Control The Polymerization Of Alkyl Cyanoacrylate Biology Essay

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Due to high reactivity and exceptional adhesive characteristics of poly (alkyl α-cyanoacrylate), in recent years, alkyl α-cyanoacrylate monomers become more and more popular and have been widely used in various aspects of daily life. The α-cyanoacrylates with short alkyl chains are usually used as glue for repairing and do-it-yourself activities [1] while longer alkyl chain α-cyanoacrylates are commonly employed in biomedical field due to their biocompatible and biodegradable properties. One important application of alkyl α-cyanoacrylates in biomedical field is as skin adhesives, surgical glues and embolitic materials with several advantages over traditional materials in surgery. [2-5] Besides, poly (alkyl α-cyanoacrylate) also have been widely employed as drug delivery materials (include nanoparticle, nanosphere, and nanocapsule drug carriers) because it is able to entrap a wide range of biological drug in vivo delivery and release through biodegradation. [6] Other applications of poly (alkyl α-cyanoacrylate) include the detection of latent fingerprints [7], as an electrolyte matrix for dye-sensitized solar cells [8] and other medical applications. [9] The structures of some common alkyl α-cyanoacrylate monomers are showed below (Figure 2.1). [1]

Figure 2.1 Structure of alkyl α-cyanoacrylate: methyl α-cyanoacrylate (MCA), ethyl α-cyanoacrylate (ECA), n-butyl α-cyanoacrylate (nBCA), isohexyl α-cyanoacrylate (IHCA), octyl α-cyanoacrylate (OCA), isostearyl α-cyanoacrylate (ISCA), hexadecyl α-cyanoacrylate (HDCA).

2.2 Synthesis of alkyl α-cyanoacrylate monomers

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Since 1949, many papers have reported the synthesis process of alkyl α-cyanoacrylate monomers. [10-13] In general, there are two steps in the synthesis of alkyl α-cyanoacrylates. In the first place, in the presence of basic catalyst, corresponding alkyl α-cyanoacrylate reacts with formaldehyde via Knoevenagel condensation reaction to form PACA oligomers. The catalyst can either be inorganic (e.g., sodium or potassium hydroxide, ammonia) or organic (e.g., quilonine, piperidine, dialkyl amines). Then the oligomers obtained in last step are depolymerized thermally and pure alkyl α-cyanoacrylate monomers are achievable as products. It is worth noting that the addition of a kind of appropriate stabilizers like loke protonic or Lewis acids and a small amount of a free-radical inhibitors used to prevent repolymerization are necessary (Figure 2.2).

From then on, except several slight modifications and improvements, like making cyanoacrylates bear longer alkyl ester chains through a transesterification approach, [14] or using a more efficient catalyst in the condensation step, [15] the synthetic method remains almost same.

Figure 2.2 Synthesis of alkyl -cyanoacrylate monomer via (a) Knoeenagel condensation reaction and (b) thermal depolymerization.

2.3 Biocompatibility and Biodegradation of Poly (alkyl α-cyanoacrylate) Polymers

The biocompatibility and biodegradation of poly (alkyl α-cyanoacrylate) nanoparticles play a quite significant role on its biomedical applications based on the fact that the drug carrier applied in vivo application must be made of biocompatible, biodegradable or at least excretal materials. In general, poly (alkyl α-cyanoacrylate) can be used as drug carrier all because of its distinguished strategies to degradation. In this field, many studies have reported mechanisms and details so far.

By hydrolysing side chain ester function of poly (alkyl cyanoacrylate), [16-18] corresponding alkyl alcohol and poly (cyanoacrylate acid) are formed as biodegradable products as the latter compound has water-soluble property and can be eliminated by kindney filtration (Figure 2.3 (a)). This strategy plays a predominant role on degradation of poly (alkyl cyanoacrylate) over other degradation mechanisms. It is noteworthy that the length of alkyl side chains and the reaction conditions can influence the hydrolysis greatly. The longer the alkyl side chains, the lower the toxicity and the slower hydrolysis. [16, 19, 20] In terms of surrounding environment, it can be strongly catalysed by esterases from serum, lysosomes and pancreatic juice. [21, 22] As a matter of fact, only low-molecular-weight PACA polymers, typically below 10,000 can be eliminated from body totally.

