A Global View on Block Copolymers

In this systematic review, a total of 45,143 publications on block copolymers, issued between 1952 and 2019, are analyzed in terms of number, source, language, institution, country, keywords, and block copolymer type, to find out their evolution and predict research trends. The number of publications devoted to block copolymers has been growing for over six decades, maintaining a consistent level throughout the last few years. In their majority, documents came out of the United States, although more recently, Chinese institutions are those displaying the largest production. Keywords analysis indicated that one-third of the publications concerned synthesis, around 20% explored self-assembly and morphological aspects, and another 20% referred to block copolymer applications in solution. In particular, 2019 confirmed the expansion of studies related to drug delivery, and in minor extent, to a deeper view of self-assembling. Styrene–butadiene–styrene block copolymer was the most popular in studies covering both basic and industrially oriented aspects. Other highly investigated copolymers are PEO-b–PPO-b–PEO (Pluronic©) and amphiphilic block copolymers based on polycaprolactone or poly(lactic acid), which owed their success to their potential as delivery vehicles. Future trending topics will concern nanomedicine challenges and technology-related applications, with a special attention toward the orientation and ordering of mesophase-separated morphologies.


Introduction
Block copolymers are a class of polymers formed by two or more homopolymer fragments joined together by covalent bonds. Due to their usual reciprocal insolubility, the chemically different blocks easily segregate to form intermolecular phase-separated morphologies either at the solid state or in solution. The size, the periodicity in the nanoscale, the ordering, and the orientation of such microphases in the solid state may, in principle, be controlled by fine-tuning block molecular weights, molecular weight distribution, composition, the interaction parameters between block components, and also the conditions of self-assembly [1]. For block copolymer thin films, additional interactions with the eventual substrate and free interfaces should also be considered [2], whereas for the assembling in solution, the formation of differently shaped micelles, lamellar structures, or vesicles also depends on solubility parameters, the possible crystallinity of the core-forming block, the addition of further molecular components, etc. [3].
All these, nowadays, apparently basic considerations on block copolymers are the result of decades of investigations and publications, which also had the effect to enable their use as a tool for nanomaterial fabrication (after some processing or as templates), model for behavioral studies, vehicle for drug delivery, additive in photovoltaic cells, precursor for nanocarbons, industrial compatibilizing agent, The evolution of the number of publications on block copolymers from the very beginning, with the first publication directly mentioning block copolymer synthesis in 1952 [13] to 2019 is shown in Figure 1. Four different trends are clearly visible. During the first period, from the beginning to 1968, the publications on this subject were infrequent, reaching a maximum of 13 in 1967. The second, until 1992, corresponded to an increase up to around 350 publications per year, whereas in the third, the interest for block copolymers rose dramatically during 20 years, reaching more than 2300 publications per year by 2012, this being a great indicator of the importance of this field in current polymer science research. Finally, from 2012 onwards, the scientific output was fluctuating around a little lower number, possibly suggesting the achievement of a maturity level. The four periods account for 0.1%, 8.8%, 58.9%, and 32.2% of a total of 45,143 documents, respectively.
Polymers 2020, 12, x FOR PEER REVIEW 3 of 16 used to create word clouds of keywords [12]. The keyword size is directly proportional to the number of documents.

Evolution of the Scientific Output and Impact
The evolution of the number of publications on block copolymers from the very beginning, with the first publication directly mentioning block copolymer synthesis in 1952 [13] to 2019 is shown in Figure 1. Four different trends are clearly visible. During the first period, from the beginning to 1968, the publications on this subject were infrequent, reaching a maximum of 13 in 1967. The second, until 1992, corresponded to an increase up to around 350 publications per year, whereas in the third, the interest for block copolymers rose dramatically during 20 years, reaching more than 2300 publications per year by 2012, this being a great indicator of the importance of this field in current polymer science research. Finally, from 2012 onwards, the scientific output was fluctuating around a little lower number, possibly suggesting the achievement of a maturity level. The four periods account for 0.1%, 8.8%, 58.9%, and 32.2% of a total of 45,143 documents, respectively. Most publications on block copolymers are articles (85.3%) or conference papers (10.3%) ( Figure  2). In third position are reviews, which are usually understood to be manuscripts that resume previous publications in a field without using original material; these represent a huge 2.6% of publications. It is worth mentioning that the first document of this type, published in 1971 [14], cited 235 works, nicely focusing on the last developments on synthesis, characterization, and properties, also including an early view on commercial uses of block copolymers. As a comparison, the year with the highest number of reviews was 2016, with 87. Other studies appear at a much lower frequency, such as book chapters (1.0%) or books (0.1%). Furthermore, as most of the studies were published in international journals, mostly English speaking, English is the predominant language; it was found in 93.9% of documents (results not reported in the form of a table or figure). Among the minority languages, the use of Chinese and Japanese, with 2.2% and 2.0%, respectively, envisages a strong presence of these countries.
