2.5 Reversible chain-transfer catalyzed polymerization
2.5.2 Catalysts
As a crucial requisite of a suitable catalyst, the affinity ofG• toward iodine should be high enough to allow frequent activation of polymer−I while on the other hand,G−Ishould still be able to readily give away its iodine
GeI4 GeI3
Figure 2.3 Exemplary catalysts used in RTCP added in the form of either the deactivating G−I or in the form of G−H, which yields the activating G• after hydrogen-atom abstraction.
atom to allow frequent deactivation ofpolymer•. To date, several different compound classes have been applied as catalysts, includingGe-,[26,77]Sn-,[26]
N-,[28,78–80,82]P-,[26,28,83,84]O-,[29] andC-centered[29] compounds. Examples are given in Figure 2.3. The catalysts can either be employed as deactivating G−I, i. e., already bearing an iodine atom, or in the form ofG−H. In the latter case, the hydrogen atom can be abstracted by fragments of the conventional initiator for example, which in situ gives the activatingG•.[26,28,29,83]
2.5.3 Kinetics
Catalyzed reversible chain-transfer versus DT
RTCP catalysts can be employed in rather low concentrations (down to about 1 mmol L−1) and still significantly improve molar-mass control compared to the corresponding catalyst-free ITP system. In common RTCP systems, iodine transfer between polymer and catalyst molecules (kda,RT andka,RT in Scheme 2.8b on the preceding page) was found to be about 102 to 103 times faster than between two polymer molecules (kex in Scheme 2.5b on page 17).[26,27,85]In this context, the addition of catalyst led to a systematical increase of the activation–deactivation frequency of living chains. For sufficiently high catalyst concentrations and well-controlled RTCP systems, DT is therefore expected to be negligible regarding its impact on molar-mass control.[85,86]
Rate retardation
In contrast to ITP systems, rate retardation was observed in all kinetic studies of RTCP systems.[26,27,85] It is ascribed to the irreversible cross-termination reaction betweenG•andpolymer• (similar to cross-termination between the intermediate radical andpolymer• in RAFT polymerizations).[87] The extent of rate retardation was found to depend on the structure of bothG• andpolymer•. Besides its adverse impact onRp, cross-termination should lead to a slow but steady depletion of the catalyst,[86] which results in reduced activation and deactivation as polymerization proceeds. Guidelines were developed to overcome this problem by a repeated addition of the catalyst.[86]
CHAPTER 3
RITPs and RITP-based RTCPs of methyl methacrylate
ITP systems have been successfully applied to a wide range of monomers forming secondary propagating radicals (e. g., styrenics,[69] acrylates,[88]
vinyl acetate[89]). While for an effective reversible deactivation of these polymer classes, the utilization of only moderately active CTAs is sufficient, monomers givingtertiary propagating radicals like methacrylates require highly activated CTAs, as it is illustrated in Scheme 2.7 on page 18. Unfortu-nately, for iodo CTAs, high activities regarding iodine transfer are inherently accompanied by weakC−Ibonds, resulting in low stability and potential decomposition of the CTAs during synthesis, isolation, or storage.[58,63]
To overcome this problem, Lacroix-Desmazes et al. invented the RITP in 2005,[24] in which a highly activated iodo CTA is produced in situ right before polymerization sets in. Details on the mechanism will be given in the following section. RITP was shown to allow for molar-mass control of methacrylates[25] and has already been used in several homo- and heteroge-neous systems.[24,90,91] Because of its well-feasible procedure and low costs applying rather basic chemicals, RITP has even become popular for several of the above-mentioned secondary-radical polymers.[8,24,92,93]
In 2008, Goto et al. suggested that RITP systems of methacrylates can be adapted to RTCP as the addition of catalysts resulted in an improved chain-growth control.[27] As presented in Scheme 2.8 on page 25, in RTCP systems, iodine atoms are expected to be transferred between living chains and
catalyst molecules. Kinetic studies[26,27,85] demonstrated that the activation–
deactivation frequency of living chains systematically increases when po-tential catalysts are added to ITPs of St and MMA. For all RTCP systems in-vestigated in the literature so far, evolutions ofMnas a function of monomer conversion indicate thatMnis mainly determined by the stoichiometry of iodine (=iodo CTA) and not of the catalyst (see Equation 2.10 on page 16 or Figure 2.1 on page 20). This is in accordance with the mechanistic theory of RTCP and indicates that the capping agents of dormant chains are indeed iodine atoms. However, to the best of knowledge, no thorough end-group analysis of polymer produced by RTCP has yet been conducted to investigate the capping-agent species or a potential impact of the catalysts on chain-end functionality of the obtained polymer.
Electrospray-ionization mass-spectrometry (ESI-MS) is based on a rather soft ionization technique and offers great accuracy and high sensitivity. It is thus a powerful tool to investigate the structure of macromolecules.[94]
Particularly the macromolecules’ end-groups—which form during initiation and termination—straightforwardly offer information about the mechanism and kinetics of a polymerization process and are hence frequently used to study into both RDRP[95,96]and conventional RP systems.[97–99]
In the present chapter, ESI-MS was chosen as the ideal technique for end-group analysis of polymer produced via RITP and RITP-based RTCP systems. As the representative of the commonly employed methacrylate family, MMA was the monomer of choice. PolyMMA is indeed especially suitable for ESI-MS analysis as its numerous carboxyl functions are readily attached by ions, which leads to a high spectral signal-to-noise ratio (in con-trast to polySt, for example). It should be noted that end-group analysis may not only give information about the nature of the capping agent in an RTCP.
