0 4.0
PPDA
b Polydispersity
PPDA can be considered as a chain initiator or a dormant chain (M m, m = 2), which is reactivated to Mm* by oxidation. The electrophilic aromatic substitution of Mm* with aniline (M) yields a dormant aniline trimer (Mm+1). The process repeats to give higher oligomers. Since aniline has a much higher oxidation potential (1.03 V vs SCE) than the dimer (0.50 V vs SCE) and subsequent higher oligomers (10), the number of growing chains could be mainly determined by the amount of PPDA initially added to the system. Therefore, an increase in [PPDA] (i.e., decreasing the [M]/[Mm] ratio) should result in a decrease in the molecular weight. Indeed, we have found that the molecular weight decreases significantly as [PPDA] is increased (Fig. 1a). At 20 mol % PPDA, the number-average molecular weight Mn was only ~2600 as determined by gel-permeation chromatography (GPC) with polystyrene calibration. The product is best described as an oligomer. Furthermore, the molecular weight distribution or polydispersity (i.e., the ratio of number- to weight-average molecular weight Mw/Mn) becomes narrower as [PPDA] is increased (Fig. 1b). This observation is consistent with the proposed mechanism. Thus, at low concentration of added PPDA, the total number of growing chains [Mm] is contributed both from [PPDA] initially added and from [PPDA] that was generated by a relatively slow oxidative dimerization of the aniline monomers (10). Consequently, the number of growing chains varies as the polymerization proceeds, leading to the observed high polydispersities. At higher [PPDA], the number of growing chains, predominantly contributed from the initially added PPDA, remains relatively unchanged, resulting in lower polydispersities. In the aniline polymerization with 1,4-phenylenediamine (PDA) as the additive (Scheme I, R = H), both the molecular weight and its distribution also decrease with an increase in the PDA concentration (Fig. 1). Again, PDA can be considered as the initiating dormant chain (Mm) because of its lower oxidation potential (0.62 V vs SCE) than aniline’s (10). The oligomers obtained were found to have two amino endgroups, which can be derivatized to yield many new organic and polymeric compounds (19, 20). It is noteworthy that at the molar ratio of PDA to aniline of 1:2, the polymerization can be stopped at the trimer stage to afford an amino-capped trianiline in one step (19). With either [PDA] or [PPDA] at ≥2–5 mol %, the GPC curve of the product changes from a bimodal pattern (21) to a single narrow peak. One of the recent achievements in synthetic polymer chemistry is the living radical polymerization of vinyl (12–14) or ring (15) monomers. Among the key features is that the propagating radical chain reacts with a capping agent such as 2,2,6,6-tetramethyl-1-piperidinyloxy radical (TEMPO) to yield a dormant species. For the polymer growth (eq 4 with polystyrene as an example [13]), the dormant chain P(St)m–TEMPO is reactivated by thermal dissociation of the TEMPO capping group to generate the radical chain P(St)mⴢ. Depending on the reaction conditions and lifetime of the radicals, one or more monomers (St) are added to the reactivated chain, after which recapping with TEMPO affords a dormant chain of higher molecular weight P(St)m+x–TEMPO. This process is characteristic of nonclassical chain polymerization (eq 3).
PDA
3.0
2.0
1.0 0
10
15
20
25
Initiator (mol−%) Figure 1. Effect of the amount (mol-% based on the aniline monomer) of PPDA and PDA as initiator in the oxidative polymerization of aniline on (a) the number-average molecular weight (Mn) and (b) polydispersity (Mw/Mn) of aniline polymers or oligomers.
The reactivation can also be accomplished photochemically (14). At high concentrations of the capping agents, the stationary concentration of reactivated radicals remains low and the occurrence of chain transfer and termination is minimized or eliminated, resulting in the living polymerization. Similarly, in the metal-catalyzed atom transfer radical polymerization (ATRP)(12b), the reactivation is accomplished through abstraction of halogen atoms by the metal complex from the dormant alkyl halide chains to afford a reactive alkyl radical, which is able to propagate and also to abstract a halogen back from the metal catalyst forming a dormant chain again. Biological polymerization has been considered as the work of nature and therefore as a separated topic, usually independent of the polymerization theories of synthetic polymer chemistry. According to the generally accepted theory in biochemistry (16 ), the synthesis of ribonucleic acids (RNAs) and proteins involves deoxyribonucleic acid (DNA) and messenger RNA (mRNA) templates. In the elongation (polymer growth) phase of RNA synthesis, RNA polymerase moves and unwinds about 17 Watson–Crick base pairs of DNA template in the 3′ to 5′ direction to form a transcription “bubble”. This bubble contains the RNA polymerase, DNA, and nascent RNA with a catalytic site at the 3′ end, where the 3′-hydroxyl of the RNA is so positioned that it can readily react with the α-phosphorus atom of an incoming ribonucleotide triphosphate (16, 22). Once the bubble is formed, the 3′-hydroxyl of the nascent RNA becomes a reactive species, which reacts with a ribonucleotide triphosphate complementary to the base on the DNA template to afford an RNA transcript of higher molecular weight. This newly formed dormant RNA chain (Mm) is reactivated to Mm* with reactive 3′-hydroxyl by moving the transcription
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Research: Science and Education
bubble one base pair in the 5′ direction on the DNA template. Addition of a complementary ribonucleotide triphosphate (M) at the catalytic site yields, again, a dormant RNA (Mm+1) to accomplish another chain growth step. The process repeats and the polymerization proceeds in accordance with the nonclassical chain mechanism (eq 3) until a terminator base sequence on the DNA is reached. Protein synthesis (translation) is achieved following the same general scheme as in RNA transcription (16 ). The movement of a nascent peptidyl-tRNA(Mm) from an A (aminoacyl) to a P (peptidyl) ribosomal site or the associated movement of the ribosome to the next codon on the mRNA template and the binding of aminoacyl-tRNA to the vacant A site can be considered as the chain reactivation. The reaction of the activated peptidyl chain (Mm*) at the P site with the aminoacyl-tRNA (M) accomplishes another step of chain growth to afford a longer dormant chain (Mm+1) at the A site. Similarly, biosynthesis of cellulose (23) involves a relative movement of cellulose synthase and binding of uridine 5′diphosphate-glucose molecules (reactivation) and a cooperative operation of two glycosyl transferase activities to add cellobiosyl units to the cellulose chain (chain growth).
3. 4. 5. 6. 7. 8.
9.
Summary In summary, a new general mechanism of polymerization, the nonclassical or reactivation chain polymerization, has been presented and demonstrated with the oxidative polymerization of aniline, living radical polymerization, and biosynthesis of RNAs, proteins, and cellulose. All the known polymer-forming processes investigated, natural or synthetic, could be interpreted on the basis of 3 general mechanisms: classical chain, reactivation/nonclassical chain, and classical step polymerization. The classical chain polymerization could be considered as a special case of the reactivation chain polymerization when the reactivation process is simultaneous or very fast. Though the full implications are still under exploration, application of the new theory is exemplified by the synthesis of aniline oligomers with controllable molecular weight and end groups as well as low polydispersity. We are now working on the quantification of the conceptual model of the nonclassical chain polymerization and toward the development of one possible unified scheme for all the known polymerization processes.
10. 11.
12. 13. 14. 15. 16. 17.
18.
Acknowledgments I thank R.O. Hutchins, W. Magee, and R. Dickstein for discussions and a cadre of students and associates for experimental measurements. This work was supported partially by NIH (DE-09848) and Akzo-Nobel Corp.
19. 20.
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