In order to discuss the impact of publications, their citations are briefly analyzed. To date (by February 17, 2020), the documents dealing with block copolymers have an h-index of 343, with 41 articles or reviews having more than 1000 citations. The three most influent publications have ca. 10,000 [15], ca. 3300 [16], and ca. 3100 [17] citations, respectively. On the other side, the top three books [18][19][20] accumulate a much lower number of citations, ca. 750, 300, and 250 (including 45 Most publications on block copolymers are articles (85.3%) or conference papers (10.3%) ( Figure 2). In third position are reviews, which are usually understood to be manuscripts that resume previous publications in a field without using original material; these represent a huge 2.6% of publications. It is worth mentioning that the first document of this type, published in 1971 [14], cited 235 works, nicely focusing on the last developments on synthesis, characterization, and properties, also including an early view on commercial uses of block copolymers. As a comparison, the year with the highest number of reviews was 2016, with 87. Other studies appear at a much lower frequency, such as book chapters (1.0%) or books (0.1%). Furthermore, as most of the studies were published in international journals, mostly English speaking, English is the predominant language; it was found in 93.9% of documents (results not reported in the form of a table or figure). Among the minority languages, the use of Chinese and Japanese, with 2.2% and 2.0%, respectively, envisages a strong presence of these countries.
In order to discuss the impact of publications, their citations are briefly analyzed. To date (by February 17, 2020), the documents dealing with block copolymers have an h-index of 343, with 41 articles or reviews having more than 1000 citations. The three most influent publications have ca. 10,000 [15], ca. 3300 [16], and ca. 3100 [17] citations, respectively. On the other side, the top three books [18][19][20]  and periodic reviews have a high impact on the scientific community, whereas the importance of handbooks or textbooks, usually conceived to spread basic knowledge or to give practical indications providing ready references, does not seem to be as easy to be measured by this citation parameter. citations of individual chapters), respectively. Similarly to other research fields, original articles and periodic reviews have a high impact on the scientific community, whereas the importance of handbooks or textbooks, usually conceived to spread basic knowledge or to give practical indications providing ready references, does not seem to be as easy to be measured by this citation parameter.

Figure 2.
Distribution of document types for block copolymers.

Geographical Distribution of Publications
All the top 15 institutions (Figure 3), each accounting for more than 400 publications, are nonprofit organizations, either universities or large public institutions dedicated to multidisciplinary research. Only two are Europeans-the French Centre National de la Reserche Scientific and the Russian Academy of Science-while five are from the United States of America, five are Chinese, and three are Japanese. The Tokyo Institute of Technology was the most productive university [21], after three large fundamental science national agencies. Among the 158 institutions with more than 120 publications until 2019, 33 (around 20%) are from the United States. In a similar way, the companies dedicating the most efforts to disseminate knowledge on block copolymers were the American IBM (Almaden Research Center), The Dow Chemical Company, and Exxon Mobil Corporation, with a lower number of documents by the French Arkema Group and the German BASF SE.

Geographical Distribution of Publications
All the top 15 institutions (Figure 3), each accounting for more than 400 publications, are nonprofit organizations, either universities or large public institutions dedicated to multidisciplinary research. Only two are Europeans-the French Centre National de la Reserche Scientific and the Russian Academy of Science-while five are from the United States of America, five are Chinese, and three are Japanese. The Tokyo Institute of Technology was the most productive university [21], after three large fundamental science national agencies. Among the 158 institutions with more than 120 publications until 2019, 33 (around 20%) are from the United States. In a similar way, the companies dedicating the most efforts to disseminate knowledge on block copolymers were the American IBM (Almaden Research Center), The Dow Chemical Company, and Exxon Mobil Corporation, with a lower number of documents by the French Arkema Group and the German BASF SE. citations of individual chapters), respectively. Similarly to other research fields, original articles and periodic reviews have a high impact on the scientific community, whereas the importance of handbooks or textbooks, usually conceived to spread basic knowledge or to give practical indications providing ready references, does not seem to be as easy to be measured by this citation parameter.