It has been postulated that 'unzipping' depolymerisation reaction which can be started by a base is another degradation strategy of PACA. [23] Consequently, it is more likely to occur in a biologic media which can offer amino acids of proteins as inducers for reaction (Figure 2.3 (b)). After the depolymerisation of parent polymers, repolymerization occurs immediately and then lower-molecular-weight polymers are formed. So far, the detailed mechanism description has not been reported, may be because the occurrence is too fast to be unambiguously observed.

At last, based on the inverse Knoevenagel condensation reaction, PACA can be degraded and produces corresponding alkyl cyanoacrylate and formaldehyde (Figure 2.3 (c)). Although the formaldehyde might also be formed from hydrolysis of -hydroxyl functions of polymer chains, the hydroxyl ions provided by it have been initially used as an initiator (Figure 2.3 (d)). [24] However, the inverse Knoevenagel condensation reaction in aqueous solution at physiological PH has been less reported, possibly because it is much slower than above-mentioned enzyme-catalyzed hydrolysis mechanism. [25-27]

Figure 2.3 Degradation pathways for poly (alkyl cyanoacrylate) polymers: (a) hydrolysis of ester functions, (b) 'unzipping' depolymerisation reaction, (c) inverse Knoevenagel condensation reaction, (d) release of formaldehyde from hydrolysis of the -hydroxyl functions.

2.4 Polymerization of alkyl α-cyanoacrylate

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Alkyl cyanoacrylate family, as the exotic class of representative vinyl monomers, have two powerful electro-withdrawing groups on the α-carbon of the double bond, ester (COOR) and cyano (CN). And because of that, in the presence of nudeophiles, like anions (hydroxide, iodide, alcoholate, etc.) or weak bases (alcohol, amine, etc.), alkyl α-cyanoacrylate monomers react remarkably and exhibit a high polymerization rate even if the content of above-mentioned additions is low. As a consequence, the pure alkyl α-cyanoacrylates are quite difficult to control and in order to keep these monomers stable, a small amount of acid stabilizers like SO2, sulfonic acid, etc. are necessary. [1]

There are three distinguished methods to synthesize poly (alkyl α-cyanoacrylate): (1) anionic, (2) zwitterionic, and (3) radical (Figure 2.4). Due to high reactivity of alkyl α-cyanoacrylate derivates, anionic and zwitterionic polymerization are considered as two predominant methods for polymerization, especially under traditional reaction conditions in terms of a pure radical process.

Figure 2.4 Initiation and propagation steps in (a) anionic, (b )zwitterionic and (c) radical polymerization of alkyl cyanoacrylate monomer with a base (B-), a nucleophile (Nu), and a radical (P-) as initiators, respectively.

2.4.1 Synthesis of homopolymers

In this field, Peter and his team accomplished an extensive work in order to get a better understanding of the influences of experiment conditions on polymerization results. [28-31] As a matter of fact, simple anions (CH3COO_, CN_, I_, etc.) or covalent organic bases (Et3N, pyridine, etc.) can initiate the homopolymerization of ethyl cyanoacrylate (ECA, Figure 2.1) and n-butyl cyanoacrylate (nBCA, Figure 2.1) in solution, resulting in anionic or zwitterionic polymerization respectively. [28] Regarding zwitterionic polymerization of nBCA, the nature properties of initiator and other reaction conditions (the kind of inhibitors, presence of water, etc.) can influence some main characteristics of the polymer products (like number-average molecular weight and molecular weight distribution) as well as polymerization kinetics (such as monomer conversion, polymerization rate and so forth). With respect to anionic polymerization, the same research group accomplished the polymerization of nBCA at 20-40℃ in tetrahydrofuran (THF), using tetrabutyl ammonium salts (hydroxide, bromide, acetate, and substitute acetates) as the initiators. And they reported a nearly ideal living polymerization can be achieved with the presence of hydroxide-based initiator. [32-34]