It can also clarify the impact of undesired side-reactions by the catalyst on the polymerization process. In this context, the activating catalyst species G• might contribute to initiation or termination (=cross-termination with polymer•, cf. Section 2.5.3 on page 26). Both reactions would lead to an adverse depletion of the catalytic species and a continuously decreasing quality of chain-growth control during polymerization. In addition, infor-mation about the impact of catalysts on chain-end functionality is crucial when further processing of living polymer is planned.
RITPs of MMA were conducted and analyzed in comparison to RTCP systems using three different catalysts, namely (1)N-iodosuccinimide, NIS, as well as theH-phosphonic acid derivatives (PADs) (2) diethyl phosphonate, (EtO)2P(O)H and (3) 4,4,5,5-tetramethyl-1,3,2-dioxaphospholane 2-oxide,
½ AIBN
CP CP–I +CP–polyMMA CP + CP–polyMMA–I ktr
Scheme 3.1 Simplified RITP mechanism on the basis of a system with (i) MMA, (ii) molecular iodine (I2), and (iii) AIBN. During the inhibition pe-riod, radicals almost exclusively react with free iodine (I2or I•) with almost no monomer conversion. When free iodine is consumed, the polymerization period starts according to a common ITP with the pre-equilibrium (top) and main equilibrium (bottom).[24,25]
PinP(O)H(for structural formulas see Figure 10.4 in the Experimental Section on page 220). While NIS and (EtO)2P(O)Hare well-established catalysts for the investigated system,[27,28]the specially designed cyclic PinP(O)Hhas shown good activity in RTCPs of St in previous studies.[83]To assure practical relevance of the obtained results, polymerization conditions were chosen following literature recommendations.[27,28,82]
In addition to the analysis of RTCP systems, general aspects of the stability iodine end-capped polymer will be presented via ESI-MS. In this regard, the potential loss of functional iodine end-groups is important for iodine-mediated polymerizations in general, including ITPs, RITPs, RTCPs, and UV-initiated iodine-mediated polymerizations presented in Chapter 4.
Eventually, a method will be shown to overcome the fairly long inhibi-tion periods prior to polymerizainhibi-tion, which are an inherent feature of the investigated RITP systems, as will be explained in the following.
3.1 Basics & mechanism of RITP
A typical RITP system consists of (i) monomer, (ii) molecular iodine (I2), and (iii) a conventional radical initiator. The RITP mechanism is based on
2 CN Δ
− N2 N N CN
NC
AIBN CP
NC I I2
CP–I or 2 I 2
Scheme 3.2 Simplified decomposition of AIBN forming cyanopropyl radi-cals, CP•, and successive reaction with free iodine, which yields the CTA cyanopropyl iodide, CP−I.
monomer conversion
time
inhibition period polymerization period
mainly I2 CP or
polyMMA CP or MMA
polyMMA or I
Figure 3.1 Schematic profile of monomer conversion versus time in an RITP system of MMA: during the inhibition period, radicals mainly react with free iodine; during the polymerization period, radicals react with MMA (cf. Scheme 3.1 on the previous page).
the fact that free iodine (I• andI2) is a very strong radical scavenger and inhibitor of RPs.[24,100,101] The reaction of carbon-centered radicals with free iodine is even faster than the diffusion-controlled self-termination of carbon-centered low-molar-mass radicals, which is ascribed to minor steri-cal hindrance and to the absence of spin effects.[102–104] As a consequence, almost no monomer conversion is observed in RITP as long as there still is free iodine present in the system. The simplified mechanism is illustrated in Scheme 3.1 on the preceding page. It will be explained based on the example of the here investigated system comprising (i) MMA, (ii) I2, and (iii) 2,20 -azobis(2-methylpropionitrile) (AIBN). AIBN is the most frequently used ini-tiator in RITP systems,[8,18]since it decomposes to give tertiary cyanopropyl radicals,CP•, which lead to the highly activated CTA cyanopropyl iodide, CP−I, after the reaction with free iodine (see Scheme 3.2). Mechanisti-cally, during the first stage—the so-called inhibition period—CP• almost
Table 3.1 Initial concentrations of substances used for RITP (without cata-lyst (cat.)) and RTCPs of MMA in bulk at 80◦C.
entrya [MMA]0 [I2]0 [AIBN]0 [cat.]0 cat.
/ mol L−1 / mol L−1 / mol L−1 / mol L−1
1 ( ) 8.7 4.4 × 10−2 8.7 × 10−2 – –
2 ( ) 8.7 4.4 × 10−2 8.7 × 10−2 1.1 × 10−2 NIS 3 ( ) 8.7 4.4 × 10−2 8.7 × 10−2 1.1 × 10−2 (EtO)2P(O)H 4 ( ) 8.7 4.4 × 10−2 8.7 × 10−2 1.1 × 10−2 PinP(O)H
aSee Figure 3.2 on the next page and Figure 3.3 on page 38.
exclusively reacts with free iodine rather than to effectively initiate poly-merization. In this regard, short oligomeric polyMMA chains are indeed formed as well[25] but promptly deactivated by free iodine (left box). At the end of the inhibition period, the system consists of (i) MMA, (ii)CP−I and oligomericCP−polyMMA−I, as well as (iii) residual AIBN. During the polymerization period,the residual AIBN then initiates polymerization and the system behaves like a common ITP exhibiting a pre-equilibrium and a main equilibrium (right box). A typical profile of monomer conversion versus time is given in Figure 3.1 on the facing page.