Figure 2.
Distribution of document types for block copolymers.

Geographical Distribution of Publications
All the top 15 institutions (Figure 3), each accounting for more than 400 publications, are nonprofit organizations, either universities or large public institutions dedicated to multidisciplinary research. Only two are Europeans-the French Centre National de la Reserche Scientific and the Russian Academy of Science-while five are from the United States of America, five are Chinese, and three are Japanese. The Tokyo Institute of Technology was the most productive university [21], after three large fundamental science national agencies. Among the 158 institutions with more than 120 publications until 2019, 33 (around 20%) are from the United States. In a similar way, the companies dedicating the most efforts to disseminate knowledge on block copolymers were the American IBM (Almaden Research Center), The Dow Chemical Company, and Exxon Mobil Corporation, with a lower number of documents by the French Arkema Group and the German BASF SE.   [21][22][23][24][25], [17], [26][27][28][29][30][31], [15], [32], [33]. The apparent leading position of the United States' investigations is confirmed by the analysis by country (Figure 4), where this entry represents around 26% of the publications. China and Japan are in the second and third positions, respectively. Other countries as Germany, South Korea, France, and the United Kingdom are well positioned in the top 10 list, and they account for more than 2000 publications each; except in the case of France, their institutions are not included amongst the top 15. This situation is similar to that of the United States, where the absence of a strong national agency formed by several institutes working on the same subject is supplied by a widespread interest in many institutions with a proportionally smaller individual output. In Figure 5, the scientific production of each country is color highlighted in a world map. Deep red indicates countries with more than 4000 publications, pale reds refer to countries with a smaller number of documents (in the range of 300-3000), down to oranges and light yellow, the latest indicating countries with at least 1 document. This field of study appears as especially relevant in North America, Europe, and the Far East, with some peaks of scientific production also in Turkey, Australia, Brazil, and India. Interestingly, normalizing the number of documents by the country population, the ranking changes completely ( Figure 6). The top position goes to Singapore with more than 82 publications per million inhabitants, followed by Belgium and the Netherlands, with more than 70. The leadership of this small country is essentially due to the excellent activity by a single recently formed institution, the National University of Singapore (the most cited publication of those therein produced is listed as reference [34]). On the other side, the following European countries have a consolidated tradition of research on polymers, which is well exemplified by their leading institutions, namely the University of Liège (Belgium) [35], the Eindhoven University of Technology (the Netherlands) [36], the ETH Zurich (Switzerland) [37], the University of Athens (Greece) [38], and the Technical University of Denmark [39], respectively. In addition, in the case of the bigger countries of this series, i.e., South Korea and Germany, their production was ascribed to the sharp activity of recent institutions such as the Pohang University of Science and Technology (South Korea) [40] or the tradition of an established research center such as the Max Planck Institute for Polymer Research (Germany) [41], respectively.