Although anionic and zwitterionic polymerization are more likely to happen for the polymerization of alkyl α-cyanoacrylates, free-radical polymerization is considered as the main approach to extend polymeric chain during homopolymerization [35-38] and copolymerization [38, 39] in bulk with a suitable inhibitor in the reaction medium. It is noteworthy that anionic polymerization still cannot be inhibited completely even under such a specific inhibition condition, but it can be neglected with respect to the timescale of the polymerization reaction. In 1960, Canale et al. [35], using boron trifluoride-acetic acid compound as inhibitor, successfully accomplished free-radical polymerization of MCA at 60-C initiated by azobisisobutyronitrile (AIBN). Bevington et al. [37] used propane-1,3-sultone as inhibitor for the free-radical polymerization of MCA in bulk or in 1,4-dioxane at 60-C, initiated by AIBN or benzoylperoxide (BPO). In 1983, Yamada et al. [38] polymerized ECA in bulk at 30-C with a small amount of acetic acid or propane-1,3-sultone and they reported a very high propagation rate constants: in the presence of acetic acid and in the presence of propane-1,3-sultone. As a comparison, methyl methacrylate (MMA) which is considered as a highly reactive monomer gave at 30-C. [40]

2.4.2 Synthesis of copolymers

Via a free radical process (using trifluoride-acetic acid complex as an efficient inhibitor against anionic polymerization), alkyl α-cyanoacrylates can react with more 'common' vinyl monomers to produce a wide range of copolymers. The kind of resulting copolymers greatly depends on the nature properties of comonomer. [39] The studies have reported that random copolymers with MMA were achievable in bulk while alternating copolymers with styrene were obtained in benzene solution at 60 with AIBN as initiator. With respect to the nature of bulk, random copolymers with 10% MMA show similar properties to PMCA homopolymer, on the contrary, alternating copolymers with styrene showed an improved thermal stability compared with random copolymers. Hall et al., who previously did research on reactions of electron-rich olefins with electron-poor olefins, [41-43] proved the alternating copolymers starting from a styrene and MCA mixture with a 1:1 proportion, can either be produced under AIBN initiation and UV light at 40 in benzene solution or be produced spontaneously at room temperature. [44] However, when copolymerize MMA with other comonomers such as isobutyl vinyl ether, p-methoxystyrene or -bromostyrene, the result is mixtures of (co)polymers and/or small adducts. [44]

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In 1978, Buck proposed a comprehensive synthetic strategy of bis(alkyl cyanoacrylate)s starting from anthancene adducts. [45] The copolymerizations of these difunctional alkyl monomers based on cyanoarylates with monofunctional alkyl cyanoacrylates like MCA and isobutyl cyanoacrylate lead to crosslinked macromolecular adhesive compositions. These adhesive compounds exhibit excellent mechanical properties under both dry and wet environments compared with noncrosslinked counterparts who could be desirable applied as pit and fissure sealant in density.

Like poly(ethylene glycol) (PEG) and PACA blocks which have more complicated macromolecular architectures like diblock and triblock copolymers can also be produced in homogeneous media through zwitterionic polymerization. [46] The synthesis involved the preparation of triphenylphosphine end-capped monohydroxyl and dihydroxyl PEGs, offering corresponding monofunctional and difunctional macrozwitterionic initiator. In the subsequent step, the polymerization of IBCA was initiated by every macroinitiator in THF at room temperature to achieve PIBCA-b-PEG diblock and PIBCA-b-PEG-b-PIBCA triblock copolymers with tuneable compositions in good match with the initia stochiometry.

Peracchia et al. have reported the synthesis process of poly[(hexadecyl cyanoacrylate)-co-methoxypoly(ethylene glycol) cyanoacrylate] [P(HDCA-co-MePEGCA)], which is a kind of comb-like copolymers showing amphiphilic properties. [47] Hexadecyl cyanoacrylate and PEG monomethyl ether cyanoacrylate react with formaldehyde via Knoevenagel condensation reaction in the presence of dimethylamine as the catalyst (Figure 2.5). Due to the slow, in situ formation of the cyanoacrylate monomers, the polymerization is easier and better to be controlled than a direct ionic polymerization. What's more, by changing the initial cyanoacrylates feed ratio, the composition of the copolymer (and thus its hydrophilicity/hydrophobicity) can be adjusted easily. [1]

Figure 2.5 Synthesis of random, comb-like poly[(hexadecyl cyanoacrylate)-co-methoxypoly(ethylene glycol) cyanoacrylate] [P(HDCA-co-MePEGCA)] copolymer via knoevenagel condensation reaction.

2.5 RAFT polymerization

2.5.1 Introduction

Reversible addition-fragmentation chain transfer (RAFT) polymerization is a kind of living radical polymerization (LRP) and exhibits many versatile advantages over other living radical polymerizations, such as atom transfer radical polymerization (ATRP) and nitroxide-mediated polymerization (NMP). It was first reported in 1998 by CSIRO. In order to mediate the polymerization through a reversible chain-transfer process, RAFT polymerization adds a chosen quality of thiocarbonylthio compounds like dithioesters, dithiocarbonates, trithiocarbonates and xanthates as RAFT agent to a conventional living radical polymerization. This is the main difference from conventional living radical polymerization. [48]

The Z and R group of RAFT agent play different roles in RAFT polymerization (Figure 2.6). [48] The Z group greatly affects the stability of the thiocarbonyl-thio radical intermediate. As a consequence, the strong stabilizing groups help to form intermediates and contribute to a high reactivity of C=S bond toward radical addition. The R group must be a good leaving group compared with growing polymeric chain so that new polymer chains can be initiated. Radical initiators, such as azobisisbutyronitrile (AIBN) and 4,4'-Azobis(4-cyanovaleric acid) (ACVA) are widely used. Due to the low concentration of RAFT agents in reaction system, the initiators usually have a lower concentration than that in conventional radical polymerization.

RAFT polymerization is applicable with a wide range of monomers and processes quite efficiently. What's more, the molecular weight of polymer can be decided in a easy way before reaction and the molecular weight distribution can be controlled under a pretty good result. Polydispersity indexes can also be controlled in a narrow and expecting rang. Besides, RAFT polymerization is useful to design polymers with desirable architectures such as linear block polymer, star copolymer, grafting polymer and so forth. As a result, RAFT polymerization has been regarded as the most promising polymerization technique.

Figure 2.6 Structure of RAFT agents.

2.5.2 Mechanism of RAFT polymerization

During reaction, transfer of the chain-transfer agent (CTA) between growing radical chains exhibits a low concentration, while the concentration of dormant polymeric chain is at a higher value in order to control the growth of molecular weight and restrict termination reaction. In general, the process of RAFT polymerization includes five steps (Figure 2.7). [49-51]

Step (: Initiation

The reaction is initiated by initiators, such as Azobisisobutyronitrile (AIBN) or 4,4'-Azobis(4-cyanovaleric acid) (ACVA). In this step, the initiator (I) reacts with the monomer (Ki) to create a radical species which is used to start an active polymerizing chain.

Step (II): Addition-Fragmentation

The active chain (Pm) quickly reacts with the reactive C=S bond of the chain transfer agent (CTA) (7) (Kadd) and then kicks out a re-initiating group (R). In this step, the radical intermediate (8) is obtained. Because it is able to lose either the leaving group (R) and a macro chain-transfer agent (macro-CTA; 9) (Kβ) or the radical species (Pm), the fragmentation of the intermediate is reversible.

Step (III): Reinitiation

The R group then re-initiate the polymerization (Kre-in) by means of reacting with the monomers and starting a new polymer chain which is then able to propagate (Kp) or react back with the macro-CTA(Kβ). When the initial CTA is finished up, the macro-CTA presents uniquely in the reaction medium and goes to the next step - equilibrium.

Step (IV): Equilibrium

This step is regarded as the fundamental step in the RAFT process. Active and dormant (thiocarbonylthio capped) chains exchange quickly to ensure equal probability for all chains to grow. As a consequence, the resulting polymers all have a narrow molecular weight distribution.