Polymers 2020, 12, x FOR PEER REVIEW 5 of 16 The apparent leading position of the United States' investigations is confirmed by the analysis by country (Figure 4), where this entry represents around 26% of the publications. China and Japan are in the second and third positions, respectively. Other countries as Germany, South Korea, France, and the United Kingdom are well positioned in the top 10 list, and they account for more than 2000 publications each; except in the case of France, their institutions are not included amongst the top 15. This situation is similar to that of the United States, where the absence of a strong national agency formed by several institutes working on the same subject is supplied by a widespread interest in many institutions with a proportionally smaller individual output. In Figure 5, the scientific production of each country is color highlighted in a world map. Deep red indicates countries with more than 4000 publications, pale reds refer to countries with a smaller number of documents (in the range of 300-3000), down to oranges and light yellow, the latest indicating countries with at least 1 document. This field of study appears as especially relevant in North America, Europe, and the Far East, with some peaks of scientific production also in Turkey, Australia, Brazil, and India. Interestingly, normalizing the number of documents by the country population, the ranking changes completely ( Figure 6). The top position goes to Singapore with more than 82 publications per million inhabitants, followed by Belgium and the Netherlands, with more than 70. The leadership of this small country is essentially due to the excellent activity by a single recently formed institution, the National University of Singapore (the most cited publication of those therein produced is listed as reference [34]). On the other side, the following European countries have a consolidated tradition of research on polymers, which is well exemplified by their leading institutions, namely the University of Liège (Belgium) [35], the Eindhoven University of Technology (the Netherlands) [36], the ETH Zurich (Switzerland) [37], the University of Athens (Greece) [38], and the Technical University of Denmark [39], respectively. In addition, in the case of the bigger countries of this series, i.e., South Korea and Germany, their production was ascribed to the sharp activity of recent institutions such as the Pohang University of Science and Technology (South Korea) [40] or the tradition of an established research center such as the Max Planck Institute for Polymer Research (Germany) [41], respectively.

Journals
The evolution of the 10 journals that published the highest number of articles on block copolymers over the last 30 years, and those standing in the top 5 positions in 2019 is visible in Figure 8. All over the analyzed period, the first place was occupied by Macromolecules [27,29,35,41], whose trend roughly corresponds to the evolution of the total number of publications visible in Figure 1 (see e.g., the correspondence between the relative minima in 1995 and 2014). This journal alone accounted for 17% of publications in 1990, and its relevance in the field grew up to 29.6% in 1994. Such level was maintained for a few years, then decreasing slowly to less than 10% from 2010 on. The initial faster than average increase of interest may be explained through the ability of the (editorial office of the) journal to intercept from the beginning the potential of block copolymers. In the following years, the transition to applicative topics and the availability of basic and characterization tools to a wider research community made other journals more competitive to collect a larger output. As an example, journals with a more general scope widely focused on materials, as Langmuir or Soft Matter, conveyed the interest in block copolymers toward a better comprehension of interface phenomena [42] and materials design and fabrication [43], respectively. Other general polymer science journals as

Journals
The evolution of the 10 journals that published the highest number of articles on block copolymers over the last 30 years, and those standing in the top 5 positions in 2019 is visible in Figure 8. All over the analyzed period, the first place was occupied by Macromolecules [27,29,35,41], whose trend roughly corresponds to the evolution of the total number of publications visible in Figure 1 (see e.g., the correspondence between the relative minima in 1995 and 2014). This journal alone accounted for 17% of publications in 1990, and its relevance in the field grew up to 29.6% in 1994. Such level was maintained for a few years, then decreasing slowly to less than 10% from 2010 on. The initial faster than average increase of interest may be explained through the ability of the (editorial office of the) journal to intercept from the beginning the potential of block copolymers. In the following years, the transition to applicative topics and the availability of basic and characterization tools to a wider research community made other journals more competitive to collect a larger output. As an example, journals with a more general scope widely focused on materials, as Langmuir or Soft Matter, conveyed the interest in block copolymers toward a better comprehension of interface phenomena [42] and materials design and fabrication [43], respectively.  Amongst the most influential journals that published a quantitatively small but qualitatively important series of articles, it is worth citing the multidisciplinary journals Nature (21 documents) [13,40,44] and Science (43) [15,26], the chemistry journals Journal of the American Chemical Society (463) [45] and Angewandte Chemie (357) [32,46], and the materials science journals Nature Materials (40) [24,36,47] and Progress in Polymer Science (109) [14,48].

Keywords and Block Copolymer Types
The analysis of keywords was carried out by discarding all those that were obvious, such as 'block copolymer', 'block copolymers', 'copolymer', 'polymer', or 'article', and gathering together those that are very similar to each other in a single unifying term. As an example, the keyword 'synthesis' also includes 'copolymerization', 'polymerization', and 'synthesis (chemical)'. Figure 9 shows the top 15 keywords that appeared in more than 2000 block copolymer-related documents. These terms are tentatively gathered in 7 groups. The most ubiquitous keyword group refers to block copolymer preparation and includes 'synthesis' and 'atom transfer radical polymerization', for a total of around 33% of publications. A second group regards molecular and structural characterization techniques (20.3%), including the terms 'molecular weight', 'transmission electron Amongst the most influential journals that published a quantitatively small but qualitatively important series of articles, it is worth citing the multidisciplinary journals Nature (21 documents) [13,40,44] and Science (43) [15,26], the chemistry journals Journal of the American Chemical Society (463) [45] and Angewandte Chemie (357) [32,46], and the materials science journals Nature Materials (40) [24,36,47] and Progress in Polymer Science (109) [14,48].