Step (V): Termination

The termination of reaction is inevitable and occurs in all free-radical polymerization systems by either combination (Ktc) or disproportionation (Ktd). However, the final resulting polymer is comprised of a large majority of polymeric chains exhibiting the re-initiating groups (R) at one end and the thiocarbonylthio groups at the other end due to the termination reactions are controlled under a minimum. This structure has been proved by various analytical techniques, such as 1H NMR, UV spectroscopy and showed in many reports. What's more, these studies also have reported that a small number of chains terminated by the thiocarbonylthio part have been initiated by the free-radical initiator. [52, 53]

Figure 2.7 Mechanism of RAFT polymerization

According to the mechanism, the following remarks can be concluded: [48]

The mechanism of RAFT polymerization is quite different from that of ATRP or NMP, because the chain grows depending on the cooperative chain transfer between polymeric chains (bimolecular reactions) rather than reversible radical capping (monomolecular reaction).

The source of radicals triggers the degenerative chain transfer, leading to the growth of polymeric chains. With the increasing concentration of radical, the rate of polymerization will increase but the probability of chain termination will also increase which may cause a higher molecular weight distribution of product polymers.

The majority of the polymeric chains are initiated by the CTA re-initiating group (R group) and terminated by the thiocarbonyl-thio group (Figure 2.8).

As conversion rate increases, the molecular weight increases linearly and can be predicted. If we assume that all CTAs have reacted and neglect the chains started by the source of radicals, via the equation bellow:

Mn,theo - the theoretical number-average molecular weight

[Monomer] and [CTA] - the concentration of the monomer and CTA, respectively

FW(CTA) and FW(CTA) - the monomer and CTA formula weight, repectively

c - the fractional conversion (Figure 2.9).

Figure 2.8 Overall reaction in RAFT polymerization

Figure 2.9 Evolution of Mn and PDI with the monomer conversion in the bulk polymerization of methyl acrylate at 60 mediated by PEDB in the following concentrations: (-), (-), (-), (-).

2.5.3 RAFT Polymerization Processes

A representative advantage of RAFT polymerization is the polymerization process. Comparing to conventional free radical polymerization, RAFT polymerization only needs the involvement of a CTA which means, under the same reaction conditions, the RAFT polymerization does not need any modification of existing setups and this makes large-production get a more satisfactory result. By far, RAFT polymerization can be conducted in many kinds of process, such as bulk, solution, emulsion and miniemulsion.

2.5.3.1 Bulk

Up to date, bulk polymerization is the simplest process for RAFT polymerization. However, during the polymerization process, the viscosity increases when conversion rate reaches a high value, leading to a negative effect on the processing of final resulting polymer. In general, polymerization in bulk is faster than that in solution. For instance, Zhu etal. [54] reported that at 60, the polymerization of glycidyl metharylate reached 96.7% conversion in bulk and 64.3% in benzene compared with 50% conversion of monomer for the same reaction time. But the polymerization in bulk does not lead to a higher polydispersity. For example, under the same experiment conditions, the polymers produced by 4-acetoxystyrene and isobutyl methacrylate in the bulk both have a narrower polydispersities than those produced from solution or emulsion. [55, 56]

2.5.3.2 Solution

Solution polymerization solves the problem for high glass-transition temperature polymers in bulk that the viscosity increases under a high conversion rate, but it is slower and can result in a little higher polydispersity than that of bulk polymerization. [55, 56] One dominant merit of solution polymerization is allowing the copolymerization of monomers which are not miscible.

2.5.3.3 Emulsion

Emulsion polymerization is a promising process for polymer synthesis and gives good heat transfer when the viscosity of reaction system is at a low value, even at high polymer loadings. [57] RAFT polymerization in emulsion is first reported by CSIRO group with the polymerization of butyl methacrylate. [58] Most researches have paid attention to polymerization of styrene while some focus on polymerization of BA [59] and methacrylates. [60, 61] Considering the polymerization of methacrylate, the conversion of monomer into polymer in emulsion was kept at a high value (often 90%). The RAFT agent with a low molecular weight was polymerized in first lab initio step. But in this step the control result was not quite good. Fortunately, the poor polydispersity achieved in this step did not influence the control in the subsequent steps of polymerization.