Keywords and Block Copolymer Types
The analysis of keywords was carried out by discarding all those that were obvious, such as 'block copolymer', 'block copolymers', 'copolymer', 'polymer', or 'article', and gathering together those that are very similar to each other in a single unifying term. As an example, the keyword 'synthesis' also includes 'copolymerization', 'polymerization', and 'synthesis (chemical)'. Figure 9 shows the top 15 keywords that appeared in more than 2000 block copolymer-related documents. These terms are tentatively gathered in 7 groups. The most ubiquitous keyword group refers to block copolymer preparation and includes 'synthesis' and 'atom transfer radical polymerization', for a total Polymers 2020, 12, 869 9 of 16 of around 33% of publications. A second group regards molecular and structural characterization techniques (20.3%), including the terms 'molecular weight', 'transmission electron microscopy', and 'nuclear magnetic resonance', and it is related to basic investigations as well. A third group includes the keywords 'self-assembly', 'phase separation', and 'morphology', which may be found in either basic or applicative publications (20.5%). Another group includes two of the most important block copolymer properties, such as 'hydrophobicity' and 'hydrophilicity' that were investigated in around 9% of publications.
Polymers 2020, 12, x FOR PEER REVIEW 9 of 16 microscopy', and 'nuclear magnetic resonance', and it is related to basic investigations as well. A third group includes the keywords 'self-assembly', 'phase separation', and 'morphology', which may be found in either basic or applicative publications (20.5%). Another group includes two of the most important block copolymer properties, such as 'hydrophobicity' and 'hydrophilicity' that were investigated in around 9% of publications. 'polyethylene oxides' also refers to the sister keyword 'polyethylene glycols'; 'micelles' also refers to the keyword 'micelle'; 'polystyrene' also refers to 'polystyrenes'; 'self-assembly' also refers to 'self assembly'; 'drug delivery' includes the keywords 'drug delivery system', 'drug delivery systems', 'controlled drug delivery', 'drug carrier', and 'drug carriers'; 'transmission electron microscopy' also refers to 'high-resolution transmission electron microscopy'.
The keyword 'nanoparticles' concerned both the direct use of block copolymers for controlling metal nanoparticle fabrication and especially their use for the formation of polymeric nanoparticles in solution, which are intended as micelles, nanospheres, nanocapsules, and polymersomes. Therefore, 'nanoparticles' and 'micelles' were considered together and jointly account for 20.4% of publications, whereas the use of block copolymers for 'drug delivery' is by far the most investigated application (around 13%). In addition, one of the keywords of the last group formed by the two most common class of copolymers as keywords, namely 'polyethylene oxides' (20.0%), mostly refers to their application in solutions. Finally, the 'polystyrene'-based copolymers include investigations on self-assembly and applications either in solid state or in solution and appear in almost 14% of documents.
A first qualitative attempt to follow the change in the focus of investigations over almost 70 years is visualized by the word clouds of the top 20 keywords in the periods 1952-1984, 1998-1999, 2009, and 2019 ( Figure 10), each of them obtained analyzing a comparable number of documents (in the approximate range 1800-1950). During the first period, the interest focused on essential aspects as those related to synthesis and characterization, being 'styrene', 'butadiene', and related keywords (namely 'polystyrenes', 'polybutadienes', 'thermoplastic elastomers', and 'butadiene-styrene block copolymers'), which is a clear reference to the few available block copolymers. In 1998-1999 and later in 2009, some other polymers ('polymethyl methacrylates' and especially 'polyethylene oxides'), polymerization methods ('anionic polymerization' and 'atom transfer radical polymerization'), and improved characterization techniques (e.g., 'light scattering' and 'transmission electron microscopy') appeared as top keywords or showed an increase in their frequency of use. The word cloud of 2009 also shows the growing concerns about the behavior of block copolymers in solution, through the size of the keywords 'micelles', 'nanoparticles', 'drug delivery' and 'amphiphilic block copolymers'. 2019 confirmed such evolution and 'drug delivery' stands as the biggest word, surrounded by strictly related keywords as comparably sized 'micelles' Figure 9. Top 15 keywords in scientific research on block copolymers. 'Synthesis' refers to the sum of the keywords 'copolymerization', 'polymerization', 'synthesis', and 'synthesis (chemical)'; 'polyethylene oxides' also refers to the sister keyword 'polyethylene glycols'; 'micelles' also refers to the keyword 'micelle'; 'polystyrene' also refers to 'polystyrenes'; 'self-assembly' also refers to 'self assembly'; 'drug delivery' includes the keywords 'drug delivery system', 'drug delivery systems', 'controlled drug delivery', 'drug carrier', and 'drug carriers'; 'transmission electron microscopy' also refers to 'high-resolution transmission electron microscopy'.
The keyword 'nanoparticles' concerned both the direct use of block copolymers for controlling metal nanoparticle fabrication and especially their use for the formation of polymeric nanoparticles in solution, which are intended as micelles, nanospheres, nanocapsules, and polymersomes. Therefore, 'nanoparticles' and 'micelles' were considered together and jointly account for 20.4% of publications, whereas the use of block copolymers for 'drug delivery' is by far the most investigated application (around 13%). In addition, one of the keywords of the last group formed by the two most common class of copolymers as keywords, namely 'polyethylene oxides' (20.0%), mostly refers to their application in solutions. Finally, the 'polystyrene'-based copolymers include investigations on self-assembly and applications either in solid state or in solution and appear in almost 14% of documents.
A first qualitative attempt to follow the change in the focus of investigations over almost 70 years is visualized by the word clouds of the top 20 keywords in the periods 1952-1984, 1998-1999, 2009, and 2019 (Figure 10), each of them obtained analyzing a comparable number of documents (in the approximate range 1800-1950). During the first period, the interest focused on essential aspects as those related to synthesis and characterization, being 'styrene', 'butadiene', and related keywords (namely 'polystyrenes', 'polybutadienes', 'thermoplastic elastomers', and 'butadiene-styrene block copolymers'), which is a clear reference to the few available block copolymers. In 1998-1999 and later in 2009, some other polymers ('polymethyl methacrylates' and especially 'polyethylene oxides'), polymerization methods ('anionic polymerization' and 'atom transfer radical polymerization'), and improved characterization techniques (e.g., 'light scattering' and 'transmission electron microscopy') appeared as top keywords or showed an increase in their frequency of use. The word cloud of 2009 also shows the growing concerns about the behavior of block copolymers in solution, through the size of the keywords 'micelles', 'nanoparticles', 'drug delivery' and 'amphiphilic block copolymers'. 2019 confirmed such evolution and 'drug delivery' stands as the biggest word, surrounded by strictly related keywords as comparably sized 'micelles' and 'self-assembly' and smaller 'polyethylene oxides' and 'nanoparticles', possibly marking the route for the following years.
Polymers 2020, 12, x FOR PEER REVIEW 10 of 16 and 'self-assembly' and smaller 'polyethylene oxides' and 'nanoparticles', possibly marking the route for the following years.  Figure 11 shows the number of publications in the period 2000-2019 by type of block copolymer as directly mentioned in the title of the publication or in the abstract, and considered as representatives of the different classes. The trend of each type over those 20 years is shown in Figure  12. As discussed with respect to Figure 1, this range includes the faster growing period of the field.  Figure 11 shows the number of publications in the period 2000-2019 by type of block copolymer as directly mentioned in the title of the publication or in the abstract, and considered as representatives of the different classes. The trend of each type over those 20 years is shown in Figure 12. As discussed with respect to Figure 1, this range includes the faster growing period of the field. Although this approach is not exhaustive, it aims to give an idea about the most investigated copolymers and how their interest is evolving throughout the golden age of block copolymers.