In recent years, a new method to RAFT emulsion polymerization has been reported. [62, 63] In the first place, in order to produce a macro-RAFT agent, a water-soluble monomer (AA) was polymerized in water phase to a low degree of polymerization. Then, a hydrophobic monomer (BA) was added at a controlled feed ratio to produce oligomers whicn are used to form rigid micelles. These comprise a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. This process is similar to the 'self-stabilizing lattices' process we have previously introduce in macro-monomer RAFT polymerization which involves sequential polymerization of methacrylic acid and non-polar methacrylates. [64] These two processes both can allow emulsion polymerization with no surfactant adding.

RAFT polymerization in mini-emulsion has also been reported. [60, 65-70] The polystyrene with a narrow polydispersity can be produced via RAFT polymerization in mini-emulsion in a batch process. [60] Some retardation is found with dithiobenzoate RAFT agents. [60, 66] However, when using dithioesters [66] or trthiocarbonate RAFT agents [67], the retardation reduces remarkably. In conventional mini-emulsion polymerization, the high concentration of surfactant and co-stabilizer which is typically used is an important issue should be noted. Recently, Pham et al. [67] have reported surfactant-free mini-emulsion polymerization. The process used amphipathic macro-RAFT agents as sole stabilizer which is synthesized in situ via polymerization of AA. This approach eliminated secondary nucleation of new particles and contributed to a latex without labile surfactant and good particle size control.

2.5.4 Application of RAFT polymerization

2.5.4.1 Block copolymers

RAFT polymerization is regarded as one of the most versatile strategies to synthesize block copolymer and so far, many studies about block copolymer synthesis have been reported. RAFT polymerization conducts with retention of the thiocarbonylthio group, allowing an easy way to synthesize AB diblock copolymer by simple introduction of another monomer (Figure 2.10). [72] Triblock copolymers (ABA, ABC, etc.) are polymerized similarly by adding further monomers one be one. One of the most interesting issues is the capability of making hydrophilic-hydrophobic block copolymers in which the hydrophilic part is comprised of unprotected polar monomers such as acrylic acid, dimethylaminoethyl methacrylate, or ethylene oxide. The hydrophilic-hydrophilic block copolymers comprised of acrylic acid and N-isopropylacrylaminde (NIPAM) has also been synthesized. [73]

In RAFT polymerization, the sequence of introducing the blocks of a block copolymer is quite significant. [73, 74] The propagating radical for the first formed block must be a good hemolytic leaving group in terms of that of the second block. For instance, when synthesize a methacrylate-acrylate or methacrylate-styrene diblock, the methacrylate block should be firstly prepared, [75, 77] because the propagating radicals of styrene or acrylate is poorer compared with methacryate propagating radicals.

Block copolymers derived from polymers synthesized by other polymerization mechanisms can be prepared firstly by producing a pre-polymer which have thiocarbonylthio groups. And then use pre-polymer as a macro-RAFT agent for the subsequent steps. (Figure 2.11). [72,76] This approach is firstly used to synthesize poly(ethyl oxide)-block-PS from commercial product hydroxyl end-functional poly(ethylene oxide) [72,76] Other block copolymers polymerized in a similar way include poly(ethylene-co-butylene)-block-poly(S-co-maleic anhydride), [77] poly(ethylene oxide)-block-poly(MMA), [78] poly(ethylene oxide)-block-poly(N-vinyl-formamide) [79] and so on.

Figure 2.10 Synthesis of AB diblock copolymer

Figure 2.11 AB diblock synthesis from end-functional polymers through RAFT polymerization.

2.5.4.2 Star copolymers

Many studies have reported the synthesis process of star copolymer throygh RAFT polymerization by using a wide range of multifunctional CTAs. It is worth noting that by functionalization of either R substituents or the Z substituents, the core of star can be formed and this is the main reason for the the RAFT process is different from other living radical polymerization like ATRP or NMP.