Polymers 2020, 12, x FOR PEER REVIEW 11 of 16 Although this approach is not exhaustive, it aims to give an idea about the most investigated copolymers and how their interest is evolving throughout the golden age of block copolymers.  Styrene-butadiene-styrene (SBS) block copolymer, one of the few industrially available, appears as the most used over the period, both in investigations covering basic aspects [49] and in industrially oriented publications [50]. As a fact, the total number of publications studying this copolymer is growing steadily, up to a maximum of 176 in 2019. Similar considerations, even though for a smaller number of documents, may be extended to styrene-isoprene-styrene (SIS) block copolymer. The interest for poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO) copolymers, and especially for the materials traded as Pluronic© , is mostly related to their biocompatibility and use as vehicles for diagnostic and therapeutic agents [51], and in lesser extent to their application as template, especially for mesoporous silica [15]. From 2002 to 2019, this family of polymers was investigated in 50-100 documents per year. Other highly investigated copolymers, generally composed of biocompatible, biodegradable hydrophobic polymer blocks covalently bonded to a biocompatible hydrophilic block, typically PEO, are those based on polycaprolactone (PCL) [52] and poly(lactic acid) (PLA) [22]. Also these copolymers owe their success to the application as delivery vehicles, confirming once more the central position of this growing sector within block copolymer investigations. During the last 5 years, around 50 articles per Polymers 2020, 12, x FOR PEER REVIEW 11 of 16 Although this approach is not exhaustive, it aims to give an idea about the most investigated copolymers and how their interest is evolving throughout the golden age of block copolymers.  Styrene-butadiene-styrene (SBS) block copolymer, one of the few industrially available, appears as the most used over the period, both in investigations covering basic aspects [49] and in industrially oriented publications [50]. As a fact, the total number of publications studying this copolymer is growing steadily, up to a maximum of 176 in 2019. Similar considerations, even though for a smaller number of documents, may be extended to styrene-isoprene-styrene (SIS) block copolymer. The interest for poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO) copolymers, and especially for the materials traded as Pluronic© , is mostly related to their biocompatibility and use as vehicles for diagnostic and therapeutic agents [51], and in lesser extent to their application as template, especially for mesoporous silica [15]. From 2002 to 2019, this family of polymers was investigated in 50-100 documents per year. Other highly investigated copolymers, generally composed of biocompatible, biodegradable hydrophobic polymer blocks covalently bonded to a biocompatible hydrophilic block, typically PEO, are those based on polycaprolactone (PCL) [52] and poly(lactic acid) (PLA) [22]. Also these copolymers owe their success to the application as delivery vehicles, confirming once more the central position of this growing sector within block copolymer investigations. During the last 5 years, around 50 articles per Styrene-butadiene-styrene (SBS) block copolymer, one of the few industrially available, appears as the most used over the period, both in investigations covering basic aspects [49] and in industrially oriented publications [50]. As a fact, the total number of publications studying this copolymer is growing steadily, up to a maximum of 176 in 2019. Similar considerations, even though for a smaller number of documents, may be extended to styrene-isoprene-styrene (SIS) block copolymer. The interest for poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO) copolymers, and especially for the materials traded as Pluronic©, is mostly related to their biocompatibility and use as vehicles for diagnostic and therapeutic agents [51], and in lesser extent to their application as template, especially for mesoporous silica [15]. From 2002 to 2019, this family of polymers was investigated in 50-100 documents per year. Other highly investigated copolymers, generally composed of biocompatible, biodegradable hydrophobic polymer blocks covalently bonded to a biocompatible hydrophilic block, typically PEO, are those based on polycaprolactone (PCL) [52] and poly(lactic acid) (PLA) [22]. Also these copolymers owe their success to the application as delivery vehicles, confirming once more the central position of this growing sector within block copolymer investigations. During the last 5 years, around 50 articles per year were reporting applications of each of these two classes of amphiphilic copolymers.
Three other diblock copolymers, such as those based on PS-PMMA [40], PS-PEO, and polystyrenepoly(acrylic acid) (PS-PAA), may be considered as model systems and have long been used for both investigations on basic aspects related with phase separation (in solution or in the solid state, depending on the blocks) or for specific applications, often as a tool for nanomaterial fabrication [53]. In particular, PS-PMMA diblock copolymers appeared in 780 publications over the period (with a maximum of 70 in 2013) and were still the focus of many studies on, e.g., lithographic applications [54]. At the end, three other promising classes of block copolymers are those based on poly(N-isopropylacrylamide) (PNIPAM), poly(3-hexylthiophene) (P3HT), and polyacrylonitrile (PAN), especially in publications related to their capacity to respond to temperature stimuli (30-50 publications per year since 2010) [55], in photovoltaic devices (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) publications per year since 2010) [56], or as precursor of nanostructured carbons [57], respectively.

Conclusions and Outlook
Analyzing the bibliometric data on block copolymer research worldwide, from 1952 to 2019, the following conclusions may be drawn: concerned synthesis (with a special mention for atom transfer radical polymerization) (Figure 9), around 20% mentioned aspects regarding molecular and structural characterization techniques, another 20% explored self-assembly, phase separation, and other morphological aspects, and around 9% investigated hydrophobicity and hydrophilicity. In addition, the keywords 'micelles' and 'nanoparticles' account for 20.4% of documents; also, the strictly related term 'polyethylene oxides', mostly referring to their application in solution, was used in another 20% of publications.
The changes in the focus of block copolymer research could be envisaged from the evolution of keywords (word clouds in Figure 10). During the first 25 years, investigations mostly concerned essential aspects, and also until the end of the 1990s, the most common keywords were those related to synthesis and basic molecular and structural characterization, being 'styrene', 'butadiene', and later 'polystyrenes' and 'polymethyl methacrylates', the most common references to specific block copolymers. In 2009, block copolymers in solution, through the keywords 'micelles', 'drug delivery', and 'polyethylene oxides', focused the attention of the research community, as well as did new types of polymerization techniques. The last analyzed year, i.e., 2019, confirmed the expansion of studies related to drug delivery and, to a minor extent, to a deeper view of self-assembly processes, also through the new developments of specific characterization techniques (X-ray diffraction, electron microscopies, etc.). (6) The most representative block copolymer types found in publications since 2000 range from the most commonly used SBS to the peculiar block copolymers based on PNIPAM (thermoresponsive systems), P3HT (application in photovoltaics), and PAN (nanostructured carbon precursor), through the commercial Pluronic© family and other amphiphilic copolymers as those based on PCL and PLA (applications as delivery vehicles) ( Figure 11). The success of other PS-based copolymers lay in their use as model systems (in particular PS-PAA and PS-PMMA diblock copolymers) or for the fabrication of nanomaterials (again PS-PMMA, and also PS-PEO), the latest application essentially due to their easy self-assembly in thin films to form ordered nanostructures with controlled orientation. With respect to the identification of growing trends, both Pluronic© and especially the industrial SBS, showed a renewed appealing, possibly associated to their easy availability and widespread field of application.
With respect to the initial Feynman-like question on the perspectives for block copolymer research, in our opinion, future trending topics will concern nanomedicine challenges and technology-related applications, with a special role played by the fine-tuning of processing conditions to control the orientation and ordering of mesophase-separated morphologies, to establish complex hierarchical features. A deep look into current investigations makes us speculate on the potential held by some hot papers. In particular, we would like to envisage a bright future for applicative challenges concerning the integration of specific block copolymers into large-scale manufacturing, namely in (i) all-polymer solar cells by fully conjugated donor-acceptor block copolymers [58], (ii) soft-solid electrolytes, as in micellar ion gels for self-healing electrochemical devices [59], (iii) nanofiltration membranes [60], and for the fabrication of (iv) N-and S-doped nanostructured carbons useful in adsorption, catalysis, and energy storage [57] or (v) nanoelectronic devices, e.g., through the optimization of infiltration techniques to be inserted into semiconductor fabrication processing [61]. Moreover, as recently discussed in a more general essay by Binder [62], additional strength to block copolymer synthesis investigations could come from the development of methodology mimicking biological systems, e.g., through the use of enzymes in enzymatic ATRP, or from a deeper tuning of "click" reactions to link large and voluminous molecules. At the end, yes, there is still plenty of room for block copolymer research, especially if the players will be able to strengthen connections, either interdisciplinary and with industry, that enable taking advantage of the huge amount of basic knowledge gathered over almost 70 years of excellent investigations.