The use of the R group from CTA produces a similar structure of star (co)polymer compared with those gained from ATRP or NMP. In general, since the polymerization of monomers is mainly terminated by combination, the conversions should be controlled under a low value in order to avoid star-star coupling. What's more, aiming to keep the number of dead linear polymeric chains to a minimum, the concentration of radicals which are used to initiate the AFCT reactions should be maintained at a very low value. When these factors are considered, polydispersities ranging from 1.1 to 1,4 can be obtained. [48]

On contrary, the use of Z group for the star copolymers gives a unique and special result to RAFT polymerization. In this way, the polymeric arms detaches from the core as they grow and then they react back onto the chain for chain-transfer reaction. Coupling happens not between stars but between arms. As a result, the polydispersity of star copolymers is quite narrow as the PDI value is around 1.1, which means a quite excellent result. The conversion of polymerization is higher than that in the growing approach. Stetic hindrance, however, may play a negative role on arms, resulting in a high molecular weight as it is difficult for chains to react back on the thiocarbonyl-thio moiety linked to the core. Furthermore, it is worth noting that in this type of architecture, the thiocarbonyl-thio is the key part for arms attaching to the core and a integrally part of the polymeric structure as a consequence. [48]

2.5.4.3 Grafting polymers

Linking grafted polymers onto the polymer backbone by non-controlled radical polymerization can contribute to a broad molecular weight distribution and a high polydispersity. Even applying other controlled living radical polymerization methods such as ATRP and NMP, the polymeric microsphere would often need derivitization. But by using RAFT polymerization, grafting from microspheres can finish in only one step and the grafted polymer with a RAFT end group can reinitiate the chains to form block copolymer shells. [80]

2.5.5 Reaction Conditions (Temperature, Pressure, Solvent, Lewis Acids)

So far, the detailed studies of the influences of temperature on the process of RAFT polymerization have not been reported. Temperatures reported about RAFT polymerization range from ambient temperature to 140. There are some data shows that the molecular-weight distribution of resulting polymer is narrower when RAFT polymerization is processed at a higher temperature. [7] Besides, at higher temperatures, the retardation with dithiobenzoates is less. Regarding MMA polymerization with trithiocarbonate RAFT agent 1 (Figure ), the temperature has little influences on molecular-weight distribution. (Figure 15). However, it is worth noting that the higher temperature provide higher rates of polymerization which means in a shorter reaction time a given conversion is achievable.

Figure Trithiocarbonate RAFT agent 1

The studies for RAFT polymerization under high pressure have been reported. [161]-[163] The termination of RAFT polymerization is in the form of radical-radical termination, allowing polymer products have higher molecular weight and higher polymerization rates comparing to that achieved under ambient pressure. [-章本身]

The RAFT polymerization is compatible with a variety of reaction media containing protic solvents like alcohols and water [13][14][164][165] and less traditional solvents like ionic liquids [166] and supercritical dioxide. [167] In aqueous media, polymers have been polymerized successfully via RAFT polymerization. However, attention should be paid to the hydrolytic sensitivity of some RAFT agents. [243] The roughly relationship between RAFT agents have been found (dithiobenzoates trithiocarbonates aliphatic dithioesters). [-章本身]

RAFT polymerization can be processed in the presence of Lewis acids. There are many studies about attempted tacticity control homopolymers [91][168-170] to polymerize stereoblock copolymers. [171] and control alternating tendency for polymerization with Lewis acids as additives. [172][173] Take MMA polymerization as an example, by adding Lewis acids scandium triflate, Sc(OTf)3, the fraction of isotactic triads increases and the rate of polymerization in traditional radical [174] and RAFT course [168][170][171] improves apparently. Molecular weight and polydispersity are poor controlled by using dithiobenzoate RAFT agent 8 [170] or 22 [168][170][171]and Sc(OTf)3. By NMR analysis, [170] the reasons to poor results can be found that Lewis acid causes degradation of the dithiobenzoate RAFT agents. In addition, further studies show that the trithiocarbonates are substantially more stable. Therefore, the polymers produced with trithiocarbonate RAFT agent 58 have narrow molecular-weight distributions (). In addition, the molecular weights as well as effect on tacticity are achieved expectedly.

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